OCS Study 

MMS 96-0022 

Mississippi River Plume 

Hydrography: 

Second Annual Report 

?y 

T E X A 5 ~ LOUISIANA MISSISSIPPI 

.{ 

-- 

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U .S. Department of the Interior AMN Minerals management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 96-0022 

Mississippi River Plume 

Hydrography: 

Second Annual Report 

Editors 

Stephen P . Murray 

Jami Donley 

Prepared under MMS Contract 

14-35-0001-30632 

by 

Coastal Studies Institute 

Louisiana State University 

Baton Rouge, Louisiana 70803 

Published by 

U.S. Department of the Interior New Orleans Minerals Management Service 

Gulf of Mexico OCS Region September 1996 
DISCLAIMER 

This report was prepared under contract between the Minerals Management 

Service (MMS) and Louisiana State University. This report has been technically 

reviewed by the MMS and approved for publication. Approval does not signify that the 

contents necessarily reflect the views and policies of the Service nor does mention of 

trade names or commercial products constitute endorsement or recommendation for use. 

It is however, exempt from review and compliance with MMS editorial standards. 

REPORT AVAII.ABII.ITY 

Extra copies of the report may be obtained from the Public Information Office 

(MS 5034) at the following address : 

U.S. Department of the Interior 

Minerals Management Service 

Public Information Office (MS 5034) 

Gulf of Mexico OCS Region 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone Number: (504) 736-2519 or 

1-800-200-GULF 

CITATION 

Suggested Citation: 

Murray, S.P. and J. Donley. 1995. Mississippi River plume hydrography: Second annual 

report. OCS Study MMS 96-0022. U.S. Dept of the Interior, Minerals Mgmt. 

Service, Gulf of Mexico OCS Region, New Orleans, La. 175 pp. 

iii 
PRINCIPAL INVESTIGATORS AND KEY PERSONNEL 

Remote Sensing and Image Processing 

Nan D. Walker, Coastal Studies Institute, LSU, Baton Rouge, LA 

Lawrence J. Rouse, Jr., Coastal Studies Institute, LSU, Baton Rouge, LA 

Oscar K. Huh, Coastal Studies Institute, LSU, Baton Rouge, LA 

Hydrographic Survey 

Stephen P. Murray, Coastal Studies Institute, LSU, Baton Rouge, LA 

Curtis C. Ebbesmeyer, Evans-Hamilton, Inc., Seattle, WA 

Neal Pettigrew, Department of Oceanography, University of Maine, Orono, ME 

Sediment Flux 

Wilford D. Gardner, Department of Oceanography, Texas A&M, College Station, TX 

Mary Jo Richardson. Department of Oceanography, Texas A&M, College Station, TX 

Benthic Boundary Layer 

L. Don Wright, School of Marine Science, VIIVIS, College of William & Mary, 

Gloucester Point, VA 

R. W. Sternberg, School of Oceanography, University of Washington, Seattle, WA 

C. Sherwood, Battelle Marine Sciences Laboratory, Sequim, WA 

Light and Nutrients 

R. Eugene Turner, Coastal Ecology Institute, LSU, Baton Rouge, LA 

Hypoxia and Pigments 

Nancy Rabalais, Louisiana Universities Marine Consortium, Cocodrie, LA 

Phytoplankton 

Quay Dortch, Louisiana Universities Marine Consortium, Cocodrie, LA 

Zooplankton 

Richard F. Shaw, Coastal Fisheries Institute, LSU, Baton Rouge, LA 

Pollutant Chemistry 

Jay C. Means, Aquatic Toxicology Laboratory, School of Veterinary Medicine, 

LSU, Baton Rouge, LA 

Debra J. McMillin, Aquatic Toxicology Laboratory, School of Veterinary Medicine, 

LSU, Baton Rouge, LA 

Data Management 

Mary L. White, Coastal Ecology Institute, LSU, Baton Rouge, LA 

Program Management 

Stephen P. Murray, Coastal Studies Institute, LSU, Baton Rouge, LA 

Field Logistics 

Rodney Fredericks, Coastal Studies Institute, LSU, Baton Rouge, LA 

v 
TABLE OF CONTENTS 

Page 

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. .. .. .. .. .. .. .. .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . xi 

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv 

I. Introduction . . . . . . . . . . . . . . . . . . .-----------. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 

II. Hydrographic Survey. .--- . .. ..---- .. .. .. . . .. .. . . .. . . . . . . . . . . . . . . . .- .--- . . . . . . . . . . . . . . . . . .--- .--- . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

A. Cruise III, April 13-22, 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . 5 

1 . Background . . . . .---- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . .----- .5 

2. Wind Velocity Time Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

3. Multiple Interface Data Acquisition System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

a. Underway Wind Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .------ . . . . . . . . . . . . . 8 

b. Near-surface Velocity and Water Property Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 

4. Vertical Sections . . . . . . . . . . . . . . . . . . . . . . . . .------- . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 

B. Cruise N, July 13-22, 1993 . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

1 . Background . . . . . . . . . . .------- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . .--- .-- . . . . . . . . . . . . . . . . .--- . . . . . . .24 

a. Background Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . 30 

b. Satellite Data Acquisition . . . . . . . . . . . . . . . . . . . . .- .------- . . . . . . . . . . . . . . . . . . . . .------- . . . . . . . . . . . . . . . . . . . 30 

c. Pre-SCi.TI.P Drifters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 

2. Property Fields at Constant Depths . . . . .---- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 

3. Acoustic Doppler Velocities at Constant Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ 

4. Vertical Sections --- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . 

a. S1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

b. 12 . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

c. S2. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-- .-- . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .---- . . . . . . .49 

d. S2B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 

e. S3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 

f. S3B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49 

g. S4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 

h. 'I"7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54 

i. SS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

j. S6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

k. S7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 

1. S8 . . . . . . . . . . . . . .---- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . .---- . . . . . . .54 

m. Return Leg . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-------- . 54 

C. Preliminary Summary and Conceptual Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 

1. The Lens-shaped Halocline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 

2. Coastal Undercurrents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .---- . . . . . . . . . . 61 

III . Biological Characteristics Survey. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

A. Phytoplankton Pigments . . . . . . . . . . .-- .- .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

1. Introduction . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .------- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .----- . . .----- . . . . . . . 63 

3. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 

a. Cruise III, April 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 

b. Cruise N, July 1993 --- . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 

vii 
B . Phytoplankton Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 

. 

Cruise 

Cruise 

C. Zooplankton Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .------ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . . O 

1 . Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .---- . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . .------ . . . . 

2. Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- 91 

a. Overview . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 

b. Cruise III, April 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 

c. Cruise N, July 1993 . . . . . . .------- . . . . .---- . . . . . . . . .--- . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . . . . . . .---- . .103 

IV. Field Measurements of Bottom Boundary Layer Processes and Sediment 

Suspension on the Louisiana Inner Continental Shelf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109 

V. Hypoxia. . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- .-- . . . . . . . . . . . . . . . . . . . . . .117 

1 . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 

2. Method . . . . . . . . . --- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 .---- . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . .. .. . . .. . . . . . . . . . .- 

3. Results . . . . . . . . . . . . . . . . . . .----. . . .. .. .. .. ..---- .. .. .. .. . . ..--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . 77 

a. III, April 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 

b. N, July 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

4. Discussion . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 

a. Using Phytoplankton as Indicators of Water Sources and Mixing . . . . . . . . . . . . . . . 85 

b. Seasonal Differences and the Effect of the 1993 Flood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 

c. Toxic and Noxious Phytoplankton Abundance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 

A. Introduction . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117 

B. Methods . .. . . . .- .------ . . . . . . . . . . . . . . . . . .---- .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .117 

C. Results . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .118 

1. Cruise III, April 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118 

2. Cruise N, July 1993 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118 

VI. Remote Sensing and Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127 

A. Methodology and Image Analysis Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127 

B. Historical Analysis: Annual and Interannual Variability 

of Surface Temperatures and Fronts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .128 

1. Introduction . . . . . . . . . . . . . . . . . . . . . . .------ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .128 

2. Shelf Temperature Changes, Frontal Characteristics and 

Circulation Observations during 1989-90 . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . .---- .132 

3. Interannual Variability of Temperature Structure and Fronts . . . . . . . . . . . . . . . . . . . . . . . . .138 

4. Frontal Summary Based on 1989-90, 1992-93 and 1993-94 Satellite Data . . . .138 

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .---- .-- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .141 

VII. Higher Resolution Survey of the Galveston Plume Area and Adjacent Area 

of Convergence . . . . . . . . . . . . . . . . . . . . . . . . .----- . .----- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149 

A. Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149 

B. Parameters Measured . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149 

C. Plume Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149 

1 . Water Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .149 

2. Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .152 

viii 
3. Phytoplankton Pigments and Associated Environmental Variables . . . . . . . . . . . . . . . . 152 

4. Zooplanlcton Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .152 

D. Convergence Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .152 

1 . Water Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .152 

2. Circulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .---- . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154 

3. Phytoplankton Pigments and Associated Environmental Variables . . . . . . . . . . . . . . . . 154 

4. Zooplankton Characteristics . . . . . . . . . .---- .---- . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .154 

VIII. Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .---- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . .157 

ix 
LIST OF FIGURES 

No. Page 

1 Map showing the LATEX B study area. .. .. .. .. .. .. .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 2 

2 Discharge of the Mississippi River at Tarbert Landing, 1988-1993, 

highlighting occurrence of fast four LATEX B cruises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

3 Freshwater discharge down the Mississippi River upstream of the 

Atchafalaya diversion and in the Atchafalaya Rive itself at Simmesport. . . . . . . . . . . . . . . . . . . . . . . . 6 

4 Shiptrack, hydrographic locations, and survey domain for LATEX B, 

Cruise III, April 13-19, 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

5 Wind vectors from four Coastal-Marine Automated Network stations 

along the coast, April 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

6 Shipboard "absolute winds," corrected for ship speed and heading, 

over the cruise survey. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--------- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . 10 

7 Shipboard survey of Doppler velocity measurements made at 4.5 m depth . . . . . . . . . . . . . . . . . . 12 

8 Contours of MIDAS (underway) measurements of near surface temperature 

for Cruise III, April 13-19,1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . 13 

9 Contours of MIDAS (underway) measurements of near surface salinity 

for Cruise III, April 13-19,1993. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

10 Salinity section at S 1 in the eastern LATEX region. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 16 

11 Salinity section from S2, which extends farthest from the coast (-80 km) 

of all the sections in this study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

12 Contour diagram of the eastward component of Doppler velocity 

measured at S2, showing predominantly westward flow with some indication 

of a weak counterflow at depth. . . . . . .. .. . . . . .. .. .. . . . . . . .- .-- .------ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

13 The inner half of the salinity section at S3 is greatly influenced 

by the fresh outflow from Atchafalaya Bay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 17 

14 Section S4 exhibited a "textbook" example of a lens-shaped halocline 

resulting from upwelling of a coastal plume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

15 Northward doppler velocity contours at S4 show a very well-developed 

upwelling circulation with strong onshore flow in the lower half 

of the water column, and strong offshore flow in the upper half. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

16 Eastward doppler velocity contours at S4 show very strong eastward flow 

throughout the section . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

17 Salinity contours along the east/west line, T7--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

18 Northward doppler velocity contours along easdwest line T7, showing 

a very well-developed upwelling circulation along this section of the inner shelf. . . . . . . . 21 

19 Nearshore salinities along section SS are apparently reduced by the freshwater 

inflow from the Calcasieu. .. .. . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

20 The contours of northward flow speed along section SS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

21 Contours of the eastward flow component at SS showing downwind (westward) 

flow in the upper half of the water column, and an upwind counter current 

in the lower layer. . . . . . . . . . . . . . . . .. . . . .------ .. .. ..----- . .. .. .. . . .. . . . . . . . . . . .-- .-- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 

22 Section SSB salinity contours. . .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

23 Crossshore velocities are weak over much of section SSB, but a strong 

onshore flow was observed over the middle portion that does not satisfy 

a two-dimensional continuity constraint. . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

24 The easdwest flow component at SSB is predominantly downwind at the surface and 

upwind at depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . .. .. .. .. .. . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

25 The salinity distribution for the first section west of Galveston Bay (S7). . . . . . . . . . . . . . . . . . . . . 25 

26 Contours of northward flow along section S7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

27 Contours of eastward flow along section S7. . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . .------ .------ . . .- .---- . . . . . . . . 26 

28 Salinity distribution along X2. . . . . . . . . . . . . .. . . . . . . . . .. . . . . .----- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- 27 

xi 
29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

45 

46 

47 

48 

49 

50 

51 

52 

53 

54 

55 

56 

57 

58 

59 

60 

61 

62 

63 

64 

65 

66 

67 

The track of LATEX-B Cruise N showing locations of meteorological 

stations and drifter start points. .. .. .. .. .. .. .. ..---. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 

Shipboard absolute winds, corrected for ship speed and heading, over 

the cruise survey. . . . . .. .. .. .. .--- .. .. . . .. .. . . .. .. .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 

Wind vectors from four Coastal-Marine Automated Network stations 

along the coast, June 1993. .. . . . . .. . . . . .. .. . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . 31 

Wind vectors from four C-MAN stations and the NDCB buoy just east 

of the Mississippi delta, July 1993. .. . . .. . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .--- . . . . . . . . . 32 

Surface temperature frontal analysis of July 8, 1993, revealing three 

distinct thermal regimes on the LATEX shelf. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 

Mississippi River discharge for January through September 1993 

in comparison with long-term monthly means, maxima and 

minima from 1930-1992. . . . . . . . . . . . .. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 

Salinity and density sections along line S2 in July 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 

Turbidity patterns as revealed by the 6 mg/I suspended sediment 

contour on July 17, 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 

Suspended sediment distributions in mg/1 as determined 

from surface water collection during the Cruise N, July 1993. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 37 

Surface temperature fronts on July 17, 1993, 

overlain with the Cruise N cruise track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 

Temperature and salinity sections along line SS in July 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 

Turbidity fronts as observed in satellite imagery of July 20, 1993, 

with near-surface ADCP current vectors superimposed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 

Surface temperature frontal analysis of satellite imagery obtained 

on August 1, 1993, depicting the location and morphology of Eddy Whopper. . . . . . . . . . . . 42 

The trajectory of pre-SCULP drifter #15. . . . . . . . . . . . . . . . . . . . . .- .-------- . . . . . .. . . . . . . . . . . . . . .- .--- . . . . . . . . . . . . . 43 

Near surface salinity along the track of Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

Salinity along the Cruise N track at the 7 m level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 

Salinity along the Cruise N track at the 15 m level . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 

Current vectors along the Cruise N track at the bin centered at 4.5 m depth. . . . . . . . . . . . . . . 46 

Salinity along line S 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 

Zonal velocity component along line S 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 

Salinity along line S2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 

Zonal velocity component along line S2. .---- . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 

Salinity along line S2B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .--- . .--- . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 

Meridional component along line S2B. . . . . . .------- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 

Salinity along line S3. . . . . . .-- .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 

Zonal velocity component along line S3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 

Salinity along line S4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

Zonal velocity component along line S4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 

Salinity along line S5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 

Zonal velocity component along line SS. . . . . . . . . . . . . . . . . . . .-- .-- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 

Salinity along line S6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 

Zonal velocity component along line S6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 

Salinity along line S7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . 57 

Zonal velocity component along line S7.- .---- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. 57 

Salinity along line S8. . . . . . . . . . . . . . . . .-------------- . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 58 

Zonal velocity component along line S8. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . .- .--- .----- . . . . . . . . .. 58 

Velocity vectors at the 4.5 m depth along the return leg cruse track. . . . . . . . . . . . . . . . . . . . . . .. .. .. 59 

Salinity along the return leg south of Atchafalaya Bay showing the two 

low salinity pulses associated with the current jets in Figure 65. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 

Distribution of surface water chlorophyll a, phaeopigments and 

total pigments for Cruise III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 64 

xii 
68 Distribution of surface water chlorophyll a, phaeopigments and 

percent phaeopigments of total pigments by longitude for Cruise III . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 

69 Distribution of near-bottom water chlorophyll a, phaeopigments and 

total pigments for Cruise III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 

70 Relationship between surface chlorophyll a collected by bucket and 

MIDAS tap-in samples and between surface chlorophyll a and 

1v[DAS scaled in vivo fluorescence voltage for Cruise III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 

71 Distribution of chlorophyll a concentrations by salinity, dissolved 

inorganic nitrogen, silicate and the silicate :DIN ratio for Cruise III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 

72 Distribution of surface water chlorophyll a, phaeopigments and 

total pigments for Cruise N. .. .. .. .. .. .. .. .. .. .. . . .. .. . . . . . . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 

73 Distribution of surface water chlorophyll a, phaeopigments and 

percent phaeopigments of total pigments by longitude for Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . 71 

74 Distribution of near-bottom chlorophyll a, phaeopigments 

and total pigments for Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 

75 Relationship between surface chlorophyll a collected by bucket 

and MIDAS tap-in samples and between surface chlorophyll a and 

MIDAS scaled in vivo fluorescence voltage for Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 

76 Distribution of chlorophyll a concentrations by salinity, 

dissolved inorganic nitrogen, silicate and the silicate :DIN ratio for Cruise N. . . . . . . . . . . . . 75 

77 Contours of percent High PI/PE cyanobacteria/total cyanobacteria 

during Cruise III : surface; bottom. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 

78 Contours of percent PC cyanobacteria/total cyanobacteria 

during Cruise III : surface ; bottom. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . 79 

79 Contours of numbers of Skeletonema costatum 

during Cruise III: surface; bottom. .. .. .. . . . . .. .. . . .. .. .. . . . . . . . . . . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . . . . 81 

80 Contours of percent Diatoms/phytoplankton > 8 Fun 

during Cruise III : surface; bottom. ..------ .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . 82 

81 Contours of numbers of Pseudo-nitzschia spp. 

during Cruise III : surface; bottom. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . . . . . . . . . . 83 

82 Contours of percent High PU/PE cyanobacteria/total 

cyanobacteria during Cruise N: surface; bottom. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . . . . . . . . . . . 84 

83 Contours of percent PC cyanobacteria/total cyanobacteria 

during Cruise N: surface ; bottom. 86 

84 Contours of Skeletonema costatum during Cruise N: surface; bottom. .. .. .. . . . . . . . . . . . . . . . . 87 

85 Contours of percent Diatoms/phytoplankton > 8 Em 

during Cruise N: surface ; bottom. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . . . . . . . . . . . 88 

86 LATEX B biological stations during Cruise III, April 1993. . . . . . . . . . . . . . . .. .. .. .. .. . . . . . . . . . . . . . . . . 97 

87 Plankton biomass for 153-dun mesh samples collected during Cruise III . .. . . . . . . . . . . . . . . . . . . 99 

88 Plankton biomass for 335-urn mesh samples collected during Cruise III . . . . . . . . . . . . . . . . . . . . 100 

89 Total larval fish density for Cruise III zooplankton samples. . . .. . . .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . 101 

90 LATEX B biological stations during Cruise N, July 1993. . . . . . . .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . 104 

91 Plankton biomass for 153-Errn mesh samples collected during Cruise N. . . . . . . . . . . . . . . . . . . 105 

92 Plankton biomass for 335-urn mesh samples collected during Cruise N. . . . . . . . . . . . . . . . . . . 106 

93 Total larval fish density for Cruise N zooplankton samples . .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 

94 Vector diagram of near-bottom flows measured in spring 1993 at 20.5 m. . . . . . . . . . . . . . . . . 111 

95 Time series of mean current and bottom orbital speeds near the bed 

at 20.5 m in spring 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .--- .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 

96 Near-bottom velocity profiles from the 20.5 m site in spring 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 

97 Time series of burst-averaged suspended sediment concentration 

measured at four elevations above the bed at the 20.5 m site in spring 1993. . . . . . . . . . . . . . 114 

98 Distribution of near-bottom dissolved oxygen for Cruise III; SeaBird CTD data. . . . . . . 119 

99 Cross-shelf profiles of dissolved oxygen for the S1 line and S2 line 

in April 1993, Cruise III. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . .----- . . . . . . . . . . . . . . .----- . 120 

xiii 
100 Distribution of near-bottom dissolved oxygen for Cruise IV; 

Hydrolab's Surveyor 3 data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 121 

101 Extent of near-bottom water hypoxia documented with the SeaBird CTD 

and the bottom water measurements made with the Hydrolab Surveyor 3 CTD. . . . . . . . 122 

102 Cross-shelf profiles of dissolved oxygen for the S 1 line and S2 line 

in July 1993, Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . 123 

103 Cross-shelf profiles of dissolved oxygen for the S4 line and S6 line 

in July 1993, Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-------- .-- . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 124 

104 CTD profiles from Station C6B during LATEX B Cruise N and 

during the NECOP cruise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .--- . . . . . . . . .. . . . . . . .--- . . . . . . . . . . . . . . . . 126 

105 Image processing procedure summary for NOAH AVHRR data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 

106 Monthly mean air temperatures compared with long-term monthly 

mean air temperatures recorded at Lake Charles, Louisiana, 

from July 1988 through June 1994. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 

107 Map depicting the location of LATEX A mooring lines along 

which SSTs wee extracted in this study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . .--- . . . . . . . . . . . . . . . . . . . . . . . . 131 

108 SST information extracted from satellite image data for line A and line D 

between September 1989 and December 1989. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 

109 Hourly air temperatures recorded at Grand Isle, Louisiana, 

during December 1989 and January 1990. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 135 

110 SST information extracted from satellite image data for line A and line D 

between December 1989 and August 1990. . . . . . .. . . .. .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 136 

111 Schematic depicting the location of major SST fronts on December 26, 1989, 

and January 10, 1990, in relation to the shelf break and 

the LATEX A mooring lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- .--- .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . 137 

112 Surface temperatures extracted from line A and line D, 

which exemplify the lowest temperatures on the inner shelf for each 

of the years 1989-90, 1992-93 and 1993-94. . . . .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . .. 139 

113 Summary figure depicting the average position of fronts along lines A through E, 

as well as one standard deviation for 1989-90, 1992-93, and 1993-94. . . . . . . . . .. .. .. .. . . . . .. 142 

114 Scattergram showing positions of fronts and their intensifies based on all data 

from 1989-90, 1992-93 and 1993-94 for line A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . 143 

115 Scattergram showing positions of fronts and their intensities based on all data 

from 1989-90, 1992-93 and 1993-94 for line B. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. 144 

116 Scattergram showing positions of fronts and their intensities based on all data 

from 1989-90, 1992-93 and 1993-94 for line C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. .. 145 

117 Scattergram showing positions of fronts and their intensities based on all data 

from 1989-90, 1992-93 and 1993-94 for line D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. .. . . . . . . 146 

118 Scattergram showing positions of fronts and their intensities based on all data 

from 1989-90, 1992-93 and 1993-94 for line E. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . . . 147 

119 Plume and convergence surveys: map of sea surface temperature 

from MIDAS, Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. . . . . . . . . . . . . 150 

120 Plume and convergence surveys : map of sea surface salinity 

from MIDAS, Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . . . . . . . . . . . 150 

121 Plume and convergence surveys: ADCP current vectors at 4.5 m depth 

and pre-SCIJLP trajectories, Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . . . . . . . . . . . . . . . 151 

122 Comparison of chlorophyll a biomass to salinity for discrete water samples 

and of MIDAS scaled in vivo fluorescence and salinity 

for the Galveston plume survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. .. . . . . . . . . . . . . . . 153 

123 Comparison of chlorophyll a biomass to salinity for discrete water samples 

and of MIDAS scaled in vivo fluorescence and salinity for the convergence survey. . 155 

xiv 
LIST OF TABLES 

No. Page 

1 Times and dates of survey lines, Cruise III. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

2 Tunes and dates of survey lines, Cruise N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 

3 Phytoplankton groups or species which can be used to trace water masses 

and mixing in the LATEX area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 

4 

. . . . . . . . 

Common phytoplankton groups counted in each size fraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. .. 77 

5 Comparison of mean phytoplankton abundance and relative abundance 

for all cruises in 1992 and 1993. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 

6 Average cell numbers and estimated biomass in all samples collected 

in July through September in the core of the hypoxic region 

in non-flood and flood years. . . . . . . . . . . . . . .. . . .. .. .. . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 

7 Abundance and frequency of occurrence of toxic and noxious phytoplankton 

observed during cruises. . . . . . . . . . . . . . . . . . . . .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 

8 Samples obtained at each station during Cruises III and N. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 

9 Estimated larval fish and squid densities by cruise from data obtained 

during Cruises III and N. . . . . . . . . . . . . . .---.-- .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 

10 List of zooplankton dry weight densities for 153-urn and 335-Am mesh 

samples, total larval fish and squid densities at each station sampled 

during Cruises III and N. . . . . . .. .. .. .. .. . . .. .. .. . . .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 102 

11 Data summary . ---- . . . . . . . . . . . . . . . . . . . .. .. .. . . ..---. .. .. .. .. .. .. .. .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 110 

12 Frontal Analyses and GIF files provided to LATEX personnel in 1993 . . . . . . . . . . . . . . . . . . . . . 129 

13 Average frontal intensity during 1989-90, 1992-93 and 1993-94. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . 140 

14 Maximum frontal intensity during 1989-90, 1992-93 and 1993-94. . . . . . . . . . .. . . . . . . . . . . . . . . . . 140 

15 Average frontal distance from the coast during 1989-90, 1992-93 and 1993-94. . . . . . . . 140 

RV 
I. INTRODUCTION 

The Mississippi River plume Hydrography study (LATEX B) is part of a larger LouisianaTexas Physical Oceanography Program, which began in October 1991. The overall objective of 

our study is to determine the fundamental aspects of the velocity structure, volume and sediment 

flux, pollutant chemistry, and biological characteristics in the coastal plume arising from the 

discharge of the Mississippi and Atchaflaya Rivers . This report covers LATEX B Cruise III (April 

1993) and Cruise N (July 1994). Figure 1 shows the LATEX study area. 

Satellite images and observations of the hydrographic and current regimes of the coastal waters 

from the Mississippi Delta west and south to the Texas-Mexico border indicate the presence of a 

brackish water, turbid plume emanating from the discharge of the Atchafalaya-Mississippi Rivers . 

This plume is strongly modulated and even reversed by the annual cycle of the winds from 

Louisiana to south Texas. The coastal current formed by this plume is locally influenced by turbid, 

low salinity outflow plumes from major coastal bays (Sabine and Calcasieu Lakes and Galveston 

Bay). Intense northerly wind events associated with frontal passages in late fall, winter, and early 

spring apparently completely disrupt the Atchafalaya source of the coastal current and its dissolved 

and suspended sediment particulate load. The Mississippi-Atchafalaya River system typically has a 

peak discharge in April in excess of 30,000 m3/s and a low in September to October of about 

10,000 m3/s. 

These river discharges, mixing with seawater and spreading sediment over the continental 

shelf, create a region of altered (lowered) salinities and temperatures, a seaward sloping pressure 

gradient, and a resulting mosaic of water masses and fronts in the estuarine, inshore, and 

continental shelf areas of the LATEX region. Inner shelf distribution of waters and sediments 

results from the interaction of momentum, buoyancy forces, winds, waves, longshore currents, 

and pressure gradients (sea surface slope and density gradients) . These runoff-altered shelf 

waters, loosely referred to as the coastal current are in fact a complex of river and estuarine 

discharge plumes, mudstreams from rivers, and local zones of resuspended bottom sediment-all 

in various stages of mixing and dispersal in the ambient gulf waters . 

Fresh water distribution has heretofore been the most valuable tracer of large-scale water 

motion in the LATEX region. The fresh water content in the coastal plume and on the shelf is 

clearly an annual cycle triggered by the spring flood of the Mississippi-Atchafalaya Rivers. The 

Mississippi-Atchafalaya discharge is adverted westerly and southerly along the LATEX coastline 

even as far as Mexico by downcoast wind components from the time of spring flood until early 

summer. The onset of song southerly and southeasterly winds in early summer off Mexico and 

south Texas then exert upcoast wind stress components on the low salinity layer near the coast, 

reversing the flow, causing a convergence in the coastal currents, and adverting low salinity water 

offshore from the convergence and offshore Ekman transport in the surface layer. The brackish 

layer is, in a sense, peeled off and plowed offshore by the northward migrating convergence zone. 

The result of this activity is that nearly the entire LATEX shelf east of Galveston Bay out to the 200 

m isobath is covered with low salinity water by late July. With the slackening of the strong 

southerly and southeasterly winds in late summer, downcast flow returns to the LATEX region 

and downwelling favorable winds re-establish the low salinity coastal current. Salinity in the south 

Texas coastal current decreases as water of northerly origin adverts back into the region (Smith, 

1980). Salinities gradually increase in October all along the LATEX coastal plume region as the 

major rivers approach low river discharges . 
w+ 

N J 

Figure 1 . Map showing the LATEX B study area. 
The link between coastal current dynamics, chemistry and biology is nowhere more apparent 

than off central Louisiana. The Mississippi and Atchafalaya Rivers (a third of the total flow of the 

Mississippi River system enters the Gulf of Mexico via the Atchafalaya River) are the major source 

of "new" nutrients to phytoplankton on the inner and mid-shelf. Since the 1950s, water quality in 

the Mississippi River has changed dramatically (Meade and Parker, 1985; Smith et al ., 1987; 

Turner et al ., 1987). Suspended sediments and silicate concentrations have decreased since the 

1950s, whereas nitrogen and phosphorus loadings have increased. These large changes, resulting 

from human activities, provide a basis for concern that this loading, combined with increased water 

clarity, have resulted in a "eutrophication" effect on the shelf. The area impacted by, and the 

duration of, hypoxia on the Louisiana shelf is of considerable concern since Louisiana fisheries are 

28% of the U.S . total. Fish, shrimp, and benthic annual densities are severely depressed in these 

hypoxic zones and critical periods of the life history of several commercially important species 

(Renaud, 1986) may be affected Of more recent concern is the content of chemical pollutants as 

they are transported via fresh water into the open Gulf waters. 

This report on the second field year of LATEX B studies documents an integrated, multidisciplinary approach that combines satellite imagery and image processing with ship surveys and 

scientific analyses (see also Murray, 1994, the annual report from the first year). The shipboard 

activities include measurements of subsurface currents, temperature, salinity, dissolved pigments, 

and phytoplankton and zooplankton systematics. The objective of this report is to present the 

results of Year 2 field observations with minimal synthesis. Field year three will be detailed in a 

future report. The final report will present an in-depth analysis and synthesis of all data sets. 

Station Nomenclature 

The C"TD stations were assigned distinct numbers that con-elate to specific tasks. For example, 

P931001 reflects the following 

P = RN Pelican 

93 =1993, the calendar year 

1 = the first cruise of that calendar year 

001 = the discrete station number assigned to each CID station and used by every 

project task taking samples at that location. 

Cruise III, April 1993, followed the basic survey, using numbers beginning with 001. For 

Cruise N, July 1993, the basic survey stations were numbers 100 through 399. The plume study 

is a 400 series, and the convergence study is a 600 series. All cruises follow this system, so that 

each basic survey stations is as close to the same location as possible. 
II. HYDROGRAPHIC SURVEY 

Figure 2 shows the discharge of the Mississippi River at Tarbert Landing, which is upstream 

of the controlled diversion of about one-third of this discharge into the Atchafalaya River channel. 

Cruise III in April 1993 occurs at a discharge of -35,000 m3/sec, reasonably representative of the 

high flows of the previous five years. Cruise N in July 1993 occurs at a discharge of 

-20,OOOm3/sec, a value 2 to 3 times the July values of the previous 5 years. 

A. Cruise III, April 13-22, 1993 (Neal R. Pettigrew) 

Cruise III of LATEX B took place the week prior to the crest of the 1993 spring "flood" 

(Figure 3). The combined discharge from the Mississippi and Atchafalaya rivers during the Cruise 

III period averaged approximately 3 x 104 m3/s (Figure 3), a value roughly twice that recorded the 

previous April during Cruise I. Comparison of the two April cruises thus provides a good 

opportunity to distinguish between regional oceanographic conditions during dry and wet years. 

While these interannual differences are clear, especially along the Texas coast, the salinity and 

density fields along the Louisiana Shelf are clearly dominated by the influence of fresh-water 

inflow from the Mississippi and the Atchafalaya during both dry and wet spring conditions. 

1. Background 

The shiptrack, hydrostations, and domain for Cruise III are shown in Figure 4. The 

principal survey sections, which run roughly perpendicular to the coast (from offshore of 

Terrebonne Bay, Louisiana to beyond Corpus Christi, Texas) are denoted S 1 through S8, and X 1 

through X3. Between these sections are "tie lines" enumerated T2 through T15 along which 

additional station and underway data were taken while steaming between successive sections. 

Even a cursory look at the experimental domain reveals several factors that may 

substantially complicate the physical oceanography of the study region . In addition to the 

influences of the Mississippi and Atchafalaya rivers, the Louisiana/Texas (LATEX) shelf is 

punctuated by a series of smaller rivers and coastal embayments that could add substantially to the 

variability of the LATEX inner shelf. In addition, there is significant coastal curvature, the 

importance of which upon the structure of both wind-driven and density-driven circulation has 

been prominent in the coastal literature since the seminal work of Csanady (1977) . 

Especially interesting, and potentially troublesome, are complications associated with the 

size of the domain. The LATEX B study area represents an inner-shelf strip approximately 700 km 

long. While the coastal survey of such a vast coastal system offers unprecedented opportunity to 

observe the transitions and interactions of significant alongshore variations in meteorological 

forcing, the data are likely to be complicated by the existence of temporal fluctuations with time 

scales shorter than the survey period In order to distinguish between genuine alongshelf 

variability and temporal abasing of the survey data, shipboard winds and survey data were 

interpreted within the context of time series wind records at multiple alongshore locations. The 

times of each of the survey lines and sections are listed in Table 1. 

5 
40000 

35000 

30000 

25000 

E 

20000 

m 

t 

V 

N 

15000 

10000 

5000 

0 

1 

Time (Date) - 

Figure 2. Discharge of the Mississippi River at Tarbert Landing, 1988-1993, highlighting 

occurrence of first four Latex B cruises. 

40000 

35000 

30000 

25000 

N 

20000 

m 

m 

15000 

U 

N 

D 

10000 

5000 

0 

94 

Figure 3. Freshwater discharge down the Mississippi River upstream of the Atchafalaya diversion 

and in the Atchafalaya River itself at Simmesport . 

6 

N N N N N N N N N N N M (") (7 ('1 ('1 ('I C'I 

of of o~ of o~ o~ o~ rn m a~ a a~ of o~ w rn of of rn o~ a~ o~ o~ o~ w a~ o~ o~ rn rn 

` N ~O I~ O~ O N c7 V f0 1~ 01 O ` c ~ I~ a0 O r N (") N t0 I~ \ N v N f~ 

r N (7 < to ^ ~- ~ ~ ~- ~ ~ 

N N N N N N ('7 C7 O \ ~ r N ~- ~ 

r r r 

fD t~ m 07 O N r N C7 < N <O 1~ OD r r (7 7 1n ~O 1~ 

Time (Date) 
30.0 

29.5 

29.0 

28.5 

28 .0 

Y 

27.$ 

27.0 

26.5 

X4 

Louisiana 

26.0-T- i i i i i 

I t 

98.0 97.0 96.0 95 .0' 94.0 93 .0 92.0 91 .0 90 .0 

Longitude 

Figure 4. Shiptrack, hydrographic locations, and survey domain for LATEX B, Cruise III, April 13-19, 1993 

S2 S1 
Table 1 . Times and dates of survey lines, Cruise III. All times are in GMT (GMT is CDT plus 5 

hours) . Dates are noted in Julian days. 

LiIIQ 

S1 1306 13 April 2156 13 April 

T2 2156 13 April 0310 14 April 

S2 0302 14 April 1123 14 April 

T3 1123 14 April 1545 14 April 

S213 1545 14 April 1956 14 April 

T4 1956 14 April 0040 15 April 

S3 0040 15 April 0717 15 April 

T5 0717 15 April 1248 15 April 

S313 1248 15 April 1717 15 April 

T6 1717 15 April 2138 15 April 

S4 2138 15 April 0337 16 April 

T7 0337 16 April 0928 16 April 

S5 0928 16 April 1700 16 April 

T8 1700 16 April 1955 16 April 

S513 1955 16 April 0133 17 April 

T9 0133 17 April 0432 17 April 

S6 0432 17 April 1104 17 April 

T10 1104 17 April 2140 17 April 

S7 2140 17 April 0107 18 April 

T11 0107 18 April 0849 18 April 

S8 0849 18 April 121 8 18 April 

T12 1218 18 April 2123 18 April 

X1 2123 18 April 0044 19 April 

T13 0044 19 April 1622 19 April 

X2 0622 19 April 1015 19 April 

T14 1015 19 April 1615 19 April 

X3 1615 19 April 2022 19 April 

2. Wind Velocity Time Series 

Time series vector stick plots of velocity are shown for April 1993 in Figure 5. The data 

are from Coastal Marine Automated Network (C1VIAN) stations located at New Orleans, Grande 

Isle, Sabine, and Corpus Christi. The data show April 1993 to have been a month of strong and 

variable winds. Wind speeds in excess of 20 knots wee common, and significant shifts in wind 

strength and direction occurred over time scales ranging from 1 to 5 days. While there is clear 

visual coherence between the records, there are also apparent phase shifts across the array with 

wind episodes generally appearing first at Corpus Christi and propagating northeastward toward 

New Orleans. There are also clear cases of significant wind convergence and divergence related in 

part to the northeastward propagation of the wind signal. 

The survey (April 13-19) occurred during a period of generally southerly and southeasterly 

winds that began on April 10 and ended April 15. During this period there were consistent 

differences in wind speed along the coast, with Grand Isle experiencing winds of roughly 10 

knots, while at the other stations winds were characteristically twice as strong . During the 

principal wind shifts on April 15 and 17, there were significant differences in wind strength and 

direction between Grand Isle and Sabine . 

3. Multiple Interface Data Acquisition System (MIDAS) 

a. Underway Wind Measurements. Data logged by the MIDAS included shipboard 

"absolute winds." These wind records, corrected for ship speed and heading, provide shipboard 

survey data of the wind field over the cruise. It is anticipated that, while these data provide much 

8 
orp s C 

I `S111~\ 

ihrislti 

~\ 

~ ~ ~ 

. !~ 

IA AINI \U1M9 aU1lR'Q LAA\1 ' 11 // TN ~. 

~ 

OA,RAl IU1~11~ 1~11 

~ 

~ ~I 

~ 

A~EYlp M111q~\" iRIY 

~ 

~ 

I 

i 

I 

~1 

I 

~.1A\\ 

I 

I 

I~/~\ 

' 

I~ I i ~ I i I I I I 

l = zo k ~ 

i 

sca , ts 

L 

Figure 5. Wind vectors from four Coastal-Marine Automated Network (C-MAID stations along the coast, April 1993. See 

Figure 1 for locations of the CMAN instruments. 
30 .5 

30 .0 

29.5 

29 .0 

B 28 .5 

a 

0 28 .0 

27 .5 

21 .0 

Latex B, P93-1, IvUDAS, Winds (4km-avg) 

Loulslana 

Scale

o---I-1~= 

20 11 US 

97 .0 96.5 96.0 95 .5 95 .0 94 .5 94 .0 93 .5 93 .0 92 .5 92.0 -91 .5 91 .0 90.5 

Longitude 

Figure 6. Shipboard "absolute winds, corrected for ship speed and heading, over the cruise survey. These data were collected 

by the Multiple Interface Data Acquisition System (MIDAS) aboard the RN Pelican . 
greater spatial resolution of the wind field than can be attained from CIVIAN station data, the Midas 

winds will be abased by the temporal variability that characterizes the meteorology of the region . 

Thus care must be exercised in interpreting the spado-temporal wind field show in Figure 6. 

Several generalizations can be made with regard to winds over the survey domain during 

the study period The winds along the Texas inner shelf were consistently directed onshore. At 

the far southwest end of the survey, winds were particularly strong (roughly 30 knots) and had a 

significant "upcoast" component; that is, in the opposite direction of Kelvin wave propagation. 

Along the Louisiana shelf the wind direction was far less consistent with wind reversals in both the 

cross-shore and alongshore components, as well as apparent convergences and divergences of 

significance occurring in the region between Sabine and Atchafalaya Bay. In the region east of 

Atchafalaya Bay, the wind was directed primarily "downcoast~" 

Comparison between MIDAS and CMAN winds, with reference to Table 1 and Figure 4, 

suggests that, while there was a significant temporal component (abasing) in the MIDAS wind 

convergences, there was also a spatial component to the wind convergence, especially between SS 

and S6 (Figure 4) in the vicinity of Sabine . These data suggest that interesting oceanographic 

consequences may be present in the Doppler velocity and/or hydrographic data. 

b. Near-surface Velocity and Water Property Fields . Figure 7 shows a plot of 

shipboard Doppler velocity measurements made at 4.5m depth. Comparison of these vectors with 

the corresponding shipboard winds measurements shows a clear correlation. This correlation, 

which holds even during periods of rapidly changing wind (see Figure 6 and Table 1), suggests 

that the inner shelf flow field responds rapidly to the variable coastal wind forcing. 

The near surface currents along the Texas shelf are consistently upcoast, although the 

cross-shore component alternates between on- and offshore for masons that are not readily 

apparent Along that portion of the LATEX shelf from Sabine to Atchafalaya Bay the Doppler 

Currents show a pattern that generally reflects the recorded winds; that is, the currents are primarily 

downwind The close relationship between the wind and current vectors along in this part of the 

survey may suggest rapid response to winds varying on time scales short relative to an inertial 

period . Currents are still largely accelerating downwind, and the rotational adjustment (Ekman 

response) has not yet fully developed in some cases. East of Atchafalaya Bay the dovmcoast 

winds have produced a downcoast surface coastal current, although the offshore component along 

72 seems to indicate an offshore surface current not simply plated to the local wind. 

Contours of MIDAS (underway) measurements of near surface temperature and salinity are 

presented in Figures 8 and 9, respectively. Temperature contrasts are very slight with only 2° C 

difference over the 400-mile-long survey area. The warmest surface temperatures are southwest of 

Corpus Christi, and the coolest are found between Sabine and Atchafalaya Bay. Overall the surface 

temperatures were 2 to 3° C cooler during Cruise III (April 1993) compared to Cruise I (April 

1992). 

MIDAS surface salinity distributions showed considerably more structure and variation 

than did the temperature field. A clear change in character of suface salinity variation occurs near 

the mouth of Galveston Bay. Southwest of the Bay, the south Texas inner shelf is characterized by 

increasing alongshore surface salinity with little or no cross-shore variation. In contrast, east of 

Galveston Bay there are strong cross-shore salinity gradients with the fresher surface waters near 

shore, as expected. In addition, there are also very significant alongshore salinity variations . Two 

regions of local salinity minima seem to be associated with fresh water outflow from the 

Atchafalaya and Lake Calcasieu. In interesting contrast to the salinity field southwest of 

Galveston, aside from the nearshore salinity minimum associated with Lake Calcasieu, the surface 

salinity gradients in the region between Galveston and Atchafalaya Bays are directed primarily 

offshore with the isohalines roughly coast parallel. 

11 
30.4 

30.0 

29 .6 

29 .2 

28 .8 

28.4 

a 

28 .0 

27.6 

27.2 

26.8 

97.0 96 .6 96 .2 95.8 95 .4 95.0 94.6 94.2 93 .8 93 .4 93 .0 92.6 92.2 91 .8 91 .4 91 .0 90 .6 

Longitude 

Figure 7. Shipboard survey of Doppler veolcity measurements made at 4.5 m depth. 
30.0 --

29.5 --

29.0 --

28.5 ~~ 

. 

~j ~.s 

2a,o 

L 2 . 

~.0 

27.5 

2 0 

27 .0 -- 

-97 .0 -96.5 -96.0 -95 .5 -95.0 -94.5 -94 .0 -93 .5 -93 .0 -92.5 -92.0 -91 .5 -91 .0 -90.5 

Longitude 

Figure 8 . Contours of MIDAS (underway) measurements of near surface temperature for Cruise 

111, April 13-19,1993. 
30.0 

29.5 

29.0 

a 

j 28 .5 

28 .0 

30 .0 

27.5 

27.0 

27 

~4 

Latex B, P93-1, MIDAS, Salinity 

-97 .0 -96 .5 -96.0 -95 .5 -95 .0 -94.5 -94.0 -93 .5 -93 .0 -92.5 -92.0 -91 .5 -91 .0 -90.5 

Longitude 

Figure 9 . Contours of MIDAS (underway) measurements of near surface salinity for Cruise III, 

April 13-19,1993. 
Since the Mississippi/Atchafalaya runoff was approximately twice as high in April 1993 

compared to April 1992, one might anticipate the observed surface salinities in Cruise III to be 

substantially lower than those observed in Cruise I. Comparison of the two surveys actually 

shows the opposite case along the coast southwest of Galveston. In this region the surface 

salinities are several PSU lower during 1992, which we have characterized as the "dry" year. This 

surprising result is a consequence of the large size of the survey domain and the difference in water 

shed areas. While 1992 was a relatively dry year for the Mississippi River Basin, it was a wet year 

for the drainage area of east Texas. In addition, all Latex B cruises indicate the influence of the 

Mississippi/Atchafalaya outflow plume is not dominant west of Galveston, and that there are 

distinct differences in the hydrography of the regions east and west of Galveston Bay. 

4. Vertical Sections 

The principal physical data set from the LATEX B cruses is comprised of vertical sections 

of salinity, temperature, density, and east-west and north-south components of the Doppler 

velocity. Below a selection of interesting and/or representative sections are presented in order to 

characterize the physical oceanographic conditions of the region and to provide preliminary insight 

into the fundamental dynamical balances operating in the LATEX inner shelf region. Within the 

LATEX B domain it was found that the temperature distributions are relatively homogenous and 

make little contribution to the density distribution. Therefore, only the salinity sections are shown 

below. 

Beginning with the eastern most section (S1) off Terrebonne Bay, Figure 10 shows a 

salinity section that is generally representative of the eastern LATEX region . The retrograde 

salinity distribution shown in the upper 10 m is characteristic of buoyant coastal plumes. In this 

particular case, the coastal plume may be characterized as a "foliated" retrograde frontal structure. 

That is, there are multiple frontal features within the coastal plume that slope upward in the 

offshore direction. From the data shown in Figure 10, the plume appears to be made up of waters 

fresher than 31 PSU. However, because of the foliated nature of the front it is very difficult to 

determine exactly how far offshore the plume may actually extend and what isohalines may 

eventually surface. Further isohaline outcropping could occur offshore of the seaward end of the 

survey section. The Doppler current sections at S 1 showed a complex spatial structure consistent 

with the surface current pattern indicated in Figure 7 and are not explored further here. 

It is interesting to compare April S 1 salinity sections for the wet and dry years of 1993 and 

1992. Qualitatively the two sections are very similar. Both clearly exhibit the foliated frontal 

structure and have their main halocline at roughly 10 m. The lowest salinity values (by - 1 PSU) 

were actually observed nearshore during the dryer year; however, the overall volume of relatively 

freshwater is substantially reduced . As an example, the 30 PSU isoline outcrops approximately 

20 km closer to shore and its greatest depth is 3 m shallower during 1992 compared to 1993. Thus 

the principal response to increased runoff during relatively wet springs seems to be a seaward 

expansion of the buoyant coastal water, rather than a dramatic salinity decrease. 

Section S2 extends farthest from the coast (-80 km) of all the sections in this study. As 

shown in Figure 11 in this section the 31 .5 PSU isohaline fully outcrops, and several deeper 

isohalines have a concave distribution . Once again the foliated nature of the front is apparent, and 

had the section stopped 70 km from shore, one could have concluded that the coastal plume was 

bounded by the 31 PSU contour. The strongest salinity gradient is observed over the inner 3/4 of 

the section at a depth of roughly 5 m. Doppler velocity data (Figure 12) show predominantly 

westward with some indication of a weak counterflow at depth. 

The inner half of section S3 (Figure 13) is greatly influenced by the fresh outflow from 

Atchafalaya Bay. Near-surface salinity values of < 20 PSU and < 10 PSU were respectively 

observed over the innermost 25 km and 12 km of the section. Reference to Figure 8 shows that 

15 
E 

s 

a 

A 

93 109 . . . . r931 i .e . . . . 

ev iio . . . . . evn~os . . . . . . . . . rs~~ios. . . . . . . .e931104 . . . . . . .-

i ~"

295 

27.5 

30.0 

A 2EL5 

2 -31 

4 -- 

310 

'---

0 

2--

4--

Is t 

35.5 

, e?3 i!RI. . . . ., .eve lol 

T 

P93-1, S1, Salinity 

0 

_2 

-4 

-6 

_g _,o 

-12 

-l4 

-16 

-18 

E -20 

-22 

c. 

A -24 

-2E 

-2E 

-3C 

-3 : 

_3< 

-3t 

-33 

v » >V 4J 4U SJ !0 2$ 20 /$ 10 

Distance offshore (km) 

Figure 10. Salinity section at S 1 in the eastern LATEX region . 

. . .~ a. . . .~. . .rvai ~ . r .I. . L . . . . . rs3 u1s . . . . . . . q 117 . . . . . . . . . . . . . nns . . . . . . .eve n 

-v ----- 

- ~ _ 

-

30 

` -

au 

325---' 

P93-1, S2, Salinity 

80 75 70 65 60 55 50 45 40 . 35 30 

Distance offshore (km) 

Figure 11. Salinity section from S2, which extends farthest from the coast (--80 km) of all the 

sections in this study. 

16 
0 

_i . . . ~ . . . . _22 

-4 8 

-6 ,~.J ~ ~ 26 

-8 X-20, 

18 -10 

. . . . . B ~~~ 

.,. -12 

-14 

-16 

_lg .'Z\. ~~ . . . .-4 

.-, 

L -22 \4 2 

~' -24 ~8 

. . . 

-2g . . . ~: _26 l 

-32 

-34 

-36 

-38 

80 75 70 65 60 

Latex B, Cruise 3, ADCP S2 

U Comp . (East Positive) Corrected (a=.1500 b=1.0275) 

55 50 45 40 35 30 

Distance offshore (km) 

Figure 12. Contour diagram of the eastward component of Doppler velocity measured at S2, 

showing predominantly westward flow with some indication of a weak counterflow at 

depth. 

E 

s 

n. 

u 

Distance offshore (km) 

Figure 13. The inner half of the salinity section at S3 is greatly influenced by the fresh outflow 

from Atchafalaya Bay. 

17 
the inner portion of S3 was within a distinct lens of fresh water. Surface salinities at the outermost 

station were < 28 PSU, and one concludes that the coastal plume has been pushed further offshore 

in this location by the outflow plume from the Atchafalaya. The outer half of the section displayed 

an upwelled salinity structure and complex velocity structure during a period of rapidly shifting 

winds. 

Section S4 shows a "textbook" example of a lens-shaped halocline resulting from 

upwelling of a coastal plume. As shown in Figure 14, the lowest salinity water is found in a band 

separated from shore by a relatively weak prograde front Figures 9 and 6 show that S4 is outside 

the near field influence of the Atchafalaya outflow and exposed to the influence of vigorous 

upwelling-favorable winds. Near the bottom the isohalines are downturned suggesting a bottom 

mixed layer several meters thick. 

Doppler velocity sections shown in Figures 15 and 16 are consistent with the above 

interpretations . Figure 15 shows a very well-developed upwelling circulation with strong onshore 

flow in the lower half of the water column, and strong offshore flow in the upper half. Figure 16 

shows very strong eastward flow throughout the section. The core of the eastward flow, located at 

4-10 m depth 5-15 km from the coast, exceeds 0.35 m/s. The strong near-bottom currents are 

consistent with the formation of a bottom muted layer. 

It is important to note that the wind-driven coastal current has a subsurface maximum, that 

it decays offshore, and that it has overcome the buoyancy-driven westward component associated 

with the runoff from the Mississippi and Atchafalaya rivers during spring runoff in a "wet" year. 

These data show clearly the dominance of the wind-driven dynamics even along what is often 

thought of as a runoff dominated inner shelf. Reference to Table 1 and Figure 5 suggests that the 

strong wind-driven upwelling and coastal current system was established within a fraction of a day 

of the onset of upwelling winds. 

It is instructive at this point to examine the tie-line, T7, that runs between the outer stations 

of sections S4 and S5. Figure 17 shows an increase in salinity toward the west revealing the 

expected presence of an eastward baroclinic pressure gradient force that may play a role in the 

bottom intensified eastward flow at S4. Figure 18 shows an alonghore section of the intense 

upwelling circulation referred to above in the discussion of the lens-shaped pycnocline. 

Section SS shows a salinity structure (Figure 19) strongly dominated by freshwater runoff. 

At this location the nearshore freshwater fraction has been enhanced by the influence of the 

Calcasieu. Over the outer 25 km, the upper 12 m is vertically well-mixed giving the retrograde 

structure a particularly steep appearance. The winds at this location and time show considerable 

variation. Over the outer section the winds were downcoast and dowwelling favorable, while at 

the innermost stations the winds shifted to upcoast, upwelling favorable . As discussed earlier, 

although there is significant abasing of the MIDAS wind measurements, there appears to be a 

spatial convergence of the winds between sections SS and SSB . 

Figure 20 shows the cross-shore flow component as measured by the shipboard Doppler 

profiler. The flow over most of the section is consistent with the shifting winds onshore except for 

an apparent local upwelling very close to the coast. The local alongshore component shows a 

fascinating structure (Figure 21). The upper half of the water column flows downwind in a strong 

westward core, centered at 6 m depth and 50 km from shore. In the lower layer there is a 

consistent counter current flowing toward the east Very near the coast are weak eastward 

currents surface-to-bottom. 

The SSB salinity section (Figure 22), in comparison to S5, is much mom saline, reflecting 

reduced influence of the Calcasieu. Winds are uniformly upwelling favorable, and a lens-shaped 

halocline is in evidence near shore. Cross-shore velocities (Figure 23) are weak over most of the 

18 
-4 

-6 

_g . 

E -10 

s 

a -i2 

-ta 

-16 

-l8 

-20 

33 50 45 40 35 30 25 20 15 10 5 

Distance offshore (km) 

Figure 14. Section S4 exhibited a "textbook" example of a lens-shaped halocline resulting from 

upwelling of a coastal plume. 

o - 

_2 

_q 

-6 

_g 

E -10 

t 

Q -12 

-14 

-16 

-I8 

-2C Latex B, Cruise 3, ADCP S4 

V Comp . (North Positive) Corrected (a=.1500 b=1 .0275) 

55 So as an is in ?s ~n is in S 

Distance offshore (km) 

Figure 15. Northward doppler velocity contours at S4 show a very well-developed upwelling 

circulation with strong onshore flow in the lower half of the water column, and strong 

offshore flow in the upper half. 

19 
0 

t 

C1 

-2 

p 

12 

28 

-6 . O ~ ~3a~ 

tt 

10 

_g 

IM4 26 

-10-A 12 

14 

20 

- 

22 

_lq 20 ~tg 

/ 

22 16 

-16- 

22 

-12 i 

-18 

-U ~ 8 Latex B, Cruise 3, ADCP S4 

-20 

U Comp . (East Positive) Corrected (a=.1500 b=1.0275) 

55 50 45 40 35 in 1) 5 In I c in c 

Distance offshore (km) 

Figure 16. Eastward doppler velocity contours at S4 show very strong eastward flow throughout 

the section. 

P97 131 P931150 P971149 P931148 P93 

32p 

_4 i ~~32\ 

_ 3~ I I I 3t5 

-8 - I I i 

i 

E -10 I -32.5 

s 

A -1z I~ i 

33.0 

-14 

-16 ~ _ 

I 33` .5 

f I /' 

34.0 ~ r 

I 

-20 

P93-1, T7, Salinity 

5 10 15 20 25 30 35 40 45 50 55 60 6 

Distance (km) 

Figure 17. Salinity contours along the east/west line, T7 . 

20 
E -

r 

A 

_ 

10 

_ e 0 a o 

2 

5 in is 1n 15 1n IS an at to ca An A c 

Distance (km) 

Figure 18. Northward doppler velocity contours along east/west line T7, showing a very welldeveloped upwelling circulation along this section of the inner shelf. 

8 

s 

6 u 

A 

P93 I3I P93II31 . . . . P911113 . .P971154 

. . .7 0.. . . . . . . .. . . . . . . . . . . 

4 

305 

6 

3t0 ' 

3t5 

8 

i 

10-- 

3 

31.5 

]2 

14 334 i 25 

.16 ~ I 

-18 ~ ~l\ 

P971133 ., . P97II36 . . . P971157 . . . . . P93II58 P97I159 P93116 

\\\ 

P93-1, S5, Salinity 

70 65 60 55 50 45 40 35 3U 25 zo i~ 

Distance offshore (km) 

Figure 19. Nearshore salinities along section SS are apparently reduced by the fresh inflow from 

the Calcasieu. 

21 
0 

_2 

-4 

-6 

E 

-10 

C1 

-12 

_,4 

-1s 

-lE 

. . . . 

:`4 

1 .0. . . . . . . . . . : $ . : . ~12 ~ . 

. . 

14 

10 

16 

12 C20 

14 

70 65 60 55 50 45 40 

.Latex B, Cruise 3, ADCP SS 

V Comp . (North Positive) Collected (a=.1500 b=1.0275) 

I I I I L . I 

35 30 25 20 IS 10 5 

llistance offshore (km) 

Figure 20. The contours of northward flow speed along section S5. 

0 

. . . . . 2 . . . . . . . . ; , 

. . . . . . : to ~ ' . . : . : . 

.~: 

: -12 

~ . . . . . 

.L~ . . . . . . . . . . . . . 

. . . 

. . 

2 

-10 ~~ t2 8 

-12 / ~ 

4 'tt ~ 2 

b 

10 ~2' 

6 

b 

_1$ Latex B, Cruise 3, ADCP S5 

U Comp . (East Positive) Corrected (a=.1500 b=1 .0275) 

70 65 60 55 50 45 40 35 30 25 20 15 10 5 

Distance offshore (km) 

Figure 21 . Contours of the eastward flow component at SS showing downwind (westward) flow 

in the upper half of the water column, and an upwind counter current in the lower 

layer. 

22 
V - 

p93 166 P971163 P931161 P971167 P97 162 

.3pp . . . . . . . . . . . . . .~0 . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. . . . . . . . .. . . .. . . . . . . . . . . . . . . 

-2 - 30_5 

1 

-4 310 

28S 

-6 

_g 310 ~2TA 

-10 

_12 g2 

-~~- 315 ~ 

925 

-14- 

-16 

P93-1, SSB, Salinity 

70 65 60 55 50 45 40 35 30 25 20 15 10 

Distance offshore (lun) 

Figure 22. Section SSB salinity contours. 

0 

_2 

-4 

-6 

E 

24 . 

. . . 

. . . . . ~-Z . . . . . . . . : . . . . . . . . .

. . 

. . . 

. 

. . 

. . . . . . . . . . . . . . . . . _q . . . . . 

2 

. ~ Latex B, Cruise 3, ADCP SSB 

S 

V Comp . (North Positive) Corrected (a=.1500 b=1 .0275) 

_g 

ts , -10 

A 

-12 

-It 

70 65 60 55 )u 4:1 9V » w c

Distance offshore (km) 

Figure 23. Crossshore velocities are weak over much of section SSB, but a strong onshore flow 

was observed over the middle portion that does not satisfy a two-dimensional 

continuity constraint . 

23 
section, but a strong onshore flow was observed over the middle section that does not satisfy a 

two-dimensional continuity constraim Although not as well organized as the alongshore flow at 

S5, the east/west flow component at SSB, shown in Figure 24, is again predominantly downwind 

at the surface and upwind at depth. However, because of the wind reversal between the two 

sections the currents at SS and SSB are basically mirror images of one another. To the extent that 

the survey measurements are synoptic, these results suggest that there is an alongshore 

convergence in the upper layer, and a divergence in the lower layer. The net onshore flow 

observed at this section may reflect its participation in a complex dim-dimensional mass balance in 

response to the convergent wind field. 

The salinity section at S6 is typical of the retrograde distributions observed east of 

Galveston Bay and is not presented here. Current data, also not presented, show moderate 

eastward flow and an upwelling cross-shore circulation. 

Figure ZS shows the salinity distribution for the first section west of Galveston Bay (S7). 

The basic nature of the stratification is fundamentally different from that observed east of 

Galveston Bay. The retrograde structure associated with the buoyant coastal plume is missing, and 

in its place is hydrography typical of most coastal regions: a tight, horizontal pycnocline that fans 

shoreward reflecting coastal mixing. Within a few internal Rossby ratio of the shore, the halocline 

(and pycnocline) may be upwelled or downwelled in response to wind-stress forcing. At S7 the 

wind was directed primarily onshore. Nevertheless, the halocline is slightly upturned, and the 

Doppler current profiles (Figures 26 and 27) show a vigorous northeastward flow in the upper half 

of the water column and a south southwestward flow in the lower region. 

The next two sections, S8 and X1, are generally similar to S7. The winds are still directed 

primarily onshore, there is no downward deflection of the strong pycnocline that appears about 10 

m off the bottom, and near-surface currents are generally upcoast (Figure 6). In contrast, along 

X2 there is some indication of a downwelling of the salinity field (Figure 28) by the action of the 

strong onshore wind. Strong coastal currents with magnitudes exceeding 0.3 m/s were observed 

within 15 Ian of shore. Currents were even stronger at X3 where they exceeded 0.5 m/s near 

shore. No hydrographic data were obtained along section X3. 

B. Cruise IV, July 13-22, 1993 (Stephen P. Murray) 

Cruise N departed Cocodrie 0000 hours GMT on July 13, 1993, and proceeded to make 

observations from east to west along the cruise track of the summer regime hydrography and 

current fields as shown in Figure 29. 

Ship-corrected winds along the cruise track (Figure 30) are in essential agreement with the 

coastal stations, with weak easterly winds at the beginning of the cruise (c.f. Grand Isle Figure 

32), changing to stronger southerly winds off the Sabine Pass area, and to stronger southeasterly 

winds off south central Texas. The ship winds often show local short period weather effects not 

apparent in the widely separated C Man stations (Figure 31, 32). 

1 . Background 

Cruise N is of special interest in that in June, the month preceding the cruise, the Surface 

Current and Lagrangian Drifter Program (SCULP), funded by MMS to Scripps Institution of 

Oceanography, released an array of test drifters on the Louisiana shelf. Some of these drifters 

were still being tracked by the time of Cruise N in mid-July. The movements of these "preSCULP" drifters significantly aid in interpreting the cruise results and especially in placing these 

results in a longer temporal and spatial context 

24 
0 

_Z 

-4 

-6 

E 

-10 

C] 

-12 

-l4 

-lb 

Distance offshore (km) 

Figure 24. The east/west flow component at SSB is predominantly downwind at the surface and 

upwind at depth . 

V 

P97 179 . . . 

_z 

-4 

E 

L 

d 

u 

A 

12 L6o 

3 

14 

16 

P931 190 P9 31181 

. . . . . . . . . . . . . . 

2&5 

-29.5- 

P 93 -1 . S7, Salinity 

30 25 20 15 lu 

Distance offshore (km) 

Figure 25 . The salinity distribution for the first section west of Galveston Bay (S7). 

P971181 . . . . P91IIY7 

25 
0 

_z 

_4 

-6 

E 

-10 

C1 

-12 

-14 

-16 

Distance offshore (km) 

Figure 26. Contours of northward flow along section S7. 

0 

_2 

-4 

-6 

E -8 

r 

Q -10 

-12 

-14 

-lE 

10 

2 

6 

J 

8 

. , . 

, . . . -6

. . . . . . . . . . . . . 12 ~ . 

-16 

. . 14 

: . .12 

. . . . . . 

. . . -8 . . . . 

. . . 

-12 

Latex B, Cruise 3, ADCP S7 

V Comp. (North Positive) Corrected (a=.1500 b=1 .0275) 

30 25 20 15 10 5 

.SU 25 20 IS 10 5 

Distance offshore (km) 

Figure 27. Contours of eastward flow along section S7. 

26 
0 

-z 

-a 

-6 

_g 

-1o 

-12 

E 

-t -14 

Q -16 

-18 

-20 

-22 

-24 

-2E 

. . . . . . . . . . . 

~~~zo~ ~312o. ~312os . . . . . . . . . . . . . . . . . ~_: . . . . _ ` . . . 

30.00' ~ 

sooo 

i 

I I 

30.0 

I 

i 

_~aos ~ I 

i 

30.5 

34.0 

P93-1, X2, Salinity 

30 25 20 IS 10 5 

Distance offshore (km) 

Figure 28 . Salinity distribution along X2. 

27 
30.50 

30. 

29. 

29. 

N 

00 28 .5 

28 

27.50 

27 

LATEX B, P93-2, Cruise 4 

Wind Station (SRST) 

95 .60 94 .60 93 .60 92 .60 91 .60 90 .60 89 .60 

Figure 29. The track of LATEX-B Cruise IV showing locations of meteorological stations and 

drifter start points . 

Drifter Start Pos. S2 s1 
30 .5 

30.0 

29 .5 

29 .0 

't7 

N 

28.5 

28 .0 

27.5 

27 .0 

Latex B, P93-1, IvMAS, Winds (41rni-avg) 

Scale ~= 20 nVs 

-V/ .U -lb .) -yb.U -yS .S -yS.U -y4 .5 -y4 .U -93 .5 -93 .0 -92 .5 -92.0 -91 .5 -91 .0 -90 .5 

Longitude 

Figure 30. Shipboard "absolute winds, corrected for ship speed and heading, over the cruise survey. These data were collected 

by the Multiple Interface Data Acquisition System (MIDAS) aboard the RN Pelican. 
a. Background Meteorology. In addition to the buoyancy forcing provided by the fresh 

water discharge from the Mississippi and Atchafalaya Rivers, the coastal wind field is another 

primary driving force. Study of the Climate Diagnostics Bulletins (NOAH, 1993) in conjunction 

with the C-MAN coastal wind stations (Figure 31), produces a good understanding of the synoptic 

scale wind forcing over the coastal current region . By 7 am June 1 CST a cold front had just 

passed southward over the entire LATEX coast from the Mississippi Delta all the way south to 

Brownsville. Figure 31 shows that the associated northerly winds behind the front blew all day of 

June 1 at Grand Isle and Sabine Pass (Sea Rim State Park) while only lasting about 12 hours at the 

southernmost station at Port Aransas. On June 2 a long period of southerly and southeasterly 

wind sets in which peak around June 9, but continued with diminishing speeds until June 13. The 

persistence of a high pressure cell OW), a manifestation of the Bermuda high pressure cell, 

centered over the eastern states (Tennessee, the Carolinas) and the adjacent shelf, was responsible 

for these southeasterly winds. 

By June 13 this HPC had dissipated to the extent that the synoptic wind almost vanished, and 

Figure 31 clearly shows a sea-land breeze circulation system at the 3 coast stations on June 14, 15, 

and 16. On June 17 and 18, a new HPC in the same region re-established the easterlysoutheasterly wind regime over the LATEX coast Abruptly on June 19, the intense cyclonic 

circulation of Tropical Storm Arlene appears off the south Texas coast The intense curvature of 

the isobars and strong winds of the stone extended 150 run north to the Galveston area, and the 

orientation of Arlene brought strong northeasterly winds to the western Louisiana and Texas 

coastal region. The signature of Arlene is clearly seen in Figure 31 in the Port Aransas winds on 

June 18 and 19, blowing from the northeast and east The Sea Rim-Sabine Pass winds shifted to 

northeasterly for a shorter interval on June 18-19, and the Grand Isle winds appear unaffected by 

the storm's passage (Figure 31). 

For the remainder of June high pressure cells persisted over the central eastern seaboard states . 

These HPCs brought strong southeasterly winds to Port Aransas, southerly winds to the Sea Rim 

Sabine Pass area, and much weaker variable winds with southerly components to the east central 

Louisiana-Grand Isle coast 

The first half of July (Figure 32) is a classic summer wind pattern The Daily Weather Maps 

repeatedly showed only a very weak atmospheric pressure gradient across the northern LATEX 

region. The central and south Texas region, however, had an appreciably higher pressure gradient 

There were strong southeasterly winds off Port Aransas, strong southerly winds off Sea RimSabine Pass, and weaker and more variable southerly and southeasterly winds off Grand Isle . A 

strong wind stress gradient acted to push the south and central Texas coastal water up to the north 

and east toward the weak wind stress region off Louisiana. 

This summer pattern is perturbed only briefly on July 21-22 when a mesoscale low pressure 

cell forms over Georgia and Alabama. The ensuing cyclonic circulation brought northwesterly 

winds to the east central Louisiana coast as seen in the Grand Isle Station data in Figure 32. In the 

last four days of July, the synoptic scale pressure gradients weakened considerably and diurnal 

land-sea breeze effects are seen in the Southwest Pass, Grand Isle, and to a lesser extent in the Sea 

Rim-Sabine Pass wind data (Figure 32). 

b. Satellite Data Acquisition. Several satellite image data sets were obtained over the 

LATEX region during July 1993, which have helped in interpreting the hydrographic 

measurements. A clear-sky image was obtained on July 8, about 6 days before the cruise started, 

which revealed an extensive region of warm water (> 30° C) over the northern portion of the 

LATEX shelf (Figure 33). Between 91* and 95° W longitude, approximately 75% of the 

continental shelf was covered with this warm water mass. It is thought that this warm water mass 

reveals the spatial extent of buoyant river waters across the LATEX shelf west of the Mississippi 

30 
Cruise 4, Winds June 

1 7 3 4 7 I 7 9 1 10 11 17 11 14 15 16 17 19 19 20 11 21 23 71 27 76 11 72 29 70 31 

w I Grand Isle 

Sea Rim State Park 

Port Aransas 

Figure 31 . Wind vectors from four Coastal-Marine Automated Network (C-MAID stations along the coast, June 1993. See Figure 1 for locations of the CMAN instruments. 
Cruise 4, Winds July (1-30) 

i 2 3 4 3 6 7 1 9 i0 11 i2 13 14 15 i6 17 is i9 20 1i 11 33 24 15 76 17 >9 29 30 31 

NDBC Buoy 30. ION 

Southwest Pass 

w 

N 

Grand Isle 

Sea Rim State Park 

Port Aransas 

Figure 32. Wind vectors from four C-MAN stations and the NDCB buoy just east of the Mississippi delta, July 1993. See 

Figure 1 for locations of the instruments. 
28N 

'goo 

- ~ ° 

, ;-

11~ 

. . . . .,. . . . . . . . . . . .~. . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . .--- . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . 

p 

f ; 1^1 ~ 

< P7 C ~ 

iF / 

`r, 

. : .J. . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . .~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Figure 33. Surface temperature frontal analysis of July 8, 1993, revealing three distinct thermal 

regimes on the LATEX shelf. Water temperatures >30° C depict the distribution of 

river waters across the continental shelf. Water temperatures < 27° C reveal the spatial 

distributionof cool upwelled waters on the south Texas shelf. Waters of 27° to 30° C 

reveal a mixing zone between the two. The Cruise N track is overlain for reference. 

'33 
delta. River discharge was abnormally high during both the spring and summer of 1993 (Figure 

34), thus helping to explain its widespread distribution across the shelf. Fortuitously, during the 

summer months, Atchafalaya and Mississippi River waters are sometimes traceable by their 

warmer temperatures, particularly in the late afternoon satellite imagery. During July 1993, cruise 

measurements obtained along S2 revealed the river water to be a thin surface lens of low-salinity 

water approximately 45 meters deep, separated from denser shelf water below by a strong 

pycnocline (Figures 35a,b) . Thus, in regions where river water is found at the surface, the surface 

mixed layer is relatively shallow and the incoming solar radiation is confined to the top few meters 

producing relatively warm surface waters. 

Further south, along the south Texas coast, cool waters (< 27° C) were observed over a 

relatively large area from south of Brownsville to 28° N (Figure 33). It is hypothesized that the 

prevalence of cool water along this coastline and on the shelf was caused by wind-forced 

upwelling as the coolest waters were located adjacent to the coast and the prevailing wind regime 

was upwelling favorable. The presence of northward extending cool filaments are suggestive of the 

prevalence of northward surface flow along the southern and mid-portions of the Texas shelf. 

Although the LATEX-B cruise did not traverse these cool features, northward flow was 

encountered along the southernmost lines, even 11 days after acquisition of this July 8 image. 

The next image yielding substantial information of surface temperature or suspended 

sediment distributions was obtained on the afternoon of July 17, 1993 (2142 Z). The 6 mg/1 

suspended sediment contour reveals that the most turbid region along the coast was located 

seaward of Atchafalaya Bay (Figure 36). The Atchafalaya sediment plume extended slightly 

further to the southeast than in any other direction, although the 6 mg/1 contour was situated about 

mid-way along the LATEX-B lines. The synoptic satellite observations compare surprisingly well 

with the (non-synoptic) surface suspended sediment measurements obtained on the cruise (Figure 

37). In both datasets suspended sediment distributions were relatively high along the Chenier 

coast, in the Sabine and Galveston plumes, and along the coast south of Galveswn. The bay/river 

plumes are often regions of high turbidity as they represent discharges from the more turbid rivers 

and bays. The high turbidity regions adjacent to the Chenier plain and south of Galveston were 

probably attributable to wind resuspension of nearshore sediments as the wind was relatively 

strong and directed onshore for most of the cruise. 

The SST information obtained on July 17, 1993, at 2142 Z (Figure 38) is particularly 

interesting as the ship crossed a coastal temperatureJsalinity/density front twice within 9 hours of 

image acquisition (between July 17, 1700 Z, and July 18, 0200 Z). The image data revealed a 

thermal front ranging from 0.4 to 0.7° C running approximately east-west, becoming narrower 

towards the west The ship's location at the time of image acquisition is depicted with an asterisk. 

The Galveston plume was observed as a warmer water mass lying to the southwest of this coastal 

from Lines SS and S6 crossed the coastal front (Locations, Figure 38). Along line S5, a surface 

thermal front was encountered in close proximity to the third station from the coast (Figure 39), 

agreeing well with the frontal position as observed in the satellite imagery. Along S6, the surface 

thermal front was encountered between the 2nd and 3rd stations from the coast (not shown). The 

satellite data depicted the front nearer to the 2nd station from the coast This slight discrepancy 

may have resulted from a small movement in the front over the 5 hours between satellite image 

acquisition and the collection of data. The cruise data revealed this relatively warm coastal water 

mass as a thin lens of low salinity water approximately 4 meters deep, which extended about 25 

km from the coast (Figure 39). It is postulated that this near-coastal water mass had its origins in 

Atchafalaya Bay to the east for several reasons. The combination of cruise measurements and 

image data revealed that this relatively warm, low salinity coastal water mass became more narrow 

and shallow in a westward direction suggesting an origin to the easy In addition, the image data 

revealed the Galveston plume as having its own distinct front, separate from that of this coastal 

water mass (Figure 38). The low salinity water mass was probably confined close to the coast by 

34 
50,000 

g 40,000 

N 

L 

m 

30,000 

.n

v 

20,000 

m tM 

s 

y 10,000 

c 

0 

- 1993 -Ar- Min (1930-1992) 

- Mean (1930-1992) ~ fig-- Max (193x1993) 

Jan. Feb. Mar. Apr. May June July Aug. Sept Oct. Nov. Dec. 

Time (Months) 

Figure 34. Mississippi River discharge for January through September 1993 in comparison with 

long-term monthly means, maxima and minima from 1930-1992 (from Walker et al., 

1994). 

35 
P972115 P972116 

-2 

-4 

-6 

-8 

-10 

-12 

-14 

-16 

-18 

-20 

-22 

-24 

_-26 

~ -28 

.~ -30 

-32 

Q -34 

-36 

-38 

-40 

-42 

-44 

-46 

-48 

-50 

-52 

-54 

-56 

P932118 P932120 P932121 P932122 P932123 P932124 

` 1 

~ z\as 

- 295 

- ~u 

325 - ~~ 

~I` 

33 .5 

- 34A 

`345 

315 

360 

P93-2, S2, Salinity 

.0 5.0 10 .0 15 .0 20 .0 25 .0 30 .0 35 .0 40 .0 45 .0 50 .0 55 .0 60 .0 65 . 

Distance (km) 

v 1s-I n~~43-''P911 ns . . . . . . . . . r"vn . . . . . . . .rss tra 

-4 

-6 a 

-10- -t9_ 

9- -

-12 201_ 

-14 ----------

-16 - 

-18 -

_Zp ~~ -

-22 -23 - 

-24- ~ 

E -26 -2a ~_ _Zg . 

-30- Za 

p -32 

-34 - 

25 -36 

-38 -

-40 _q2 . -/25 

q. .t . . .

.r":uo . . . . . . ..n 21 . . . . ..rri-. .. . . ( r9~ rrr . . . .~ . . 

72~ ~ ~-- 

P93-2, S2, Density, sigma-theta, kg/m^3 

vv 8s 80 75 70 65 60 55 50 45 40 35 30 

Distance offshore (km) 

Figure 35. Salinity (upper panel) and density sections (lower panel) along line S2 in July 1993. 

36 
Figure 36. Turbidity patterns as revealed by the 6 mg/1 suspended sediment (or seston) contour on 

July 17, 1993. Cruise N track has been overlain for reference. 

30 .5 

Latex B, P94-2, Suspended Sediment 

300 

29 .5 

29 .0 

28 .5 

v 

U 

10.0 

fl0 

7.0 

8.0 

r J \ 

l 

1 

7.0-~ 

?3 .0 

-95 .5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -9? .5 -920 -91 .5 -91 .0 -90 .5 

Longitude 

Figure 37 . Suspended sediment distributions in mg/1 as determined from surface water collection 

during Cruise N, July 1993 . 

37 
Figure 38. Surface temperature fronts on July 17, 1993 (2142 Z), overlain with the Cruse N 

cruise track. The position of the ship is depicted with an asterisk. 

38 
E 

o. 

A 

0 

P93 137 . . . . . . .- I38 P9]1139 P9)1160 P971161 . P931162 . . . . . . P9)1167 . . . . . . . P911164 P971163 P911 I" P91 167 .2 . . . . "~~.~. . .~ . . . . . . . . . . . . . . . . . . . . .f . . . . . . . . . . . .~ . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . 

29.2 

,4 I , I \ \ i b 

I 

292 

29.4 

\, 292 

292 ~ ~ 29A ~ . 

~10 j -1 2 

292 I 

-14 

-16 

i 29.0 

-18 

288 

-20 2B4/ 

272 

~2~1 MINIA 

P93-2, S5, Temp . (C) 

85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 

Distance offshore (km) 

1-97 137 P97113! P931139 P931160 P931161 P971161 P931163 P931164 P971161 P971166 P97 1 

z 

t . . . . . . . . . . . . . . . . . . .Z .0 . . . . . . . . . . . . . . . . . .r . . . . I . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

( ~ 

. . . . . . . . 

i ~ ~ 

I I I\ 24A 

9A0 

25.0 

B i ;-

MO 

2&0 

10 

29,0 

12-- i 

14--

16 --

~ soo- - 

20 - --f i 

22 

E 

n. 

u 

A . P93-2, S5, Salinity 

85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 

Distance offshore (km) 

7 

Figure 39. Temperature (upper panel) and salinity (lower panel) sections along line SS in July 

1993. 

39 
the persistent southerly winds, which became relatively strong (10 m/s) about 24 hours before 

image acquisition. 

Along the return leg of the cruise, strong currents were measured south of Atchafalaya Bay 

between July 21 (2300 Z) and July 22 (0400 Z). The ADCP current vectors are overlain on the 

suspended sediment fronts of July 20, 1993 (2109 Z) in Figure 40. These relatively 

contemporaneous datasets reveal that the strong currents wee encountered just seaward of the 

main Atchafalaya plume frond In fact, a note in the cruise log reported crossing a distinct front at 

2300 on July 21 . This observation suggests that the Atchafalaya plume front had moved south 

over the 26 hours between the tune of image acquisition and the onset of song currents along the 

return leg. Abnormally strong southeastward setting currents of 0.8 to 1.5 lrn were experienced at 

the Bullwinkle rig (Location, Figure 40) between July 19 (1300 Z) and July 20 (1700 Z) (G. 

Forristall, OMNET Communication, July 20, 1993). Maximum currents occurred at 0600 Z on 

July 20. Although this rig was approximately 80 km southeast of the LATEX-B cruise track, the 

close correspondence in time of the measurements may reveal important information on the spatial 

scales of circulation on the LATEX shelf. The strong shelf-edge currents at Bullwinkle were 

attributed to Eddy Whopper, which detached from the Loop Current in early June (Walker et al., 

1993). By mid-July/early August Eddy Whopper was oriented north-south such that the northern 

margin was in close proximity to the shelf edge between 90° and 92° W longitude (Figure 41). If 

anticyclonic circulation associated with Eddy Whopper was indeed responsible for the strong 

currents encountered along the return track of the July 1993 LATEX-B cruise then one can 

conclude that mid- and inner shelf circulation in this region is sporadically affected by detached 

Loop eddies . It is relevant to point out that Eddy Vasquez affected circulation south of Atchafalaya 

Bay on the outer and middle shelf regions during October 1992 (Walker et al. in Murray, 1994; 

Walker et al ., in press). 

These comparisons of satellite imagery with LATEX-B cruise measurements have revealed that 

the temperature and turbidity patterns observed in the atmospherically corrected satellite data are 

often indicative of the surface mixed layer, rather than just the "skin" of the ocean. The satellite 

data can, in many instances, be used to identify the boundaries between distinct water mass types. 

c. Pre-SCULP Drifters (June-July 1993). A group of 15 surface layer drifter were 

released across the Louisiana shelf in early June 1993 by an SIO/NIIV1S oil spill-drifter program. 

The range of the initial positions (the drifter patch) is included in Figure 29. Analysis of the drifter 

tracks suggest four major flow episodes that are related to the wind records and are illustrated by 

drifter #15 in Figure 42. 

Episode 1. From June 2 through June 9, the surface layer motion across the drifter patch was 

onshore to the northeast This is illustrated by drifters 14, 15, 19, and 21 . Beginning about June 

11, the wind decelerated steadily into a lull period ending mid-day of June 16. 

Episode 2. Drifter hacks 14, 18, 20, and 21 for June 11 through 17 indicate that during this 

lull period, the surface layer moved back downcoast to the west, probably because the westwarddirected pressure gradient relaxed after being strengthened by the strong upcoast winds prevalent 

during the first half of June. Note drifter 19 appears trapped just south of the Atchafalaya Bay 

mouth during this episode. 

Episode 3. Strong easterly winds begin on June 17 at all three C man stations from South 

Texas to Grand Isle. The strong (25-30 knots) winds from the northeast and north of Tropical 

Storm Arlene on June 18-19 drove all the drifters downcoast to South Texas at high speeds (1-2 

knots) . As Arlene moved ashore between Brownsville and Port Aransas, early on June 20, its still 

strong trailing edge winds from the southeast now opposed the inertia of the coastal jet and 

dramatically slowed and stopped the drifter movements to the south and west 

40 
20 July 2109 Z 

Turbidity Fronts and 

Near-surface ADCP currents 

-SI ; ~.. s 

. . . a . . . . . . ., 

v 9 1 r 

ship* 

; .~.- .`." . . . . . . . . . . . . . . . .

.A£chafa3'aya . . . . .~ . . . . : . . . . . . . /1 

. . . . . . . . . . . . . : . . . . . . . . . . . . . . 

ume 

sissippi 

,Plume 

28N . . ., . . . . . . . . . . . . . . . . . . . . . . . . . . . ; . . . . . . . . . . . . . . . 28N 

-20d o :brig 

-200 ; o 

3. g~ ;" 

v" o" 

Figure 40. Turbidity fronts as observed in satellite imagery of July 20, 1993 (2109 Z), with nearsurface ADCP current vectors superimposed. The ship was positioned in the 

Galveston plume at the time of image acquisition. The position of the Bullwinkle rig is 

shown south of the Atchafalaya plume and just seaward of the 200 m isobath. 

41 
N 

313 - I-9 b - I-94 - 92- ~ - - - b -84 - 8 I-8~ 

28 

COOL F ILAf1ENIT 

W 

C U p- C L -U-P W ¬-L- L E -G-- W AT ¬ R - 

EDDY 

VIAS ULJEZI 

HOPPER 

- 

I F ~ - COC]L LJPWELLL D I 

r 

I 

- 

WATER 

~4 

I I I 

2 2 

- - I - ~ ---I- - - - - I - 

~ ~'~ 

-~ ---(_ - 

21 I AUGUST 9,3 I 04 b ~CH 3 ~ - 

COASTAL S TUD IE5 INST ITLJTE 

LL JLJ IS IANA STATE UN I VERS I T Y 

Figure 41 . Surface temperature frontal analysis of satellite imagery obtained on August l, 1993 

(1046), depicting the location and morphology of Eddy Whopper. 
30.5 

30.0 

29.5 

29 .0 

u 

v 7 

W 

28.5 

28 .0 

27.5 

27.0 

Drifter dspk20.015 

-97.0 -96.5 -96.0 -95 .5 -95.0 -94.5 -94.0 -93 .5 -93 .0 -92.5 -92.0 -91 .5 -91 .0 -90 .5 

Longitude 

Figure 42. The trajectory of pre-SCULP drifter #15. (Data courtesy of the SCULP 

program, P Niiler and J. Gaukel.) 
Episode 4. The normal summer wind regime set in again on June 23, and by June 25 the 

drifters resumed the summer current regime of a slow steady march up the coast of Texas to the 

northeast and eastward along the Louisiana coast Several pre-SCULP drifters displayed this 

eastward motion until well into August. 

2. Property Fields at Constant Depths 

The near-surface salinity field (Figure 43) mapped by the underway thermosalingraph system 

(NMAS) presents an excellent integral representation of the hydrographic conditions during Cruise 

IV. The Atchafalaya River plume is clearly seen along the coast from 91° W to 93° W. The 

extension of these lowest salinity contours suggests a nearshore flow to the west. There is a 

marked increase of salinity in the S 1-T2-S2 box to the southeast indicating a strong front between 

the Atchafalaya outflow and the ambient coastal current water. 

In the extreme southwest of the study area, surface salinities at S8 and S7 are 33-34 psu, 

indicating the presence of relatively undiluted Gulf water. The tongue-like distribution of the 23-

27 psu isohalines in the S4-SS region suggests a penetration of this high salinity water from 

central-south Texas toward the east 

The salinity at the 7m level from the CID casts shows a pattern similar and proportionally 

higher to the surface pattern (Figure 44). The salinity at the 15 m level is more uniform, ranging 

from 3x35 psu, and generally high throughout the whole study area (Figure 45). 

3. Acoustic Doppler Velocities at Constant Depth 

The velocity field along the cruise track in the 2m bin centered at the 4.5m depth level is shown 

in Figure 46. The overall velocity field is quite consistent with the circulation pattern suggested by 

the salinity distribution. Clearly there was a dominant tendency for upcoast or eastward flow all 

along the Louisiana and Texas coast covered by the cruise track This is in agreement with the 

advection of higher salinity water from the Texas coast onto the Louisiana shelf indicated by the 

salinity field (Figure 43). Off the Atchafalaya Bay the ADCP vectors suggest the eastward coastal 

current was deflected around the buoyant plume emanating from the Atchafalaya River. This is 

also consistent with extrapolating the 22-25 psu isohalines from line S3B around the bulge of fresh 

water formed by the Atchafalaya River linking up with these outflows, and with the salinity values 

on S I . 

There is no evidence in the ADCP data in Figure 46 for the nearshore westward flow of very 

low salinity Atchafalaya River water toward S3B and S4 as suggested by the salinity field 

Several possibilities could explain this discrepancy such as (a) the inshore westward flow of low 

salinity water was an episodic event that occurred prior to the cruise or (b) the doppler velocities in 

this very shallow water are unreliable and require further screening. 

There are specific zones along the cruise track that exhibit strong cross-shore motion (e .g ., 

intense onshore flow along the western end of T10 and along nearly the entire length of T7). 

Zones of strong offshore flow are seen along the east end of T9 and at the south end of S3B. 

Further data screening is necessary, but indications are these are probably real features. The 

SCULP drifter data indicate a wealth of cyclonic and anticyclonic eddys of a variety of sizes to be 

common in the region between S4 and S8. It is noteworthy that the strong offshore flow at the 

seaward end of S3B is associated with a 90° bend in the 22-23 psu isohalines, which we presently 

interpret as the eastward returning coastal current reflecting around the Atchafalaya fresh water 

bulge. 

44 
Latex B, P93-2, MIDAS, Salinity 

30.0 

21 

29 .5 

N 

29.0 

28 .5 

28.0 

30 .5 

TlA 

\\ D 

32.0 \ 

21 

-95 .5 -95.0 -94.5 -94.0 -93 .5 -93 .0 -92.5 -92.0 -91 .5 -91 .0 -90.5 

Longitude 

Figure 43. Near-surface salinity along the track of Cruise N. 

Latex B, P93-2, CTD, Salinity 7m 

30.0 

29 .5 

.= 29 .0 

a 

28 .5 

r

2a .0 2 

. .029 \ 

1 .0 . 

28 .0 

-95 . S -95.0 -94 .5 -94 .0 -93 .5 -93 .0 -92 .5 -92 .0 -91 .5 _91 .u _9U .) 

Longitude 

Figure 44. Salinity along the Cruise N track at the 7 m level. 

45 
Latex B, P93-2, CT'D, Salinity 15m 

30 .0 

29 S 

29 .0 

u 

a 

28 .5 

28 .0 

27 .5 

U/ ~ '~`J 

30D 

32A / .9~ 3tA ~O 

33.0 o ~ 00 

310 

34 A 

U 

-95 .5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -92 .5 -92 .0 -91 .5 -91 .0 -90 .5 

Longitude 

Figure 45. Salinity along the Cruise N track at the 15 m level. 

30.2 

29 .8 

29.4 

v 

^a 29 .0 

28 .6 

282 

27.8 

P93-2, ADCP, 4.5 meters, 2 km, (W=00.4632 Amp= 1 .0229 ),Iterative Method 

Scale 20 cm/s v--+ 

-95 .0 -94 .6 -94 .2 -93 .8 -93 .4 -93 .0 -92 .6 -92 .2 -91 .8 -91 .4 -91 .0 -vU 

Longitude 

Figure 46. Current vectors along the Cruise N track at the bin centered at 4.5 m depth (2 m bins). 

46 
4. Vertical Sections 

Table 2 shows the start and end time of each line for Cruise N. 

Table 2. Times and dates of survey lines, Cruise N. All times are in GMT (GMT is CDT plus 5 

hours). Dates are noted in Julian days. 

LinQ 

S1 Hypoxia 0747 13 July 1058 13 July 

Hypoxia 1058 13 July 0750 14 July 

S1 1022 14 July 1642 14 July 

T2 1642 14 July 2352 14 July 

S2 2352 14 July 0816 15 July 

T3 0816 15 July 1310 15 July 

S213 1310 15 July 1821 15 July 

T4 1821 15 July 2332 15 July 

S3 2332 15 July 0621 16 July 

T5 1621 15 July 1121 16 July 

S313 1121 16 July 1512 16 July 

T6 1512 16 July 1900 16 July 

S4 1900 16 July 1325 17 July 

T7 1325 17 July 1008 17 July 

S5 1008 17 July 1928 17 July 

T8 1928 17 July 0119 18 July 

S6 0119 18 July 1001 18 July 

T9 1001 18 July 1820 18 July 

S7 1820 18 July 2155 18 July 

T10 2155 18 July 0532 19 July 

S8 9532 19 July 0905 19 July 

GaNeston Plume 1713 19 July 1345 20 July 

Convergence Study 1708 20 July 1027 21 July 

a. SL The salinity distribution at S 1 (Figure 47) shows an 8- lOm thick broad band of low 

salinity water (<30 psu) that extends across the entire 80 km long section. This fresh water layer is 

underlain by a Sm thick halocline, which itself is underlain by a deep, fairly homogeneous layer of 

high salinity (3436 psu) Gulf water. The currents (Figure 46) are strongly eastward (upwelling 

favorable), which is consistent with the up-to-the coast slope of the pycnocline and the top of the 

deep layer. Within the first 25 km from the coast, low salinity isohalines (21-25 psu) outcrop at 

the sea surface, suggestive of a very local regime of downwelling in the shallowest water near the 

coast This interpretation is supported by the ship winds that are clearly onshore during the S 1 

transect (Figure 30). The dominant flow component is in the tonal (roughly alongshore) 

direction, and Figure 48 shows this component to the depth where bottom packing was lost . The 

surface and halocline layers are flowing strongly eastward, from the coast out to 60 km. An 

intense jet exceeding SO cm/sec is centered over the sharp break in slope 45 Ian offshore . There is 

strong indication of a counter current in the high salinity deep layer. 

b. 72. The deep water tie-line 17 (data not shown) displays, over its entire 85 km length, a 

salinity structure very similar to the outer end of S1 ; namely, (1) a pycnocline zone near the surface 

with salinities of 25 psu increasing to 33 psu at depth of 1418 m and (b) a relatively homogeneous 

deep layer of 3436 psu extending from the base of the pycnocline to the bottom at 48-58 m water 

depths. Because of the lack of bottom track along 72 there is, as of yet, no ADCP velocity data. 

47 
2 7 11~~"""""" P9I1 11Y """"" . . " P93Fl09 """""" P91j108 """""""" P93 

6 

. g - 3pA - 

10 310 

\ 

12 

\. 

`_ 32 ~ 

_-__-_- 

34 0 -~~ 

. . 

1

q . 

. 16 

18 

20 

35.0 - 

-22 

-24 

-26 

-28 

-30 

-32 - 

Y 

-34- - 

-36 - - 

-38 - 

40 . 

- 

- -------- 

-42 

-44- - 

-46- - 

E 

s 

a 

u 

Q 

" P93IIB6 """""""" p9)/py """" , !~M ` ~ ~ / B] "" P9)r~ W2~ """ P97~10' 

_- 

32.0 

P93-2, S1, Salinity 

75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 

Distance offshore (km) 

Figure 47 . Salinity (psu) along line S1 . 

_, 

=5 

_9 

-13 

-IS 

-17 

-19 

-21 

E -23 

-25 

c 

27 

-29 

-31 

-33 

-35 

-37 

-3S 

-41 

-43 

-4_` 

-4 

o 

/ _2 : 

P93-2, S1, ADCP 

U Comp . (East Positive) Corrected 

i 

75 70 65 60 55 50 45 40 !5 SU G3 to 13 1 u 

Distance offshore (km) 

Figure 48 . Zonal velocity component (positive-east) along line S 1 . 

48 
c. S2. The salinity structure at S2 (Figure 49) shows essentially the same three layers as 

observed at S 1 except that the surface layer salinities are at least 4 psu lower because of proximity 

to the Atchafalaya River mouth. The intense upcoast jet in the along-shore ADCP speed observed 

at S 1 is absent at S2. Instead we see in Figure 50 a broad area of moderate up coast (eastward) 

speeds extending from the inshore shallows out to the 65 km mark. A surface counter current 

centered at the 75 km offshore mark coincides with a deep zone of the well-mixed surface layer 

(Figure 49). It is notable that the vertical shear in the current in the upper lOm is such that upcoast 

speeds increase with increasing depth. Inspection of Figure 30, the slop winds, indicates that a 

strong wind stress, opposing the regional upcoast current during the S2 leg, is likely decelerating 

the top of the water column. 

d. S2B. Line S2B which cuts obliquely through the Atchafalaya River outflow plume 

shows strong southeastward currents on Figure 46 (the near surface ADCP map). Figures 51 and 

52 show the salinity and meridional (roughly cross-shoes) ADCP speeds along S2B in vertical 

section . The salinity clearly shows the low salinity (11 psu) river plume entering at the inshore end 

of S2B and extending about 70 km offshore with a vertical thickness of about 10 m. The flow in 

S2B is strongly southeastward over nearly the entire section . The plume water has a strong 

southward component (Figure 52), as does the water underlying the plume. 

e. S3. Line S3 is also dominated by the 8-10 m thick Atchafalaya River outflow as shown 

in the salinity distribution (Figure 53). The river plume water overlies an 8 m thick halocline in 

which the salinity increases from 24 psu up to about 32 psu. A wedge of high salinity (32-35 psu) 

water occupies the deep offshore part of this section. Currents are generally south eastward over 

the section (Figure 46). The eastward speed component reaches 30 cm/sec in a subsurface jet at 

the offshore edge (Figure 54). An anomalous westward counter current in the surface layer 

between the 40 and 55 km mark is associated with a lens of low salinity water centered at the 40 

km mark. The ship winds along S3 (Figure 30) indicate a - 5 m/sec easterly wind (counter to the 

prevailing coastal current), which is consistent with the vertical shear observed along most of the 

section, i.e., eastward speeds increasing with depth. 

f. S3B. Section S3B is filled over its entire length and down to the 8 m depth with nearly 

isohaline water of 23-24 psu. This unusual salinity distribution is associated with very strong 

southwestward currents at the seaward ends of both S3B and TS (Figure 46). It is not clear what 

is causing this seaward jetting of the 23-24 psu water. One possibility is that the intrusion of 

higher salinity water from the west illustrated in Figure 43 (the surface salinity map) could be 

squeezing this modified Atchafalaya River water seaward around the more recent Atchafalaya 

plume water. 

g. S4. Further west at S4 the complexity of structures in the salinity distribution (Figure 55 

suggests the presence of a similarly complex velocity field. Salient features are (1) a nearshore 

low salinity wedge with downwelling inclined isohalines confined to within 20 km of the coast; (2) 

a weak, low salinity feature in the surface layer centered at 35 km offshore mark; (3) a salinity in 

version in the surface layer offshore where a lens of 2426 psu water is located in the upper 8 m 

between 70 and 40 km offshore; (4) a down-to-the-coast slope of the base of the halocline (cf. the 

30 and 31 psu isohalines): and, (5) a large volume of homogenous water - 26 psu occupying the 

center of the section from 412 m depth range between 20-45 km offshore. 

The tonal velocity component at S4 (Figure 56) does indeed reflect these complexities in the 

salinity field. Inshore of the 45 Ian mark and beneath the 17 m depth level offshore there is a 

broad area of upcoast flow, which reflects the coastward dip of the halocline (item 4 above) and 

occupied by the homogenous water mass of item 5 above. The offshore lens of low salinity water, 

item 3 above, is directly associated with a intense downcoast jet reaching speeds of 40 cm/sec. 

Clearly, this is an eddy or filament of low salinity water that has intruded into the coastal current 

domain, perhaps from the mid shelf. The nearshore low salinity wedge and the patch of low 

49 
1s Ih D . . . . . . . . N) """""" P9l1 11) 493troll,- """ ) ly """"""" f931110 . . . . . . . . P7) ill . 12R "" . . " . 7 

-4 ~- 

0 

_g 

_10 `30D~_ 

-12 

-14- ~ 3gp__ 

-18- 

-20-

-22 

asn _2q ~ -

-26 -350 

-28 

-30-

_3Z f3HA 

-34 

-36 

-39 

-40 

-42 

-44--

-46--

-48 

-50 --

-52 

-54 

-56 P93-2, S2, Salinity 

t 

u 

A 

90 85 80 75 70 65 60 55 50 45 40 35 30 

"Distance offshore (km) 

Figure 49. Salinity (psu) along line S2. 

_, 

-3 

-5 

_7 

_9 

-13 

-15 

-17 

-19 

-21 

-23 

E -2S 

-27 s 

d -29 

q -31 

_33 

-35 

-37 

-39 

-41 

-43 

-45 

-47 

-49 

-51 

-53 

-55 

P93-2, S2, ADCP 

U Comp . (East Positive) Corrected 

90 85 80 75 70 65 60 55 50 45 40 3s su 

Distance offshore (km) 

Figure 50. Zonal velocity component (positive-east) along line S2. 

50 
P97 Z 136 

. . . . . . . . . . . . . ., . 

4 

6 

B 

27A---- ` 

10 

12- - 

14 

16 - 

IS 310- 

20- 

22- - 

24 _ . ._ .yip-____, 

- 26 / 

E 

s 

n. 

u 

Q 

P93-2, S2B, Salinity 

80 75 70 65 60 55 50 45 40 35 30 

Distance offshore (km) 

Figure 51 . Salinity (psu) along line S2B. 

_, 

-3 

-S 

_7 

-9 

-I1 

E -13 

r 

u -15 

L1 

-17 

-1S 

-21 

-22 

'2= 

P93-2, S2B, ADCP 

V Comp . (North Positive) Corrected 

80 75 70 65 60 55 50 45 40 35 70 

Distance offshore (km) 

Figure 52. Meridional component (positive-north) along line S2B. 

51 
E 

s 

n 

A 

P97 130 P971171 P971132 P971137 P931134 P93113S P911136 

~" \\\ I CCC 

\ V 

s ~_ I 

zoo 

10 24.0 

250 

12 

~?BA 

14 

340 

16- , 

_ 32A 

18- 

20 -

22- 

1-11, P93-2, S3, Salinity 

70 65 60 1; 1; S!1 AC en z~ ~n 1c 1n 

Distance offshore (km) 

Figure 53. Salinity (psu) along line S3. 

_, 

-3 

-5 

_7 

_g 

E -11 

r 

a 

D -1J 

-15 

-17 

-19 

-21 

,u 

1) 

K 

P93-2, SJ, ADCP 

U Comp. (East Positive) Corrected 

70 65 60 55 50 45 40 35 30 25 20 

Distance offshore (km) 

Figure 54. Zonal velocity component (positive-east) along line S3 . 

52 

o 

: . : . 

f7 
P93 133 P931152 P9)1131 P971130 P971149 P931 HS P971177 P971146 P971143 !?3 1 144 

-2 ` . . . . . . . . .;I . . . . . . . . . . . . . . . . . . .~ . . . . . . . .I . . . . . . . . . . . . .i . . . .,~ . . . . . .~ . . . . . . . . . . . . . . . . . . . i . . ~~.~. .~. . . . . . . 

._q 

-6 - 

1 

_g . 27A 

28A 

-10 

E 

L -12 ~ ^--I 

o. 30.0 

Q -14 ~ 

UWO 

2gq f_ 

-l6 

i 

1 

-22 

I / P93-2, S4, Salinity 

I I 

70 65 60 55 50 45 40 35 30 25 20 15 10 5 

Distance offshore (km) 

Figure 55. Salinity (psu) along line S4. 

_, 

-3 

-5 

_7 

.9 

-11 

E 

s -13 

-l 

_, 

-19 

_Z 

70 65 60 S5 SO 4S 40 i5 1n 25 20 15 10 5 

2 

. 

4 

\\\ O u ~ 

. . ~ .~ . 

. : . ' . ' . ~ I Ltg ~ 

O 

"=ti' : 

17 

: . 

8 

6 

~Z~f 

U Comp . (East Positive) Corrected 

Distance offshore (km) 

Figure 56 . Zonal velocity component (positive-east) along line S4. 

53 
salinity at 35 km offshore (items 1 and 2 above) are both local zones of distinct downcoast 

currents flowing counter to the upcoast flow they are imbedded in. 

h. 77. The salinity and ADCP velocities along T"7 are consistent with the presence of an 

andcyclonic eddy in the S4-T7-S5 region . Salinity in the upper 10 m is < 27 psu, and this water is 

adverting westward on the south limb of the eddy. The flow turns northward on the west end of 

T7 where isohalines rise abruptly. For example, the 27 psu isohaline rises from 12 m to 1 m over 

the westernmost 20 Ian of 17, signaling the outer edge of the eddy. 

i. S5. The SS line (Figure 57 and 58) is similar to S4 in that its outer 30 km are dominated 

by the anticyclonic eddy present along S4 and T7. There is a sharp discontinuity in the velocity 

field at about km 63 (Figure 58) between the eddy and the coastal current located shoreward. The 

water in the eddy appears to have a higher salinity (Figure 57) (28-29 psu), compared to 25-27 psu 

in the eddy in S4. On shore of km 63 along SS there is a prevalent and deep eastward flow (Figure 

58) of water with salinities in the 20-28 psu range. Local (ship) winds are westerly during the ship 

transit of S5, maximizing the tendency toward eastward flow. 

Ship winds indicate a shift to moderate (5-7 m/sec) southerly winds along T8-S6-T9. The 

inshore tie-line T8 shows a distinct parabathic current flowing eastward, returning low salinity (20-

28 psu) water from Texas that apparently had adverted down coast earlier. 

j. S6. The onshore winds along S6 (Figure 59 and 60) appear to have strongly affected the 

structure of the flow. The upper 8 m of the water column is dominated by the onshore flow 

component, while below 8 m the prevailing eastward flow is clearly dominant over the entire 

section (Figure 60). The isohalines reflect the impact of the downwelling favorable winds, the 

inner SS Ian being filled with a homogenous mass of water of 28-29 psu (Figure 59). Steep 

down-to-the-coast slopes of the 29, 30, and 31 psu isohalines also reflect the downwelling regime. 

The strong tonal upcoast flows below 8 m, however, illustrate the continued importance of the 

larger scale forces during the summer regime which produce the prevailing eastward flow. 

k. S7. It is very instructive to inspect the salinity distribution in S6, T9 and S7 in sequence 

to see the rapid transition from lower salinity water south of Sabine to the higher salinity (32-33 

psu) that completely occupies the S7 section (Figure 61). The 30 km section of S7 is very wellmixed in accord with the higher wind energy levels exhibited in Figure 32. The upcoast flow that 

dominates S7 (Figure 62) is illustrated with the more energetic tonal component which features a 

10 km scale intense jet (24 cm/sec) confined within 12 km of the coast 

L S8. Sixty km along the coast to the southwest, line S8 demonstrates a penetration into 

even higher salinity waters . In Figure 63 the 35 psu isohaline is seen only 23 km offshore at the 

10 m depth level. This high salinity water at S8 is also returning upcoast (Figure 64), but S8 is 

also heavily influenced by strong onshore flows of high salinity shelf water. These filaments of 

offshore water intruding into the near-coast water are clearly seen as a major feature along T10 

(Figure 46). This feature is continuous with the onshore flow at the seaward edge of S8. It is not 

clear whether this feature is an isolated intrusive filament or is associated with an anticyclonic eddy 

which is impinging on the coast 

m. Return Leg. After finishing the survey of the Galveston plume, the ship transited back 

to the east to home port on July 21 and 22. On this return leg a large saw tooth pattern was run 

through the outer edge of the SS-S6 box to investigate in more detail the strong cross-shore 

motions frequently observed in this region . Figure 65 shows the ADCP vectors at the 4.5 m depth 

level. There are, as anticipated, very strong (40-50 cm/sec) cross-shore motions along the saw 

tooth. South of the Atchafalaya Bay at 91°W and 91.6°W there are also zone of high speed current 

on the return track. The surface salinity trace (Figure 66) indicates there are high speed jets of very 

low salinity (5-8 psu) emanating from the Atchafalaya River outflow plume. Several of the pre54 
Depth (m) P93-2, S5, Salinity 

_Z 

-4 

-6 

_g 

-10 

E 

a -12 

v 

A - 

14 

-l6 

-I8 

-20 

-22 

85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 

_, 

-3 

_5 

_, 

_9 

-13 

G1 _,S 

-17 

-19 

-21 

-23 

Distance offshore (km) 

Figure 57 . Salinity (psu) along line SS . 

22 

I ~ ~ v O 

32 

-12 24

. 

b 

12 / 

l 

20 

1=x 

0 

I 

P93-2, S5, ADCP 

U Comp . (East Positive) Corrected 

I I I I I I I I I 

t85 80 75 7n FS SS to ac an 7r it) ~c In is 

Distance offshore (km) 

Figure 58. Zonal velocity component (positive-east) along line S5. 

55 
0 

_z 

-4 

-6 

_g 

E 

s 

10 

ca 

. 12 

_14 

_lE 

.. . . .P971176 . P971175., . . . . . . . P9]1171. . . . . . . . . P931173' . . . . . . . . . P931 

__ i . . . ` 

29A 

28A 

w 

320 

75 70 65 60 55 50 45 40 35 30 25 2a 

Distance offshore (km) 

Figure 59. Salinity (psu) along line S6. 

-3 

-5 

_7 

E -9 

s 

c . u 

-13 

-IS 

-l7 

0 

P97-2, S6, ADCP 

U Comp . (East Positive) Corrected 

i 

7S 70 65 60 55 50 45 40 35 30 25 20 

Distance offshore (km) 

Figure 60. Zonal velocity component (positive-east) along line S6. 

P93-2, S6, Salinity 

173. . . . . . . . . .P971171 P931170 P93 11 9 

56 
s 

n 

P93-2, S7, Salinity 

30 25 20 15 10 5 

Distance offshore (km) 

Figure 61 . Salinity (psu) along line S7. 

-3 

-5 

_7 

E 

t 

n -9 

u 

-ll 

-13 

_1< 

-l . 

tt 10 

I \ 

e 

10 

2 

-g 

11 

U 

r_~> 

P93-2, S7, ADCP 

U Comp . (East Positive) Corrected 

30 25 20 15 10 5 

Distance offshore (km) 

Figure 62. Zonal velocity component (positive-east) along line S7 . 

57 

v93 1e1 e931192 rums P9311 U r93 185 

Z . 

4 

8 

33.0 

10 

I 

12-- 

14--

16 -- 
0 

P9J 189 

. . . . . . . . 

P9J1190 P9J1191 P9J1191 P9] I! 

. . . . .i . . . . . . . . . . . .i 

1 

4 

_34

8 - gSp 

.0 10 - 

12--

-14 

~ _ 

- 

.fig 

-20 

-22--

-24--

P93-2, S8, Salinity 

E 

r 

Q 

30 25 20 15 10 

Distance offshore (km) 

Figure 63. Salinity (psu) along line S8 . 

_, 

-3 

-5 

_7 

_9 

-ll 

E 

a -13 

A 

-li 

-is 

-21 

'Z5 

. . . . 

P93-2, S8, ADCP 

U Comp . (East Positive) Corrected Il : : . 

30 25 20 IS iu 

Distance offshore (km) 

Figure 64. Zonal velocity component (positive-east) along line S8. 

58 
302 

29 .8 

29 .4 

29.0 

.W 

a 

28 .6 

28 .2 

27 .8 

Scale -2o cm/3 P93-2, ADCP, 4.5 meters, 2 km, (W = 00 .4632 Amp= 1 .0229 ), Iterative Method 

-95 .0 -94 .6 -942 -93 .8 -93 .4 -93 .0 -92 .6 -v2.z -vl .s -9 1 .4 -vi .u -

Longitude 

Figure 65. Velocity vectors (ADCP) at the 4.5 m depth along the return leg cruise track. 

35 

30 

25 

20 

o. 

1 S 

!1 

10 - 

s 

Latex B, Cruise 4, MIDAS Salinity, Return leg 

0 

94 93 .5 93 92 .5 92 91 .5 91 90.5 90 

Figure 66. Salinity along the return leg south of Atchafalaya Bay showing the two low salinity 

pulses associated with the current jets in Figure 29. 

59 
SCULP drifters, trapped for several weeks on the western edge of the Atchafalaya plume, broke 

free during this outflow event and transited rapidly to the southeast and east The cause of these 

fresh outflow plumes being so far offshore when they were absent on the outgoing leg is not 

certain. A drop-out of the winds along the ship track and northerly winds along the coast (Figure 

32) points to the responses to change in wind forcing as the likely mechanism. 

C. Preliminary Summary and Conceptual Synthesis 

The hydrographic conditions in the LATEX B study region are discontinuous in the vicinity 

of Galveston Bay. To the east, the hydrographic distributions may be characterized as retrograde 

and are characteristic of a coastal region that is dominated by a freshwater plume. In contrast, to 

the west of Galveston the 1993 hydrographic field shows little influence of the massive runoff 

from the Mississippi and Atchafalaya Rivers. In this region the stratification is typical of other 

coastal regions in which the offshore stratification is altered and deformed by coastal mixing and 

upwelling and downwelling cycles. This partial decoupling of the east and west LATEX inner 

shelves is not a feature peculiar to the spring season of 1993. However, the location and degree of 

easdwest discontinuity apparently varies year to year and season to season . 

The freshwater outflow from the Mississippi and Atchafalaya Rivers during Cruise III 

(April, 1993) was approximately double that which occurred during Cruise I (April, 1992). While 

there were local areas of lower salinity along the Louisiana coast during the 1993 survey, the 

principal effect of the greater fresh water input was the broadening and, to a lesser extent, 

deepening of the buoyant coastal plume. 

The presence of a buoyant coastal boundary layer along the Louisiana coast may be 

expected to be associated with a downcoast buoyancy-driven coastal current (e.g., Pemgrew and 

Murray, 1986). By making a two-layer approximation, the strength of the near-surface flow 

within in the buoyant plume may be estimated via simple scaling arguments. The elevation of the 

sea surface associated with the coastal freshening (q) may be scaled as eH (Csanady, 1971) where 

e is the proportional density defect across the plume front, defined as ep/p, and H is the 

characteristic water depth. The relevant form of the geostrophic equation is u' _ - g/f o-,q / ay, 

where u' is the upper layer velocity, f is the Coriolis parameter, and q is the sea-surface elevation. 

We may express the relevant velocity scale as (g/f) d-3/L, where L is the characteristic width of the 

buoyant coastal boundary layer. Using empirical parameter values (obtained from the April 1993 

cruise data) of: H= 20 m; E= 5 z 10-3 ; L=50 km, the velocity associated with the buoyant plume 

along the Louisiana shelf is scaled as 0.25 m/s. Thus, under high spring runoff conditions, 

massive westward thermohaline transport along the Louisiana shelf is the underlying circulation to 

which may be added components arising from wind-stress driving and/or pressure forcing from 

the offshore regime. 

Although the freshwater plume was, at all locations and times, a dominant feature of the 

Louisiana coastal hydrography, the associated thermohaline circulation was not the dominant 

circulation process in this region Direct wind-stress forcing had a clear and considerable impact 

upon both the hydrographic and velocity fields. In fact, upcoast wind driving was generally able 

to quickly halt and reverse the background downcoast buoyancy-driven flow. During the early 

stages of shifts in wind direction and magnitude, the coastal currents were observed to respond 

with a downwind acceleration At greater elapsed time, the effects of rotation exerted influence and 

the signatures of coastal upwelling and downwelling became more apparent. 

Because the study survey area is so large and the meteorological time scales short, the 

survey data are a complex mix of spatial and temporal variation. In order to analyze such a 

complex mixture of temporal and spatial variability, a crucial first step is the comparison of the 

60 
observations to elementary theoretical models that can to help organize the information in teams of 

the lowest-order dynamics. Several simple constructs are examined in the discussion below with 

the goal of helping to organize and summarize some of the main features of the data set 

1. The Lens-shaped Halocline 

The salinity section shown in Figure 14 is a clear example of the lens-shaped hydrographic 

distributions originally reported to occur in the coastal boundary layer of the North American Great 

Lakes (Csanady, 1971). During data collection, winds along the S4 line were upwelling favorable 

varying from westerly (inshore) to northwesterly (offshore) at approximately 20 knots. Under the 

influence of a strong enough upwelling favorable wind-stress impulse (defined as the time integral 

of the wind-stress forcing) the retrograde front associated with coastally-trapped buoyancy driven 

current systems can be upwelled to intersect the surface, and the buoyant water mass moved bodily 

offshore (Csanady, 1974). Shipboard Doppler currents confirm an upwelling transverse 

circulation pattern (Figure 15). These data show that the full upwelling of the 

Mississippi/Atchafalaya plume can occur along the Louisiana shelf. During this process, the 

alongshore flow reversed to the downwind direction as is consistent with the theory. 

2. Coastal Undercurrents 

One of the most striking phenomenon observed during Cruise III is the presence of 

undercurrents in the eastern region of the LATEX inner shelf. These undercurrents were observed 

to occur both as bottom intensified upcoast currents and as "deep" counter currents, in either the 

up- or downcoast direction, opposing a near-surface downwind flow. The bottom intensified 

upcoast currents were observed to occur after a switch from the downcoast to upcoast wind 

direction. 

The best example of the intensified undercurrent was observed at S4 (see Figure 16). A 

plausible explanation of this flow structure may be found by considering the interplay of two 

forcing functions in the alongshore momentum balance. As was shown in Figure 17, along this 

section of the Louisiana inner shelf, the increasing salinity toward the Texas coast contributes to an 

eastward baroclinic pressure gradient force. Because the salinity gradient exists at all depths along 

line 17, the strength of the pressure gradient force toward the east increases with increasing depth. 

Under typical conditions, the eastward alongshore pressure gradient opposes the westward 

buoyancy-driven flow toward the west The result is a westward flow that decreases in magnitude 

with depth. 

A second (related) mechanism for the generation of transient undercurrents deals with the 

difference in time scales associated with modification of barotropic and baroclinic pressure fields. 

After a coastal wind shift, changes in the pressure field are generated at the coastal boundary and 

propagate seaward (Pettigrew, 1981). Sea-surface adjustment occurs very rapidly since the signal 

propagates at the barotropic long-wave speed. However, internal adjustments take more time to 

extend offshore since they travel at the much slower internal wave speeds . The result is that, at 

distances from shore exceeding the internal Rossby radius, the modified surface pressure field can 

begin geostrophic adjustment before the changes in the baroclinic field have begun. Thus, within a 

few hours of the onset of upcoast winds, the barotropic pressure field may have reversed while the 

baroclinic field remains, temporarily, in the prior condition. The result is a transient state in which 

the barotropic and baroclinic flow components add constructively, resulting in subsurface maxima 

in the downwind current. Determining which, if either, of the above scenarios describes the basic 

dynamical origin of these exciting observations awaits further study and theoretical development 

A second, even more striking, rendition of coastal undercurrents are those that flow counter 

to the direction of the surface cunenL A clear example of this phenomenon is illustrated in Figure 

21, which shows the alongshore component of the Doppler currents at S5. The surface layer 

61 
shows downwind flow in the core exceeding 0.16 m/s, while in the lower layer upwind currents as 

high as 0.14 m/s arse found. The strong current reversal is especially striking m that it occurs in 

shallow (< 20 m ) water. The picture is even more intriguing when one looks at the currents at 

section SSB, some 30 miles to the west (Figure 24). Not only do the currents at this section 

reverse with depth , but they are also 180° out of phase with the currents at S5. Thus between 

sections SS and SSB there existed an alongshore convergence in the upper layer and a divergence 

in the lower layer. 

Reference to MMAS winds in Figure 6 shows that there was a convergence in the wind 

field between these two locations, so that in both instances the surface currents were downwind, 

and the deeper currents upwind. Reference to time series measurements of CIVIAN stations at 

Grand Isle and Sabine suggests that this meteorological convergence is real during the survey 

period, rather than merely an artifact of abasing of a time-varying field. 

The wind-stress convergence near the Louisiana/Texas border appears w be the dynamical 

origin of these spectacular lower-layer counter currents in the LATEX inner shelf region . 

Alongshore variations in the windstress or in coastline orientation may be expected to generate 

longshore pressure gradients with high pressure in regions of convergence and lows in regions of 

divergence (e.g ., Pettigrew and Murray, 1986). As shown in a theoretical paper by Crepon and 

Richez (1982), in regions of convergence the adverse pressure gradient so established propagates 

back along the coast, as a combination of barotropic and baroclinic waves. Since the barotropic 

wave has far greater phase speed, its influence is quickly felt as it slows the otherwise linear 

growth of the downwind coastal jet (see Csanady, 1977; Pettigrew and Murray, 1986). The 

barotropic wave establishes an adverse pressure gradient that acts uniformly over the water 

column. Because the coastal jet is a surface-intensified feature, the adverse pressure gradient 

overpowers the flow in the lower layer, generating an upwind under current This undercurrent 

continues to accelerate upwind until the arrival of the baroclinic pressure wave that arrives in a time 

scale given by T=Y/f R2, where Y is the distance from the convergence, f is the inertial frequency, 

and R2 is the internal Rossby radius of deformation defined as the ratio of the internal wave 

celerity to f. 

The mechanism outlined above would generate oppositely directed undercurrents on either 

side of the convergence and would be operable whether the convergence were caused by wind 

variability or changes in coastline orientation It is interesting to note that the result is a 

convergence of upper layer flow and a divergence of the lower layer flow, as was observed in the 

data. If these flow patterns are characteristic of the region, they may provide a partial explanation 

for the abrupt change in hydrography between the eastern and western LATEX regions. 

62 
III. BIOLOGICAL CHARACTERISTICS SURVEY 

A. Phytoplankton Pigments (Nancy N. Rabalais) 

1. Introduction 

A useful, standard and easy method for determining phytoplankton biomass in sea water is to 

estimate the amount of chlorophyll (usually as chlorophyll a). Our methods followed those of 

Parsons et al . (1984) fluorometric determination. Chlorophyll degradation products, 

phaeopigments, were also determined fluorometrically after acid treatment of the chlorophyll 

extract. Fluorometry is not a good indicator of phycocyanin- or phycourobilin, phycoerythrincontaining organisms in the phytoplankton community, individuals of which may often be 

abundant (e.g ., July 1993). The small coccoid cyanobacteria have less chlorophyll and more of 

the other photosynthetic pigments than the larger-sized phytoplankton. And, while fluorometric 

measurements do not provide the detail of pigment speciation available from high performance 

liquid chromatography, most of the larger-sized fractions of phytoplankton, which usually 

contribute proportionally more to phytoplankton biomass, do contain chlorophyll a. Thus, the 

fluorometric method of estimating phytoplankton biomass is suitable for comparison to electronic 

measures (CTD and MIDAS) and for comparison with salinity and nutrient data, to identify trends 

in biological communities. 

2. Methods 

Water samples were collected by bucket for the surface or by 5-1 Niskin bottles for near-bottom 

or selected depths. The middle of the Niskin bottle was approximately 1-m above the CTD Sea 

Tech fluorescence probe. MIDAS samples were collected from tubing (outflow or inflow) closest 

in proximity to the point of measurement of in vivo fluorescence in the system (Turner Designs 

fluorometer) . The volume of water filtered through GF/F filters was 100 ml, or less if turbid. The 

filters were extracted in S ml of 40/60 DMSO (dimethyl sulfoxide)/90% acetone solution for a 

minimum of 1 hour and not more than 5 hours. Extracts were measured, before and after 

acidification, on a Model 10 Turner fluorometer. The fluorometer was calibrated with a 

chlorophyll a standard (Sigma, Anacystis chlorophyll a) on a Varian DMS80 W, visible 

spectrophotometer. 

Electronic measurements of in vivo fluorescence were collected from the LUMCON SeaBird 

CTD Sea Tech fluorometer, and the LITMCON MIDAS system Turner Designs fluorometer. 

Surface water salinity values were determined on an AutoSal, maintained and calibrated by the 

LSU Coastal Studies Institute Shop. 

3. Results 

a. Cruise III, April 1993 . Surface chlorophyll a, phaeopigments, and total pigment 

distributions indicated strong cross-shelf gradients from offshore of Terrebonne Bay (S1) to 

offshore of Sabine estuary (S6) (Figure 67). To the west of Sabine estuary, surface chlorophyll a 

concentrations were uniformly below 5 gg/1 . Similar trends in surface water chlorophyll a and 

phaeopigments are visible in a longitudinal plot (Figure 68). Surface pigments were composed 

primarily of chlorophyll a. The percentage of phaeopigments in the surface water samples 

increased with distance from the east (below 25%) to the west (20 to 45%) (Figure 68). 

Bottom water chlorophyll a concentrations also displayed similar strong cross-shelf gradients 

as far west as Sabine, but higher phaeopigment concentrations were restricted to the area of the 

Atchafalaya River plume (Figure 69). Bottom water phaeopigment concentrations were higher 

than surface water values within the Atchafalaya River plume. 

63 
m 29 . 

r 

J v 28 .5 - 

v 

Di .i 

28 .0- 

~5.0 °0 0 t /

-O,0 

Surface Chlorophyll a (ug/l) 

LATEX April 1993 

-93 .0 .92 .5 -92 .0 -91 .5 -91 .0 -90 .5 -90 .0 27.5 

-97 .5 -97 .0 -96 .5 -96 .0 -95 .5 -95 .0 -94 .5 -94 .0 

Longitude 

m 

v 

J 

~. u 

',ON 

0 10 

0 

----~,o 

"'0.0 

v 

28. 0- Surface Phaeopigments (ug/I) 

LATEX April 1993 

z~. 

r 

-97.5 -97.0 -96 .5 -96,0 -95 .5 -95 .0 -94 .5 -94.0 -93 .5 -93 .0 -92 .5 -92.0 -91 .5 -91 .0 -90 .5 -90 .0 T -7- 

-- Longitude 

29. 

15 ~o ~` 

J 

-l% 

r 

2a . 5.0- 

28 . 

Surface Total Pigments (ug/l) 

LATEX April 1993 

2~ . 

-97,5 -97.0 -96.5 -96.0 -95.5 -95.0 -94.5 -94.0 -93.5 -93.0 -92.5 -92.0 -91 .5 -91 .0 -90.5 -90.0 

Longitude 

Figure 67. Distribution of surface water chlorophyll a, phaeopigments and total 

pigments for Cruise III. 

64 
OIN 

N 

Mean Surface Chlorophyll a vs Longitude 

LATEX April 1993 

-9B -91 -96 -95 .94 "93 " 92 -91 

90 1 I i +- 1- . . . . . I .,-. . . .^.--.r 

25 

70 

L 

u` 

10 

5 

G 

Mean Surface Pliaeoplgmonts vs Longitude 

LATEX April 199] 

_90 ~98 -07 ~O6 -95 ~94 -97 .92 ~01 90 

1 

r 

Mean Phaeoplgment Percentage of Total Pigments vs Longitude 

LATEX April 1993 

-9e ~97 ~e6 95 ~04 ~93 ~e2 ~91 ~90 

45% - 

pox 

]SX 

d nx ~ ~ 

zox 

a 15% ~ 

tox 

6% 

.~ _- 

1 

1 6 

a 

a 

E 

d 

nQ 

L ' 

L 

~ 7 

c 

Z 

I 

I 

Figure 68 . Distribution of surface water chlrophyll a, phaeopigments and percent 

phaeopigments of total pigments by longitude for Cruise III. 
(D 2 

r 

J 

2 

(-,5*) 

27. 5 

-97 .5 -97 .0 -96 .5 -96 .0 -95 .5 -95 .0 -94 .5 -94 .0 -93 .5 

Longitude 

v 

r 

J 

5.0 2~ 

O \ , ,tiyo 

r 

Near Bottom Chlorophyll a (ug/I) 

LATEX April 1993 

33 .0 -92 .5 -92 .0 -91 .5 -91 .0 -90 .5 -90 .0 

5.0 J ~ ~o.o e 

5/ 

OA 

2e. Near Bottom Phaeopigments (ug/I) 

LATEX April 1993 

27.s 

-97 .5 -97 .0 -96 .5 -96 .0 -95 .5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -92 .5 .92.0 -91 .5 -91 .0 -90 .5 -90 .0 

Longitude 

29 . 

50 ~~ ADO 

5.0 

O. 

29 . 

7 J ' ; 

r 

28.5-

28.0-

Vo Near Bottom Total Pigments (ug/I) 

LATEX April 1993 

27 . 

-97 .5 -97 .0 -96 .5 -96 .0 -95 .5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -92 .5 -92 .0 -91 .5 -91 .0 .90 .5 -90 .0 

Longitude 

Figure 69. Distribution of near-bottom water chlorophyll a, phaiopigments and 

total pigments for Cruise Ell. 

66 
Chlorophyll a concentrations in surface water samples (bucket-collected) were very similar to 

those obtained from the MIDAS flow-through system (Figure 70). Similar relationships were 

obtained for phaeopigments and total pigments (not illustrated) . These relationships indicate that 

the flow-through system was collecting the same water mass as the surface bucket. On the other 

hand, the continuously-cording, electronically-derived in vivo chlorophyll a values from the 

MIDAS system did not correspond well with the absolute values obtained from the water samples 

(Figure 70), although the relationships were much more linear than the parabolic relationships 

established for Cruises I and u (Rabalais in Murray, 1994). 

A similar comparison of the LUMCON C"TD in vivo fluorescence probe to absolute 

concentrations of chlorophyll a showed fairly linear relationships below concentrations of 10 jig/I, 

but the relationships were different for surface and near-bottom waters (data not shown). Thus, 

caution must be taken in interpreting cross-shelf contours of CTD in vivo fluorescence as 

representative of the chlorophyll a biomass in the study area 

Surface water salinities were mostly in the 20 to 35 ppt range, with a few values between 5 and 

13 ppt. Highest chlorophyll a biomass values were between 5 and 28 ppt, a rather broad range 

compared to other chlorophyll a biomass vs. salinity distributions (Figure 71). 

The dissolved nutrients generally decreased with an increase in salinity away from the 

freshwater sources (data not shown). The dissolved inorganic nitrogen pool was dominated by 

nitrate (cf. Figure 71 . Chlorophyll a biomass was high across a broad range of nutrient 

concentrations but generally decreased as nutrients were diluted on the seaward end of the plume, 

or coastal current (Figure 71). 

b. Cruise IV, July 1993. Surface water pigment distributions for July 1993 were very 

similar to those of Cruise III in April 1993 (Rabalais in Murray, 1994). Surface chlorophyll a, 

phaeopigments, and total pigment distributions indicated strong cross-shelf gradients from 

offshore of Terrebonne Bay (S 1) to offshore of Sabine estuary (S6) (Figure 72). To the west of 

Sabine estuary, surface chlorophyll a concentrations were uniformly below 5 jig/1. Similar trends 

in surface water chlorophyll a and phaeopigments are visible in a longitudinal plot (Figure 73). As 

on Cruise III, surface pigments were composed primarily of chlorophyll a; however, the 

percentage of phaeopigments in surface waters was much greater in July 1993 than in April 1993. 

Extremely high percentages of phaeopigments were found in surface waters on the hypoxia 

transect off Terrebonne Bay and as far west at 92°W off the Atchafalaya River plume (Figure 73). 

Bottom water chlorophyll a concentrations also displayed similar strong cross-shelf gradients 

as far west as Sabine and were similar to those of Cruise III in April 1993 (Figure 74). Bottom 

water phaeopigments were surprisingly low, since they are often elevated during mid-summer, 

especially during hypoxic conditions as documented on this cruise (see Section VII). Bottom 

water phaeopigment concentrations in the vicinity of the Atchafalaya River plume were actually 

higher in April 1993 than in July 1993. Bottom water phaeopigment concentrations were similar 

to surface water values in July 1993 . 

As on Cruise III, chlorophyll a concentrations in surface water samples (bucket-collected) were 

very similar to those obtained from the MIDAS flow-through system (Figure 75) . Similar 

relationships were obtained for phaeopigments and total pigments (data not shown). These 

relationships indicate that the flow-through system was collecting the same water mass as the 

surface bucket. On the other hand, the continuously-recording, electronically-derived in vivo 

chlorophyll a values from the MIDAS system did not correspond well with the absolute values 

obtained from the water samples (Figure 75), and was similar to the parabolic relationships 

established for Cruises I and II (Rabalais in Murray, 1994). 

67 
30 .00 

25 .00 

20 .00 

m 

a t 

a 0 

t 15.00 

U 

c 

a 

ca 

F- 

,v 10.00 

5.00 

0 .00 

0 .00 

0.7 

0.6 

N 

01 

R 

O U " 5 

J 

E 

0 0.4 

0 

0.3 

a 

N 

U 

0.2 

a 

0.1 

5.00 10.00 15 .00 20 .00 25 .00 

Bucket Sample Chlorophyll a (ug/L) 

Surface Chlorophyll a vs Midas in vivo Fluorometer Voltage 

LATEX April 1993 

30 .00 

0 

0 5 10 15 20 25 3C 

Surface Chlorophyll a (ug/L) 

Figure 70. Relationship between surface chlorophyll a collected by bucket and 

MIDAS tap-in samples (upper panel) and between surface chlorophyll a 

and MIDAS scaled in vivo fluorescence voltage (lower panel) for 

Cruise III. 

i 

I 

., 

. . 

I "" 

68 

Chlorophyll a: Bucket Sample vs Midas Tap-in 

LATEX April 1993 
Sudan Chlorophyll m we Salinity 

LATEX Apill 199 

w o0 

25 00 

2000 

2 

fi ISOo 

e 4 

1000 

$ Do 

coo 

000 500 IQ DO 1500 2000 2500 3000 3500 00 

Salinity (PPl) 

Surlace Chlorophyll a vs Slllcale :llllrale flallo 

LATEX Apill 1991 

30 

ZS 

20 

T. IS 

s 

.. 

w 

ss 

20 

IS 

e 

10 

S 

0 

ZS 

ZO 

31 IS 

4 

u 

to 

s 

Swlaco Chlorophyll a vs Dissolved Inorganic flllrogen 

LATEX Apill 1993 

I 

S~ 

goo 2oo 30o 4oo 50o 6oo 

uiu~nox,uo1 ( .g .111.) 

Swlace Chlorophyll o vs Silicate 

LATEX Apil 1993 

i 

" IN 

00 200 400 so 0 boo 1000 1200 1400 00 too 200 300 400 Sao 600 700 boo 

sia .un3 0.110 smc-1 . p-a .vy 

Figure 71 . Distribution of chlorophyll a concentrations by salinity, dissolved 

inorganic nitrogen (N03+NOZ+NH4), silicate and the silicate:DIN 

ratio for Cruise III. 
29.5 /J 10,0 ---~~ 

15 

29. 

f ~ U 0.0 ~~ 

o/ Surface Chlorophyll a (ugll) 'x_50" 

28_ i ' LATEX July 1993 

.0 -94.5 -94.0 -93.5 -93.0 -92.5 -92.0 -91 .5 -91 .0 -90.5 -90.0 

Longitude 

29 .5- \ 20 

m ' o L 29 . 

J 

Surface Phaeopigments (ug/I) 

2e, ~op`' LATEX July 1993 

-95 .5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -92 .5 -92.0 -91 .5 -91 .0 -90 .5 -90 .0 

Longitude 

29 .5 10 .0 

N 

m ~r w 

.0~ 

40 

J 29. o0.0 

Surface Total Pigments (ug/I) 

za . ~' LATEX July 1993 

-95 .5 -95 .0 -94 .5 -94 .0 -93.5 -93 .0 -92 .5 -92.0 .91 .5 -91 .0 -90 .5 -90 .0 

Longitude 

Figure 72. Distribution of surface water chlorophyll a, phaeopigments and total 

pigments for Cruise N. 

70 
J 

Mean Surface Chlorophyll a va Longitude 

LATEX July 1997 

-96 ~95 ~91 ~93 ~92 " 91 ~90 .96 

3000 

2500 

^- 20 00 

r 

T 

c 

Q I 500 

0 

u` 

= 1000 

5 00 

000 

1000 

goo 

goo 

7.00 

a 

n 

600 

nQ 

500 

i 

4 DO 

C 

300 

200 

I .W 

000 

Mean Surface Pliacoplgmenls vs Longitude 

LATEX July 1993 

~95 94 -93 ~92 91 ~90 

r . , . . 

Mean Phaeoplgmenl Percentage of Total Pigments 

LATEX July 1997 

96 95 91 97 92 91 90 

lonqlluJo 

Figure 73 . Distribution of surface water chlorophyll a, phaeopigments and percent 

phaeopigments of total pigments by longitude for Cruise IV. 

80% 

E 

' e40 

E 307. 0 

0 

E 

d 20% 

1 

10 

0:: 
m 

-o 

2s . O 

J 

~ \~O 

28 .5-I 

m 

J 

m 

J 

-95 .5 -95.0 

\ O 

ao ~~ 

Near Bottom Phaeopigments (ug/L) . 

LATEX July 1993 

-95.0 -94.5 .94.0 -93.5 -93.0 -92.5 -92.0 -91 .5 -91 .0 -90.5 -90.0 

Longitude 

29. 

` ~- 6.0 

0 (2-0 \'° p 

) 2.0 \ Near Bottom Chlorophyll a (u9/L \ 

LATEX July 1993 

-94.5 .94.0 -93.5 -93.0 -92.5 -92.0 -91 .5 -91 .0 -90.5 -90.0 

Longitude 

V ~~/ 3 \ ~ \ ~0 

~ 

~~o To 

l '0 

.5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -92 .5 -92 .0 .97 .5 -91 .0 -90 .5 .90 .0 

Near Bottom Total Pigments (ug/L) 

LATEX July 1993 

7 r 

Longitude 

Figure 74. Distribution of near-bottom chlorophyll a, phaeopigments and total 

pigments for Cruise N. 

72 
Chlorophyll a : Bucket vs Midas Tap-in 

LATEX July 1993 

30.00 

25.00 

J 

7 20 .00 

N 

T 

L 

a 

a 

15 .00 

U 

c 

a 

t0 

Fm 10 .00 

5.00 

0.00 

0.00 

6.000 

5.00 10 .00 15.00 20 .00 25 .00 

Bucket Sample Chlorophyll a (ug/L) 

Surface Chlorophyll a vs Midas in vivo Fluorometer Voltage 

LATEX July 1993 

30.00 

5.000 

m 

a 4.000 

`m 

E 

0 

03 3.000 

a 

m 

U 

N 

2.00 

a 

1 .000 

0.000 

0.00 5.00 10 .00 15 .00 20 .00 25 .00 

Surface Chlorophyll a (ug/L) 

Figure 75. Relationship between surface chlorophyll a collected by bucket and 

MIDAS tap-in samples (upper panel) and between surface chlorophyll 

a and MIDAS scaled in vivo fluorescence voltage (lower panel) for 

Cruise N. - 

. . 

L~ 

30 .00 

73 
A similar comparison of the LLTMCON C'TD in vivo fluorescence probe to absolute 

concentrations of chlorophyll a showed somewhat linear relationships, but the relationships were 

different for surface and near-bottom waters (data not shown). Thus, caution must be taken in 

interpreting cross-shelf contours of CTD in vivo fluorescence as representative of the chlorophyll a 

biomass in the study area. 

Surface water salinities were mostly in the 20 to 35 ppt range, with a few values between 2 and 

18 ppt. Highest chlorophyll a biomass values were between 8 and 20 ppt, a rather broad range 

compared to other chlorophyll a biomass vs. salinity distributions (Figure 76). 

The dissolved nutrients generally decreased with an increase in salinity away from the 

freshwater sources (data not shown). The dissolved inorganic nitrogen pool was dominated by 

nitrate (cf. Figure 76). Chlorophyll a biomass was high across a broad range of nutrient 

concentrations but generally decreased as nutrients were diluted on the seaward end of the plume, 

or coastal current (Figure 76). 

B. Phytoplankton Survey (Quay Dortch) 

1. Introduction 

The primary purpose for measuring phytoplankton species composition was to use 

phytoplankton as tracers of water masses and to extend information gained from other programs 

focused on the near river plume to the extended plume. In addition, this program offered an 

opportunity to compare the effects of the 1993 Mississippi River flood with non-flood periods and 

to compile more data on the occurrence of toxic and noxious algae in the region . 

Some phytoplankton are characteristically found in certain types of water masses and can be 

used to trace the extent of movement of these water masses. This is especially true in the northern 

Gulf of Mexico where extremely different kinds of water often mix and range from highly 

eutrophic freshwater to extremely oligotrophic and saline waters offshore . 

Dortch (1994a) has proposed a series of indicators, the most important of which are 

summarized in Table 3. The small (1-2 p.m), coccoid cyanobacteria of the genus Synechococcus 

have a variety of photosynthetic pigments, besides chlorophyll a, which give them distinctive 

colors under epifluorescence microscopy (Dortch in Murray, 1994a). One group, containing the 

pigment phycocyanin (PC) (Waterbury et al ., 1986; Vaulot and Ning, 1988), is usually the only 

cyanobacteria at low salinities in this area (Table 3). Phytoplankton in offshore waters are 

generally dominated by another group of small coccoid cyanobacteria, the high phycourobilin 

phycoerythrin-containing Synechococcus (High PU/PE) (Table 3) (Olson et al ., 1988). Another 

group of cyanobacteria, those containing low phycourobilin and phycoerythrin (Low PU/PE), are 

intermediate in color and habitat. The pigment differences between these three groups have been 

hypothesized to be related to either salinity or light quality (Wood et al., 1985; Waterbury et al ., 

1986; Vaulot and Ning, 1988 ; Pick, 1991), but, regardless of the cause, they are specific 

indicators of water sources in this area (Dortch in Murray, 1994a). Thus, the degree to which the 

high-PU/PE or PC cyanobacteria dominate and are mixed with each other and the low-PU/PE 

cyanobacteria can indicate what types of water are muting. Unfortunately, a time-scale cannot yet 

be put on the mixing because we do not yet have any data on survival rates after mixing. 

74 
N 

3000 

25 00 

2000 

d 

1500 

e s 

8 

10 00 

s o0 

000 

Swlaca Chlorophyll a vs Salinity 

LATEX July 1997 

~~ . 

i 

]0 00 

25 00 

2000 

0 

a 

i, IS00 

n 

e s 

5 

1000 

$ Do 

goo 

3000 

2500 

m o0 

J 

a 

isoo 

s 

u 

10 00 

s o0 

0 00 

S 00 1000 1500 2000 .22500 3000 3500 0 0 

Sal4aly (Pill) 

Smlace Chlorophyll a VS Silicals :1111(ale Ratio 

LATEX July 199] 

1 . S . 

.r 

00 

Jw U. 

25 09 

20 00 

d 

Swlaca Chlorophyll a vs Dissolved inorganic Illlrogan 

LATEX July 1997 

.. `S 

~soo 

e 

s 

~ooo 

s o0 

0 00 

200 400 600 B00 

111171102,1107 jug .111.1 

Swlace Cldolophyll a vs Silicate 

LATEX July 199] 

1000 1200 

200 400 600 e00 1000 1200 00 200 400 600 000 1000 0a o 1400 

$IOJ :110] Ratio Sdicalo b~9-aVlj 

Figure 76 . Distribution of chlorophyll a concentrations by salinity, dissolved 

inorganic nitrogen (N03+N02+NH4), silicate and the silicate :DIN 

ratio for Cruise IV . 
Table 3. Phytoplankton groups or species which can be used to trace water masses and 

mixing in the LATEX area. 

Group/Species Indicator 

Phycocyanin-containing 

Synechococcus(PC) 

Skeletonema costatum 

DiatomsJT'otal Autotrophs 

High Phycouribilin/ 

Phycoerythrin-containing 

Synechococcus (High PU/PE) 

Direct river input 

High productivity core of plume 

River plume 

High salinity water off shore 

Because of the very low silicate concentrations in offshore waters, there are very few diatoms, 

especially in the large size fractions. Phytoplankton, especially diatom, biomass and growth is 

usually maximal at some distance from the river mouth because it is necessary for the sediment to 

settle out so there is enough light to stimulate photosynthesis . The first species to respond is 

almost always Skeletonema costatum, an opportunistic, fast-growing diatom. Growth of this 

species is then followed by that of other diatoms. Further down the axis of the plume the nutrients 

become depleted, and although there is plenty of light, there is insufficient nutrients to support high 

phytoplankton biomass. Thus, it can by hypothesized that high numbers of S. costatum will 

indicate the high productivity core of the plume, but that diatoms will dominate along the axis of 

the plume until the nutrients become depleted or diluted. 

2. Method 

The method for preserving and counting phytoplankton was adapted from Murphy and Haugen 

(1985), Shapiro and Haugen (1988), and Shapiro et al . (1989) . Phytoplankton samples were 

collected at the surface using a bucket and at the bottom using 10 liter Niskin bottles on a CTD 

Rosette. If there was a mid-water fluorescence maximum (Sea Tech Fluorometer on seabird 

CTD), samples were also taken with a Niskin bottle. Between 0.1 and 1 liter was preserved with 

50% glutaraldehyde to a final concentration of 0.5% (by volume) and refrigerated for at least 1 

hour and not more than 24 hours prior to size fractionated filtration (Dortch in Murray, 1994a) . 

Polycarbonate filters were used throughout. Phytoplankton in the 3 to 8 Nrn and > 8 dun size 

fraction were stained with proflavine monohydrochloride (Sigma P-4646, 0.33 g/200 ml distilled, 

deionized water) . Before staining, samples < 25 ml were made up to 25 ml using 0.2 Em filtered 

seawater of approximately the same salinity . If possible, samples were filtered without vacuum, 

but if necessary < 100 mm vacuum was applied. The filters were transferred to slides and 

mounted with low fluorescence, low viscosity immersion oil. The 0.2-3 N.rn size fraction was 

counted immediately, the 3-8 N.m size fraction was stored in the refrigerator and counted within 

several days. The > 8 tun filters were stored frozen and counted back in the lab within 60 days. 

Phytoplankton were counted on a BH2-RFA epifluorescence microscope with blue and green 

excitation (excitation filters BP-490 and BP-545, barrier filters 0-515 and 0-590, and dichroic 

mirrors DM500 and DM580). Table 4 outlines the types of organisms counted in each size 

fraction. 

76 
Table 4. Common phytoplankton groups counted in each size fraction . 

Size Phytoplankton Groups 

0.2 to 3 tLm Coccoid cyanobacteria--mostly Synechococcus 

(count immediately Autotrophic eukaryotes 

on ship) Heterotrophic eukaryotes 

3 to 8 dun Phytosynthetic flagellates and non-flagellates 

(count on ship Heterotrophic flagellates and non-flagellates 

within several Cryptomonads 

days) Athecate dinoflagellates 

Diatoms 

Coccoid cyanobacteria 

>8 Wm 

(freeze and count 

in lab, detailed 

taxonomy to 

species, where 

possible) 

3. Results 

Diatoms 

Dinoflagellates 

Ciliates 

Cryptomonads 

Colonial cyanobacteria 

Colonial, freshwater chlorophytes 

a. Cruise III, April 1993. Size Distribution. The distribution of total phytoplankton in 

the study area mirrored the distribution of the smallest phytoplankton (0.2-3 Em) because they are 

numerically the most abundant in both surface and bottom waters (data not shown). The highest 

concentrations are observed on the shelf between the Atchafalaya River and Galveston Bay and at 

the western end of the study area. Maximum concentrations of 3 to 8 Nxn phytoplankton occur in 

the vicinity of the Atchafalaya River (data not shown). Large phytoplankton concentrations exceed 

106 cells L-1 over much of the shelf east of Sabine Lake (data not shown). Thus, all size fractions 

were most abundant in the region between the Atchafalaya River and Sabine Lake (S2-S6), the 

smallest phytoplankton were also abundant off of south Texas (S8-X2), and the large 

phytoplankton were also abundant to the east of the Atchafalaya river (S2-S1). 

Distribution of Specific Groups. As in the previous cruises (Dortch in Murray, 1994a), the 

two most abundant groups of phytoplankton were cyanobacteria and diatoms (Table 5). The 

cyanobacteria were relatively most abundant (# cyanobacteria/total #phytoplankton x 100) offshore 

(data not shown). The %High PU/PE cyanobacteria, indicative of inputs of offshore water, were 

relatively low and increased from east to west (Figure 77, upper panel) . At the bottom the °kHigh 

PU/PE was generally higher and the contours of similar concentrations paralleled the coast line, 

suggesting that offshore water pushed further up on the shelf at depth and was overlain by water 

that was strongly influenced by riverine input even at long distances from the river. In contrast, 

the %PC cyanobacteria, indicative of direct riverine input, was highest off of Terrebonne Bay, the 

Atchafalya River, and Calcasieu Lake and were similar at both surface and bottom (Figure 78). 

77 
30 . 

29 . 

0 

J 

28. 

Longitude 

30 . 

29 . 

0 

J 

28 . 

0 

loo E so D so C 40 B zo A o 

n 96_0 95_0 94.0 93.0 92 .0 91 .0 

Longitude 

Figure 77. Contours of percent High PI/PE cyanobacteria/total cyanobacteria (0.2-

3.0 dun) during Cruise III: surface (upper panel) ; bottom (lower 

panel). 

0 

.0 

78 
30 . 

29 . 

0 

J 

v. . .w..-

" 

~ ._. .,.w.' N 

1 

too E so D 60 C 40 8 20 A 0 

' M M w~ 

A7 n qF n 95 0 94.0 93.0 92 .0 91 .0 

28 . 

Longitude 

30 . 

`A ,-, 

29 

0 

J 

28 . 

S 

100 E so D 60 C 40 g 2o A o 

97 .0 96.0 95 .0 94.0 93 .0 92 .0 91 .0 

Longitude 

Figure 78. Contours of percent PC cyanobacteria/total cyanobacteria (0.2-3 .0 dun) 

during Cruise III: surface (upper panel) ; bottom (lower panel) . 

79 

90 .0 
Table 5. Comparison of mean phytoplankton abundance (#cells L-1) and relative abundance Wells 

L-1 for a particular group/total #cells L-1 autotrophs x 100) for all muses in 1992 and 

1993 . 

April October April July 

1992 1992 1993 1993 

Abundance 

0.2 to 3 gm 7.50 x 107 2.14 x 108 3.06 x 107 6.91 x 108 

3 to 8 dun 5 .36 x 106 1 .12 x 107 1 .31 x 106 2.62 x 107 

> 8 1im 4.32 x 106 1 .41 x 106 2.04 x 106 4.20 x 106 

Total 8.46 x 107 2.26 x 108 3.40 x 107 7.21 x 108 

Relative Abundance 

Diatoms 

> 8 Wm 56 .2 22 .4 63 .4 6.6 

Total 7 .0 0.4 9.8 0.1 

anobacteria 

Total 82 .9 95.3 74 .5 97 .1 

Number Samples 79 79 93 130 

Skeletonema costatum, an indicator of the high productivity core of riverine%stuarine plumes, 

were most abundant off of the Atchafalya River and Calcasieu Lake (Figure 79). The larger plume 

area is indicated by high % diatoms/autotrophs >8pun (Figure 80). This corresponds to the same 

areas where large phytoplankton were most abundant (data not shown). 

On this cruise a potentially toxic diatom genus, Pseudo-nitzschia, was especially abundant 

(Figure 81). Concentrations exceeded 106 cells L-1 over large portions of the shelf to the east of 

Sabine Lake (S6), in the areas most influenced by riverine input. Concentrations in the surface 

layer greatly exceeded those at depth (Figure 81). 

b. Cruise IV, July 1993. Size Distribution . Phytoplankton concentrations were more 

than an order of magnitude higher and even more dominated by small phytoplankton in July than in 

April (Table 5). At the surface the highest concentrations of total phytoplankton and small 

phytoplankton (0.2 to 3 gm) were observed from transect S2 to the east, i.e ., in the region between 

the Atchafalaya and Mississippi Rivers (data not shown). The area of high abundance of the two 

larger size fractions was restricted to a smaller area closer to the Atchafalaya River (data not 

shown). In the bottom water, elevated concentrations of all size fractions were confined to shallow 

stations near the Atchafalya (data not shown), perhaps reflecting the very strong stratification. 

Distribution of Specific Groups. The cyanobacteria were by far the most numerically abundant 

phytoplankton group, averaging 97°l0 of the phytoplankton (Table 5) and exceeding 98% of the 

phytoplankton over most of the shelf at the surface and 95% in the bottom water (data not shown). 

At the surface the %High PU/PE cyanobacteria was high in the region strongly influenced by the 

Atchafalya plume in the eastern part of the study area (Figure 82). Instead of the usually gradual 

increase in %High PU/PE to the west, there is an abrupt increase, suggesting intrusion of Gulf 

80 
30 . 

29 . 

0 

J 

28 . 

0 

Longitude 

30 . 

29 . 

0 

J 

28 . 

0 

Longitude 

Figure 79. Contours of numbers of Skeletonema costatum (number L-1) during 

Cruise III: surface (upper panel) ; bottom (lower panel) . 

81 
30 . 

29 . 

0 

J 

28 . 

Longitude 

30 . 

29 . 

0 

J 

28 . 

0 

Longitude 

Figure 80. Contours of percent Diatoms/phytoplankton > 8 arm during Cruise III: 

surface (upper panel) ; bottom (lower panel) . 

82 
30 . 

29 . 

0 

J 

28 . 

0 

Longitude 

30 . 

29 . 

7 

O 

J 

28 . 

~. 

40x105 J0 .105 20x106 10x103 5K105 

1 .105 

- s ,`~ ~Y7 

97 .0 96 .0 95 .0 94 .0 93.0 

Longitude 

92 .0 91 .0 90.0 

Figure 81 . Contours of numbers of Pseudo-nitzschia spp . (number L-1) during 

Cruise III: surface (upper panel) ; bottom (lower panel) . 

83 
30 . 0-

29 . 5-

.= 29 . 0-

J 

0 

28 . 

96 .0 

30 . 

29. 

0 J 

28 . 

ioo so eo 40 

95.0 94.0 93 .0 

Longitude 

Longitude 

A 

zo 0 

??~ 

92.0 91 .0 

Figure 82. Contours of percent High PU/PE cyanobacteria/total cyanobacteria 

(0.2-3 .0 dun) during Cruise N: surface (upper panel) ; bottom (lower 

panel) . 

90 .0 

84 
water at the surface on the shelf (Fig. 82, upper panel) . The July 17 AVHRR SST images show 

similar structures in that area (Walker, pens. comm.). The bottom contours do not show the same 

structure (Fig. 82, lower panel) . The %PC outline the areas most strongly influenced by the high 

river runoff that was occurring during this cruise (Fig. 83). 

Because of large increases in cyanobacteria, and to some extent, other phytoplankton, diatoms 

were relatively less abundant, both in the large size fraction (>8 Em) and in the total phytoplankton 

(Table 5). Thus, in this unusual year with high river flow in mid-summer, neither the distribution 

of S. costatum (Fig. 84) nor % diatoms (Fig. 85) are good indicators of the location of the river 

plume. 

4. Discussion 

a. Using Phytoplankton as Indicators of Water Sources and Mixing. It was 

originally hypothesized that the coastal boundary current would separate the shelf into areas that 

were strongly influenced by either riverine inputs or offshore Gulf water with distinctive 

phytoplankton communities and sharp, strong gradients between them. The gradients are less 

strong than expected, although phytoplankton distributions, especially the distribution of 

cyanobacterial pigment types, clearly indicates the areas of riverine and offshore influence. 

Especially interesting was the intrusion of Gulf water onto the shelf at the surface in the summer, 

which was clearly shown by the % High PU/PE cyanobacteria, hinted at in the remote sensing 

data, and not apparent at all in the hydrographic data. In the two 1992 cruises (Dortch in Murray, 

1994a) and the April 1993 cruise, the abundance of Skeletonema costatum, an opportunistic 

diatom, and the relative abundance of diatoms, also delineated the areas of riverine influence, but in 

the summer of 1993, this pattern was not repeated, despite the huge riverine input. 

b. Seasonal Differences and the Effect of the 1993 Flood. The seasonal 

differences observed on the four cruises to date are summarized in Table 5. Overall phytoplankton 

numbers are higher in summer and fall than in spring . The species composition also changes 

substantially from spring to summer/fall. Large phytoplankton, primarily diatoms, become much 

less abundant, and small phytoplankton, primarily cyanobacteria, become much more abundant 

The effect of the 1993 Mississippi River flood was to create spring-like conditions of water and 

nutrient input during the summer when temperatures and light levels were high. Between samples 

collected as part of LATEX B and other programs (NOAH NECOP), sufficient data exist for one 

station in the core of the hypoxic zone (C6B; Dortch, 1994b; Rabalais et al ., 1994) to evaluate the 

impact on phytoplankton populations. Total phytoplankton, cyanobacteria, and diatom abundance 

all increased (Table 6). However, cyanobacteria abundance increased much more than any other 

group. Calculations of biomass, based on crude estimates of cell volume from cell counts in 

different size fractions, showed that in summer 1993, the biomass of cyanobacteria exceeded that 

of diatoms. Consequently, the flood resulted in increased overall phytoplankton biomass, but also 

a change in the size distribution of the biomass. Both of these could have impacts on higher 

trophic levels and on the extent, severity, and duration of hypoxia. 

85 
30 . 

29 . 

v 

J 

28 . 

Longitude 

30. 

29 . 

28. 

Longitude 

Figure 83. Contours of percent PC cyanobacteria/total cyanobacteria (0.2-3 .0 gym) 

during Cruise N: surface (upper panel) ; bottom (lower panel) . 

86 
30 . 

,.-

" " " . 

' ,.. _ A 29 . " ~ ~ a ;1 

J 

10x103 5a10 3a105 2110 

28 . 

96 .0 95.0 94.0 93 .0 92 .0 91 .0 90 .0 

Longitude 

30 . 

, 

~- 29 . -k = o 

. . ; 

4Qa104 20x104 IOa104 Za104 

28 . 

96 .0 95 .0 94 .0 93.0 92 .0 91 .0 90 .0 

Longitude 

Figure 84. Contours of Skeletonema costatum (number L-1) during Cruise IV: 

surface (upper panel) ; bottom (lower panel) . 

87 
30 . 

30 . 

29 . 

a 

J 

28. 

Longitude 

Figure 85 . Contours of percent Diatoms/phytoplankton > 8 dun during Cruise N: 

surface (upper panel) ; bottom (lower panel) . 

88 
Table 6. Average cell numbers and estimated biomass in all samples collected in July through 

September in the core of the hypoxic region in non-flood and flood years. Adapted from 

Dortch (1994b). 

Non-Flood Flood1990-92 % Change 1993 

Cell Numbers (#L'1) 

Total Phytoplankton 1 .3 x108 18 x 108 1385 

Cyanobacteria 0.8 x 108 18 x 108 2250 

Diatoms 0.4 x 106 1 .6x 106 400 

Biomass (JigCL'1)* 

Cyanobacteria 46 .8 1007 2140 

Diatoms 263 953 360 

'Estimated from approximate cell volume 

c. Toxic and Noxious Phytoplankton Abundance. These cruises provide an 

opportunity to survey the Louisiana and Texas continental shelves for the occurrence of toxic and 

noxious phytoplankton. These data have been supplied to both the Barataria-Terrebonne Bay and 

the Corpus Christi Bay National Estuary Programs to aid in determining the potential threat from 

toxic and noxious algae in those areas, as part of their assessment of Current Status and Trends. 

No blooms resulting in water discoloration were observed on these cruises. However, 

Pseudo-nitzschia spp., potentially toxic diatoms, reached bloom concentrations during the April 

cruise (Figure 81 ; Table 7), especially on the shelf in the areas most immediately influenced by the 

Mississippi and Atchafalaya Rivers and were present in 97% of all samples. They were present in 

almost 50% of samples in the summer as well, but were much less abundant (Table 7). Dortch et 

al. (submitted) have shown that peak abundance of this organism occurs every April. Determining 

if the species that are present are toxic requires scanning electron microscopy or toxin analysis, 

neither of which have yet been done on a routine basis, so it is unknown whether the April peak 

contains toxic species. Both toxic and sometimes toxic forms have been identified in the northern 

Gulf of Mexico (Fryxell et al ., 1990; Dortcli et al ., submitted) . 

A number of other potentially toxic and noxious phytoplankton were also observed on these 

cruises (Table 7). In particular species possibly associated with Diarrhetic Shellfish Poisoning in 

humans occurred frequently in samples, both in the spring and the summer. In addition, 

Gymnodinium sanguineum, a dinoflagellate associated with fish kills and oyster mortality, which 

caused a major red tide and fish kill in the spring of 1994 (Robichaux and Dortch, submitted), was 

also observed at many locations. 

89 
Table 7. Abundance) and frequency2 of occurrence of toxic and noxious phytoplankton observed 

during cruises. A ? by Problem indicates that there is evidence implicating this species in 

causing this problem, but the link is not conclusive. 

Taxon April 13-19,1993 July 13-21,1993 Problem 

Maximum % Maximum 

Abundance Frequency2 Abundance Frequency2 

Pseudo-nitzschia spp. 4.40 x 106 96 .8 8.89 x 104 47.7 ASPS 

Prorocentrum compressum 8.12 x 104 79 .6 2.47 x 104 53.9 DSP4? 

Prorocentrum micans 3.25 x 104 54.8 7.55 x 103 32 .3 DSP4? 

Dinophysis caudata 6.67 x 102 8 .6 5.41 x 103 27.7 DSP4? 

Gymnodinium sanguineum 1 .66 x 103 9 .7 2.99 x 103 14.6 Ichthyotoxic 

Oyster mortality 

Dinophysis ovum 4 x 102 4.3 2.4 x 101 0.8 DSP4? 

Ceratium spp. 3.25 x 103 39.8 9.44 x 103 37.7 DSP4? 

Hypoxia 

Oscillatoria spp. 0 0 1 .73 x 106 16.9 Discolored Water 

Ichthyotoxic 

~ Maximum abundance observed on cruise in # cells L'1 

2% Frequency = # samples with species/total # samples x 100 

3Amnesic Shellfish Poisoning, short term memory loss, seizures, death (Hallegraeff, 1993) 

4Diahretic Shellfish Poisoning, gastrointestinal distress, but not fatal; also a tumor promoter (Hallegraeff, 1993) 

C. Zooplankton Survey (Richard F. Shaw) 

1. Materials And Methods 

Oblique plankton tows were made with a 60-cm diameter paired-bongo frame modified to 

accommodate a 20-cm ring. Bongo-net meshes were 153 and 335 lam, while a 63-p.m mesh net 

was lashed to the 20-cm ring. Nets were lowered to the bottom as rapidly as possible before 

retrieving at a ship speed of 1 knot and an ascent of approximately 1 m/s. The 153- and 335-Nm 

nets were fitted with a torpedo-type flowmeter for volume-filtered estimates. Volume-filtered 

estimates were not determined for the 63-N.rn net because of the potential of clogging ('Taylor, 

1976; Tranter and Smith, 1968). Upon retrieval, nets were thoroughly rinsed from the outside, 

concentrating the sample within the cod-end. The 63- and 153-N.m samples were initially preserved 

in 4% formaldehyde and transferred to 70% ethanol after 24 hours. The 335-gym samples were 

preserved in 70% non-denatured buffered ethanol, placed on ice, and transferred to fresh 70% 

ethanol 24 hours later. This method is suitable for larval fish otolith and growth analysis (Butler, 

1992; Methot and Kramer, 1979). 

Biomass estimates were performed on one-half of all samples. All plankton splits were made 

with a Folsom plankton splitter (McEwen, et al ., 1954). Halves were randomly selected for 

analysis. The three mesh sizes used in the study were analyzed in order to evaluate differences in 

zooplankton-size fractionadons. To determine the relationship between volumetric and gravimetric 

90 
techniques, volume displacements (Yentsch and Hebard, 1957) were conducted on a randomlyselected third of the stations from each cruise prior to dry weight determination Dry weights were 

determined for all samples by oven-drying at 60° C to constant weight (Lovegmve, 1966). 

Zooplankton density was calculated for both gravimetric (mg/m3) and volumetric (ml/m3) biomass 

estimates for the 153- and 335Nru samples as: 

Zooplankton Density = Biomass EstimateNolume Filtered, 

where Volume Filtered--volume filtered (m3) by the net Volume filtered was calculated as: 

Volume Filtered = Net Area x D Counts x Rotor Constant, 

where Net Area=area of net's mouth (m2), 0 Counts-number of flowmeter rotor revolutions, and 

Rotor Constant=0.5102. Zooplankton density was not determined for the 63-Em (20-cm net) 

samples since volume filtered estimates were not recorded for these samples. 

Larval fish and squid were removed from halves of all 335-Pin samples and were identified to 

the lowest taxonomic level possible. If a 335-dun mesh sample was not available at a particular 

station, the 153-dun sample was sorted if available. If the number of fish sorted from a half sample 

was less than five, the other half of the sample was sorted if available. For each station, larval 

density (#/m3) was calculated for each species as: 

Density = (# SpeciesxNolume Filtered), 

where # Speciesx = the number of larvae of the xm species at that station. For each cruise, percent 

catch was calculated for each species as: 

Rb Catch = (# Speciesx/E(# Speciesx)) X 100. 

All halves (minus the fraction used in the zooplankton analysis for the 153-pcn samples and 

larval fish and squid for the 335-Fun samples) not used for biomass estimates were archived in 

70% ethanol for future ecological studies. 

2. Results 

a. Overview. Two hundred and eighty-seven samples were obtained from 96 biological 

stations during LATEX B Cruises III and N (Table 8) at station depths ranging between 2.9 and 

57.7 m. Forty-four genera of larval and juvenile fish and 2 squid genera, from 30 families, were 

identified collectively (Table 9). From the half-split samples, 2268 fish were identified and 2073 

were measured, while 19 squid were identified and measured Curiously, squid were only present 

during the April cruise . An estimated 4495 larval and juvenile fish and 38 squid were captured 

after taking splits into account 

Zooplankton dry weight densities were significantly higher in April than in July within the 153-

size fraction; however, no difference was found between the cruises for the 335-size fraction (ttest, a=0.05) . Since the data violated the assumptions for parametric regression even after 

appropriate transformations, the relationship between zooplankton dry weight and volume 

displacement was evaluated by Kendall Partial Rank-Order Correlation Coefficient (T Z9- Siegel 

and Castellan, 1988). Analysis revealed a strong positive relationship (x.81, p<O.W1) between 

the two biomass determinations. Thus, the two biomass estimates are linearly related with little 

variability. However, dry weight may lead to the best estimate since it tends to be less variable and 

more reproducible statistically, and more useful for ecological studies from an energetic standpoint, 

than volume displacement estimates (Lawrence, 1976). 

b. Cruise III, April 1993. One hundred and thirty-one samples from 44 biological 

stations were obtained during the April 1993 cruise (Table 8), a high-river discharge period. 

Samples were collected from the Mississippi Delta (S1) to Corpus Christi, Texas (X3), the 

southwestern-most portion of the plume's range (Figure 86). 

91 
Table 8. Samples (63-, 153-, and 335-p.rn mesh) obtained at each station during Cruises III and N. 

X=sample collected. PL= Galveston Plume Survey. CV--Convergence Survey. 

APRIL JULY 

Mesh (N,m) Mesh (Nm) 

Station Transmit 0 M M SIaI14d ICaIIsect U 1`~ 

931101 S1 X X X 932101 S1 X X X 

931103 S1 X X X 932103 S1 X X X 

931105 S1 X X X 932105 S1 X X X 

931107 S1 X X X 932107 S1 X X X 

931109 S1 X X X 932111 S1 X X X 

931113 S2 X X 932115 S2 X X X 

931114 S2 X X X 932117 S2 X X X 

931116 S2 X X X 932119 S2 X X X 

931118 S2 X X X 932122 S2 X X X 

931120 S2 X X X 932124 S2 X X X 

931127 S3 X X X 932130 S3 X X X 

931129 S3 X X X 932131 S3 X X X 

931131 S3 X X X 932133 S3 X X X 

931133 S3 X X X 932135 S3 X X X 

931140 S4 X X X 932137 S3 X X X 

931142 S4 X X X 932144 S4 X X X 

931144 S4 X X X 932146 S4 X X X 

931146 S4 X X X 932149 S4 X X X 

931147 S4 X X X 932151 S4 X X X 

931151 S5 X X X 932153 S4 X X X 

931153 S5 X X X 932157 S5 X X X 

931155 S5 X X X 932159 S5 X X X 

931157 S5 X X X 932162 S5 X X X 

931159 S5 X X X 932165 S5 X X X 

931168 S6 X X X 932167 S5 X X X 

931169 S6 X X X 932169 S6 X X X 

931171 S6 X X X 932171 S6 X X X 

931173 S6 X X X 932173 S6 X X X 

931175 S6 X X X 932175 S6 X X X 

931179 S7 X X X 932177 S6 X X X 

931181 S7 X X X 932181 S7 X X X 

931183 S7 X X X 932183 S7 X X X 

931187 S8 X X X 932185 S7 X X X 

931189 S8 X X X 932189 S8 X X X 

931191 S8 X X X 932191 S8 X X X 

931195 X1 X X X 932193 S8 X X X 

931197 X1 X X X 932401 PL X X X 

931199 X1 X X X 932403 PL X X X 

931202 X2 X X X 932405 PL X X X 

931204 X2 X X X 932407 PL X X X 

931206 X2 X X X 932408 PL X X 

931209 X3 X X X 932409 PL X X X 

931211 X3 X X X 932413 PL X X X 

931213 X3 X X X 932415 PL X X X 

932420 PL X X X 

932423 PL X X X 

932601 CV X X X 

932604 CV X X X 

932605 CV X X X 

932606 CV X X X 

932610 CV X X X 

932613 CV X X X 

92 
w 

Table 9. Estimated larval fish and squid densities by cruise from data obtained during Cruises III and IV. Both cruises were 

combined for number caught and measured. Length statistics were based upon number measured. Also included by cruise 

are the number of stations with volume filtered estimates; vo lume filtered, and total estimated num ber of fish and squid 

caught. If the median of the density was not equal to zero, it was included after pos stn. Total catch, number caught, and 

densities were calculated after splits were accounted for. sd=standard deviation. pos sm=number of stations at which that 

species was identified. 

Apr-93 Jul-93 

# STATIONS 44 50 

VOLUME FILTERED (M3) 1067 928 

TOTAL CATCH 1634 2899 

SPECIES # # Size Range Mean Length Mean Density Mean Densfty 

Caught Measured (mm) (mm; s.d.) (#/100 m3) (sd, pos stn) (#/100 m3) (sd, pos stn) 

ANGUILLIFORMES (eels and morays) 4 2 5.3-6 .5 5.90 (0 .71) 0.43 (2 .20; 2) 

CLUPEIFORMES (herrings and anchovies) 51 25 1 .8-7 .0 4.08(l .53) 4.47 (15.09; 6) 1 .21 (7.24; 2) 

OSTEICHTHYES (bony fishes) 28 12 1 .0-3 .0 1 .50 (0 .67) 2.18 (7.50; 5) 

PERCIFORMES (perciform fishes) 46 19 1 .3-4 .9 1 .84 (0 .96) 4.21 (8.55; 13) 0.76 (5.36; 1) 

ATHERINIDAE (sibersides) 2 1 6.2 6.2 0.12 (0 .79; 1) 

BLENNIDAE (biennies) 18 9 2.0-3 .2 2.22 (0 .44) 0.55 (2.59; 2) 1 .18 (4 .14; 4) 

Ophioblennius atlanticus (redlip blenny) 4 2 2.2-2.5 2.35 (0 .00) 1 .02 (6.77; 1) 

Scartella cristata (molly miller) 2 1 9.8 9.8 0.17 (1 .24; 1) 

BOTHIDAE (lefteye flounders) 27 14 1 .6-2 .6 1 .79 (0 .43) 2.27 (7.15; 7) 0.17 (1 .23; 1) 

Citharichthys spilopterus (bay whiff) 2 1 8.5 8.5 0.17 (1 .24; 1) 

Citharichthys spp. (whiffs) 10 5 1 .7-9 .0 5.00 (3 .81) 1 .61 (7.56; 2) 0.33 (2.36; 1) 

Engyophrys septa (spiny flounder) 2 1 6.4 6.4 0.09 (0.66; 1) 

Etropus crossotus (fringed flounder) 57 32 2.0-9 .1 4.16(l .97) 1 .12 (3 .44; 5) 3.82 (10.14; 10) 

Syacium gunteri (shoal flounder) 40 20 1 .8-10.5 2.55 (2 .39) 3.09 (20.76; 2) 

Syacium spp. (lefteye flounders) 40 20 1 .1-5 .8 1 .60(l .05) 1 .04 (5 .74; 2) 1 .87 (7.94; 4) 

Trichosetta ventralis (sash flounder) 4 2 2.4-2 .9 2.65 (0 .00) 0.31 (2.18; 1) 

BREGMACEROTIDAE 

Bregmaceros cantori (codlet) 64 32 1 .5-48.0 12.09 (10.45) 4.44 (13.63; 7) 

Bregmaceros spp. (codlets) 10 5 2.0-11 .4 7.00 (4.64) 0.52 (2 .28; 3) 

CARANGIDAE (jacks) 2 1 2.9 2.9 0.21 (1 .47; 1) 

Chloroscombrus chrysurus (Atlantic bumper) 130 68 1 .2-4.9 2.15 (0.65) 13.84 (29.17; 18) 

CLUPEIDAE (herrings) 2 1 1 .8 1 .8 0.82 (5 .77; 1) 
Table 9 cont'd 

# STATIONS 

VOLUME FILTERED (M3) 

TOTAL CATCH 

SPECIES # # Size Range Mean Length 

Caught Measured (mm) (mm; s.d .) 

Brevoortia patronus (gulf menhaden) 142 

Harengula jaguana (scaled sardine) 94 

Opisihonema oglinum (Atlantic thread herring) 128 

CONGRIDAE (conger eels) 2 

ELEOTRIDAE 

Dormitator maculatus (fat sleeper) 4 

ENGRAULIDAE (anchovies) 2052 

Anchoa cubana (Cuban anchovy) 26 

Anchoa hepsetus (striped anchovy) 82 

Anchoa nasuta (longnose anchovy) 14 

Anchoa spp. (anchovies) 180 

Anchoviella perlasciata (flat anchovy) 132 

GOBIESOCIDAE 

Gobiesox strumosus (skilletfish) 6 

GOBIIDAE (gobies) 98 

Gobiosoma bosc (naked goby) 2 

LOLIGINIDAE 

Loligo spp. (inshore squids) 30 

Lo!liguneula brevis (w. Atlantic brief squid) 8 

LUTJANIDAE (snappers) 4 

Lutjanus spp. (snappers) 8 

MICRODESMIDAE 

Microdesmus lanceolatus (lancetail wormfish) 11 

MUGILIDAE 

Mugil curema (white mullet) 2 

OGCOCEPHALIDAE (batfishes) 2 

OPHICHTHIDAE (snake eels) 6 

Aplatophis chauliodus (tusky eel) 1 

Myrophis punctatus (speckled worm eel) 4 

Ophichthus spp. (snake eels) 2 

Apr-93 

44 

1067 

1634 

Mean Density 

(#/100 m3) (sd, pos stn) 

Jul-93 

50 

928 

2899 

Mean Density 

(#/100 m3) (sd, pos stn) 

67 5.2-23.5 20.15 (2.69) 10.09 (55.86; 4) 

47 3.3-17.0 9.96 (3.75) 0.11 (0 .75; 1) 6.27 (22.15; 8) 

64 3.6-13.8 7.47 (2.62) 0.07 (0 .48; 1) 15.18 (40.88; 10) 

1 38 38 0.14 (1 .01 ; 1) 

2 1 .7-2 .4 2.05 (0 .71) 0.23 (1 .49; 1) 

858 1 .7-16.2 5.33 (2 .47) 83.18 (174 .32; 26; 17.97) 160.6 (280 .18; 37; 67.42) 

12 4.5-12.3 6.67 (2 .19) 6.66 (44.18; 1) 

41 3.5-19.5 6.78 (3 .17) 0.12 (0.79; 1) 7.72 (50.46; 3) 

7 15.0-35.0 19.71 (6 .87) 1 .53 (8 .00; 2) 

83 1 .8-11 .5 5.14 (2.07) 20.82 (61 .31 ; 7) 4.05 (20.35; 2) 

66 5.0-25.0 8.56 (4.66) 0.84 (4 .77; 2) 8.25 (53.88; 3) 

3 3.7-9 .7 7.00 (3 .46) 0.45 (2 .10; 2) 

48 1 .5-10.5 5.67 (3 .51) 4.84 (11 .52; 1) 2.41 (6.41 ; 9) 

1 9 9 0.12 (0.87; 1) 

15 1 .0-1 .9 1 .00 (0 .00) 2.25 (6 .21 ; 7) 

4 1 .1-2 .6 1 .50 (0.58) 0.98 (3 .97; 3) 

2 2.4-6 .5 4.45 (2.83) 0.35 (1 .89; 2) 

4 3.0-8 .4 4.75 (2.36) 0.54 (2 .39; 3) 

6 2.5-13.0 5.83 (4.62) 

1 2.9 2.9 

1 3.5 3.5 

3 5.0-49.5 22.67 (23.25) 

1 38 38 

2 51 .0-71 .0 61 .00 (14.14) 

1 75 75 

0.88 (3 .82; 3) 

0.16 (1 .08; 1) 

0.12 (0 .82; 1) 

0.13 (0.88; 1) 0.25 (1 .26; 2) 

0.07 (0 .52; 1) 

0.73 (3 .66; 2) 

0.16 (1 .03; 1) 
110 

Table 9 cont'd 

# STATIONS 

VOLUME FILTERED (M3) 

TOTAL CATCH 

SPECIES # # Size Range Mean Length 

Caught Measured (mm) (mm; s.d .) 

OPHIDIIDAE (cusk-eels) 2 

Lepophidium spp. (cusk-eels) 12 

Ophidion selenops (mooneye cusk-eel) 2 

Ophidion spp. (cusk-eels) 12 

SCIAENIDAE (drums) 30 

Cynoscion arenarius (sand seatrout) 385 

Cynoscion nebulosus (spotted seatrout) 16 

Cynoscion nothus (silver seatrout) 8 

Cynoscion spp. (seatrouts) 4 

Larimus fasciatus (banded drum) 2 

Menticirrhus spp. (kingfishes) 16 

Micropogonias undulatus (Atlantic croaker) 26 

Pogonias cromis (black drum) 16 

StelINer lanceolatus (star drum) 68 

SCOMBRIDAE 

Auxis spp. (mackerels) 16 

Euthynnus alleKeratus (little tunny) 4 

Scomberomorus cavalla (king mackerel) 2 

Scomberomorus maculatus (Spanish mackerel) 20 

SCORPAENIDAE 

Scorpaena spp. (scorpionfishes) 2 

SERRANIDAE (sea basses) 44 

Serraniculus pumilio (pygmy sea bass) 2 

SOLEIDAE 

Achirus lineatus (lined sole) 2 

Symphurus plagiusa (blackcheek tonguefish) 11 

Symphurus spp. (tonguefishes) 190 

SPARIDAE (porgies) 4 

STROMATEIDAE 

Peprilus burti (gulf butterfish) 4 

Apr-93 

44 

1067 

1634 

Mean Density 

(#/100 m3) (sd, pos stn) 

Jul-93 

50 

928 

2899 

Mean Density 

(#/100 m3) (sd, pos stn) 

1 2.5 2.5 0.27 (1 .93; 1) 

6 2.9-7 .5 4.00(l .67) 1 .01 (3 .14; 5) 

1 35 35 0.13 (0 .90; 1) 

6 2.1-13.3 4.50 (4.32) 1 .18 (4 .52; 4) 

14 1 .2-4 .3 2.14 (1 .03) 2.95 (9 .20; 5) 0.52 (2 .72; 2) 

194 1 .4-21 .5 2.75 (2.18) 25.77 (39.64; 24; 5.88) 15.00 (52.88; 16) 

8 2.4-3 .9 2.38 (0 .52) 1 .31 (5 .88; 3) 

4 2.9-4 .5 3.00 (0.82) 0.78 (3 .35; 3) 

2 4.4-71 .5 7.95 (4.95) 0.29 (1 .90; 1) 0.09 (0 .66; 1) 

1 2.3 2.3 0.15 (1 .07; 1) 

8 1 .4-4.2 2.38 (0 .92) 1 .04 (3 .17; 5) 0.70 (3 .08; 3) 

13 2.2-7.2 4.31 (1 .49) 0.98 (5 .78; 2) 

8 2.4-4.7 2.88 (0 .64) 2.86 (15.13; 2) 

35 2.1-6.3 3.46(l .07) 8.88 (24.93; 11) 

8 2.0-7.5 2.88 (1 .73), 1 .16 (5.75; 3) 

2 4.1-4.3 4.20 (0 .00) 0.31 (1 .51 ; 2) 

1 4.5 4.5 0.15 (1 .07; 1) 

10 1 .2-4.5 2.60(l .17) 0.11 (0 .75; 1) 2.25 (8.38; 6) 

1 8.5 8.5 0.15 (1 .09; 1) 

22 1 .2-5 .0 2.14 (0 .94) 0.42 (1 .96; 2) 3.03 (10.02; 6) 

1 2.1 2.1 0.12 (0.84; 1) 

1 3.3 3.3 0.12 (0 .87; 1) 

6 2.0-12.5 4.33 (3.88) 1 .22 (5 .94; 3) 

98 1 .3-10.7 2.54(l.37) 0.72 (2.33; 4) 15.83 (33.75; 23) 

2 4.5-6 .1 5.30 (1 .41) 0.22 (1 .02; 2) 

2 2.1-2.2 2.15 (0.00) 0.35 (1 .66; 2) 
10 ON 

Table 9 cont'd 

# STATIONS 

VOLUME FILTERED (M3) 

TOTAL CATCH 

SPECIES 

SYNGNATHIDAE 

Syngnathus spp. (pipefishes) 

SYNODONTIDAE (lizardfishes) 

Synodus foetens (inshore lizardfish) 

Synodus spp. (lizardfishes) 

TETRAODONTIDAE 

Sphoeroides spp. (puffers) 

TRICHIURIDAE 

Trichiurus lepturus (Atlantic cutlassfish) 

TRIGLIDAE 

Prionofus spp . (searobins) 

Apr-93 Jul-93 

44 50 

1067 928 

1634 2899 

# # Size Range Mean Length Mean Density Mean Density 

Caught Measured (mm) (mm; s.d .) (#/100 m3) (sd, pos stn) (#/100 m3) (sd, pos stn) 

2 1 11 11 0.14 (0 .95; 1) 

16 8 3.2-9 .0 5.13 (2 .03) 0.94 (5 .78; 2) 

8 4 2.5-3 .2 2.55 (0 .50) 0.42 (2 .82; 1) 

14 7 2.4-7 .3 4.71 (1 .70) 0.26 (1 .32; 2) 0.47 (3 .29; 1) 

4 2 2.3-8 .2 5.25 (4.24) 0.18 (1 .20; 1) 0.09 (0.66; 1) 

2 1 7.5 7.5 0.07 (0 .48; 1) 

2 1 8.6 8.6 0.13 (0 .88; 1) 
30 

29 

LA 

TX 

t = t 

S6 Sg S4 ~ 

S7 

S8 

ss si 

" Xl 

zs 

N = 44 

N 

7 

N 

J 

28 

27 t98 97 96 95 94 93 92 91 90 

Longitude 

Figure 86. LATEX B biological stations during Cruise III, April 1993. 

97 
Zooplankton biomass tended to be high in the central portion of the study area, from 

Atchafalaya Bay (S2) to slightly west of Sabine Lake (S6), for both the 153- and 335-dun samples 

(Figures 87 and 88). Zooplankton dry-weight density for the 153-pcn samples ranged from 16.77 

to 1213.70 mg/m3 (mean=149.21, s.d .=201.89), while the 335-N.rn samples ranged from 6.91 to 

400.69 mg/m3 (mean=40.07, s.d.=61 .37; Table 10). 

An estimated 1596 larval and juvenile fish and 38 squid were collected during the April cruise . 

Larval fish densities were highest off Atchafalaya Bay (S2 and S3) and along the Texas coast (S7 

through X2; Figure 89), ranging from 0.05 to 9.361arvae/m3 (mean=1 .91, s.d .=1 .95; Table 10). 

April samples were dominated by the unidentified engraulids (%Catch = 36.34 %), although 

Cynoscion arenarius (14.79 %), Anchoa spp. (11 .66 %), and Brevoortia patronus (8.90 %) were 

also abundant (Table 9). These percentages are similar to Cruise I (April 1992) results, where 

engraulids (74%) and Cynoscion arenarius (14.4 %) dominated the catch. Listed below is a 

breakdown of the cruise by transect 

S1. The 153-N.m dry-weight densities were fairly low and increased with increasing distance 

from shore, ranging from 49.48-93 .40 mg/m3. The 335-N.m biomass estimates showed no 

discernible trend, ranging from 24.23-45 .16 mg/m3. Total larval fish density also increased with 

increasing distance from shore, ranging from 0.99-2.08 larvae/m3. 

,U. No trend was observed for zooplankton biomass. The 153-Prn zooplankton densities 

ranged from 43.74-281 .89 mg/m3, while the 335-Fun dry weights ranged from 22.68-70 .18 

mg/m3. However, larval fish densities decreased with increasing distance from shore, ranging 

from 1.92-4.42 larvae/M3. 

52. The highest zooplankton densities were found along the coast for both mesh sizes. Dry 

weights ranged from 26.90-627.73 mg/m3 for the 153-Nxn samples and from 13.53-400.69 mg/m3 

for the 335-urn samples. This was a maximum for the 335-urn mesh size and the second largest 

value recorded for the 153-Em samples; however, the net hit substrate during towing and mud in 

the sample probably increased the estimate unrealistically. Ichthyoplankton densities were high 

throughout the transect, ranging from 3.61-7.591arvae/m3. 

SA. The 153-N.m biomass estimates were highest in the center portion of the transect, ranging 

from 68.94-214.78 mg/m3. The 335-gym biomass density decreased as distance from shore 

increased, decreasing from 57.93 to 15.08 mg/m3. Larval fish density also tended to decrease 

from shore and ranged from 0.45-1.701arvae/m3 . 

S5. No noticeable trend was observed for the density estimates. Zooplankton biomass 

estimates ranged from 16.77-332.12 mg/m3 for the 153-urn samples and from 6.91-48.57 mg/m3 

for the 335-pzn samples. Ichthyoplankton densities ranged from 0.16-1 .SS larvae/m3. 

a. While no trend was discernible for the densities, zooplankton biomass estimates were high 

throughout the transect The 153-gym samples ranged from 122.44-1213.70 mg/m3, a cruise 

maximum, while the 335-gm biomass estimates ranged from 13.38-157.29 mg/m3. Again, this 

cruise maximum was affected by mud in the sample. Larval fish densities remained low, ranging 

from 0.06-1 .37 larvae/M3. 

,U. . The 153-urn biomass estimates ranged from 88.44-125.43 mg/m3. The 335-N.m biomass 

densities decreased with increasing distance from shore, ranging from 9.65-35 .10 mg/m3. Larval 

fish density also increased in the shoreward direction to a cruise maximum, increasing from 0.48-

9.36 larvae/m3. 

98 
30 

29 

m 

' 28 

27 

! . ! 

~`rr`~ . 

". " < 60 mg/m3 

60 - 120 mg/m3 

121 - 180 mg/m3 

~ > 180 mg/m3 

98 97 96 95 94 

Longitude 

93 92 91 90 

Figure 87. Plankton biomass (mg/m3) for 153 um mesh samples collected during Cruise III. 

99 
30. 

29 

m v 

28 

27 

~-`r- LA 

" < 15 mg/m3 

15 - 30 mg/m3 

" 31- 45 mg/m3 

~ > 45 mg/m3 

98 97 96 95 94 93 92 91 90 

Longitude 

Figure 88. Plankton biomass (mg/m3) for 335-p.m mesh samples collected during Cruise III. 

100 
30 

29 

m 

v 

28 

27 

I^ 

. . ' ~ u 

" < 1 .0 mg/m3 

1 .0 - 2.0 mg/m3 

00' 

0 2.1 - 3.0 mg/m3 

~ > 3.0 mg/m3 

98 97 96 95 94 93 92 91 90 

Longitude 

Figure 89. Total larval fish density (#/m3) for Cruise III zooplankton samples. 

101 
Table 10. List of zooplankton dry weight densities (mg/m3) for 153-N.m and 335-Em mesh 

samples, total larval fish and squid densities (#/m3) at each station sampled during 

Cruises III and N. ("-"= data not available; PL.-alveston Plume Survey; CV = 

Convergence Su rvey. 

APRIL Densities JULY Densities 

Sta t i on Transact ]w ry 3. .35IL 

,EX 

Larval Station Transact 1Dry 3WIL Dry Larval 

931101 S1 49 .48 29.75 0.99 932101 S1 239.98 44 .39 4.44 

931103 S 1 93 .40 45.15 1 .37 932103 S 1 117.07 34 .36 16.85 

931105 S1 59 .39 31 .27 1 .77 932105 S1 46.55 25 .37 0.52 

931107 S1 70 .80 24.23 2.08 932107 S1 44.83 15 .54 0.30 

931109 S 1 71 .18 28.25 0.99 932111 S 1 - 10 .33 0.41 

931113 S2 - 28.78 1 .92 932115 S2 27.52 14 .47 1 .50 

931114 S2 43 .74 22.68 2.27 932117 S2 67.49 8 .37 1 .07 

931116 S2 159 .10 70.18 2.94 932119 S2 66.59 12 .60 1 .26 

931118 S2 74 .77 24.86 4.42 932122 S2 171 .49 10 .02 1 .67 

931120 S2 281 .89 45.52 4.14 932124 S2 141 .46 90.70 0.26 

931127 S3 111 .04 27.97 7.59 932130 S3 28.36 17 .86 0.07 

931129 S3 106.30 40.90 3.88 932131 S3 47.24 6.43 0.06 

931131 S3 26 .90 13.53 3.61 932133 S3 151 .12 8 .87 2.36 

931133 S3 627.73 400.69 4.77 932135 S3 261 .11 108 .11 0.64 

931140 S4 69 .01 57.93 0.97 932137 S3 24.65 0 2.40 

931142 S4 120.02 37.93 1 .70 932144 S4 91 .43 51 .60 9.03 

931144 S4 214.48 30.42 1 .01 932146 S4 44.05 17.80 0.11 

931146 S4 138.73 16.18 0.81 932149 S4 47.26 12 .44 0.10 

931147 S4 68.94 15.08 0.45 932151 S4 189.01 88.34 1 .04 

931151 S5 42.70 6.91 1 .55 932153 S4 30.16 21 .17 1 .44 

931153 S5 16.77 25.13 1 .08 932157 S5 50 .99 25 .39 2.67 

931155 S5 182.14 24.99 0.16 932159 S5 71 .68 39 .96 1 .00 

931157 S5 332.12 48.57 0 .69 932162 S5 81 .93 11 .77 0.74 

931159 S5 178.04 40.44 1 .35 932165 S5 105 .56 25.95 2.16 

931168 S6 122.44 21 .02 0 .10 932167 S5 75 .39 35.34 4.08 

931169 S6 212.64 13.38 0 .43 932169 S6 39 .90 20.14 2.85 

931171 S6 270.61 157.29 1 .04 932171 S6 100.23 29.31 4.06 

931173 S6 1213.70 44.74 0 .06 932173 S6 176.22 73.34 8.52 

931175 S6 368.48 96.09 1 .37 932175 S6 133.84 79.38 5.29 

931179 S7 92 .34 9.64 0 .48 932177 S6 71 .56 14.85 4.71 

931181 S7 88.44 16.37 0 .91 932181 S7 45 .87 16.42 6.69 

931183 S7 125.43 35.10 9 .36 932183 S7 41 .00 19.10 5.19 

931187 S8 49 .36 10.66 0 .74 932185 S7 16 .19 8.15 0.82 

931189 S8 73.14 15.65 2 .61 932189 S8 28 .00 10.09 3.03 

931191 S8 83.36 18.99 4 .07 932191 S8 45 .76 23.94 0.27 

931195 X1 30.93 8.53 0 .71 932193 S8 80 .44 15 .34 3.07 

931197 X1 66.15 19.53 0 .94 932401 PL 42 .66 63.56 1 .19 

931199 X1 76.70 38.05 0 .54 932403 PL 58 .71 26.46 1 .50 

931202 X2 33.51 13.94 0 .62 932405 PL 39 .00 16.95 0.94 

931204 X2 38.11 31 .23 0 .31 932407 PL 46 .73 6 .60 0 

931206 X2 87.99 8.33 3 .33 932408 PL 53 .04 10.15 0.30 

931209 X3 186.16 30.49 0 .05 932409 PL 57 .04 13.10 2.43 

931211 X3 40.04 21 .48 2 .85 932413 PL 7.89 16.67 6.27 

931213 X3 17.72 15.12 0 .88 932415 PL 143 .35 28.90 1 .38 

932420 PL 72 .76 21 .19 7.37 

932423 PL 76 .13 56.61 0.84 

932601 CV 30 .33 13.93 1 .22 

932604 CV 123 .13 25 .17 1 .36 

932605 CV 56 .33 12 .59 0.84 

932606 CV 38 .17 8.62 0.57 

932610 CV 171 .45 88.21 4.81 

932613 CV 36 .97 9 .43 6.73 

102 
,U. All density estimates increased as distance from shore increased. Dry weight densities 

ranged from 49.36-83 .36 mg/m3 for the 153-N.m samples and from 10.66-18 .99 mg/m3 for the 

335-}m samples. Ichthyoplankton density increased from 0.74 to 4.071arvae/m3 . 

xj.. Dry weight estimates decreased with increasing distance from shore. Biomass estimates 

ranged from 30.93-76 .70 mg/m3 for the 153-p.cn fraction and from 8.53-38 .05 mg/m3 for the 335-

.mm samples. Larval fish density varied between 0.54 and 0.941arvaeJm3. 

2.Q. The 153-N.m dry weight density decreased with increasing distance from shore, ranging 

from 33.51-87 .99 mg/m3. The 335-pun biomass estimates ranged from 8.33-31 .23 mg/m3, while 

larval fish densities ranged from 0.31-3.33 larvae/M3. 

2U. Dry weight estimates increased with increasing distance from shore. Biomass densities 

ranged from 17.72-186.16 mg/m3 for the 153-urn samples and from 15.13-30 .49 mg/m3 for the 

335-N.m samples. Larval fish densities ranged from 0.05 to 2.851arvaeJm3 . 

c. Cruise IV, July 1993. One hundred and fifty-six samples were collected from 52 

stations during July 1993 (Table 8), typically a period of high biological activity. Samples were 

collected from the Mississippi Delta (S1) to south of Galveston Bay, Texas (S8), as well as during 

the Plume and Convergence surveys (Figure 90). 

High zooplankton biomass estimates for both the 153- and 335-PM mesh samples were found 

scattered throughout the central and eastern portion of the cruise (S 1-S6; Figures 91 and 92). 

Biomass for the 153-gym samples ranged from 16.19-261.11 mg/m3 (mean=79.72, s.d.=57.13) 

and 335-Nm samples ranged from 0-108.11 mg/m3 (mean=28 .18, s.d.=25.53; Table 10). 

An estimated 2899 larval and juvenile fish were obtained during the July 1993 cruise (Figure 

93). Larval fish densities were highest west of Sabine Lake (S6-S8), ranging from 0-16.85 

larvae/M3 (mean--2.85, s.d .=3.25; Table 10). Again, unidentified engraulids made up the bulk of 

the catch (%Catch = 50.78 %; Table 3). Symphurus spp. (6.21 %), Cynoscion arenarius (5.14 

%), Chloroscombrus chrysurus (4.48 %), Opisthonema oglinum (4.35 %), and Anchoviella 

perfasciata (4.28 %) were also common in July samples. Listed below is a breakdown of the 

cruise by transect. 

,U. Zooplankton biomass decreased with increasing distance from shore. Dry weight 

densities for the 153-pxn samples ranged from 44.83-239.98 mg/m3, while the 335-Nm samples 

ranged from 10.33-44.39 mg/m3. No trend was discernible for larval fish densities, ranging from 

0.30-16 .85 larvae/M3, the cruise maximum. 

52. Dry weight density for the 153-pin samples was higher at near-shore stations, ranging 

from 27.52-171 .49 mg/m3. The 335-Fun biomass estimates ranged from 8.37-90 .70 mg/m3, 

while ichthyoplankton densities ranged from 0.26-1 .67 larvae/M3. 

,U. Cruise maximums were obtained for both the 153- and 335-pun fractions at station 135 . 

Dry weight densities ranged from 24.65-261 .11 mg/m3 for the 153-urn samples and from 0-

108.11 mg/m3 for the 335-N.m fraction. Larval fish densities ranged from 0.06-2.40 larvae/M3. 

SA. No obvious trend was noted for the three density estimates. Zooplankton biomass 

estimates for the 153-dun fraction ranged from 30.16-189.01 mg/m3, while the 335-dun estimates 

ranged from 12.44-88.34 mg/m3. Larval fish densities ranged from 0.10-9.031arvaeJm3 . 

103 
m 

30 

2s 

28 

27 

" - ~ LA 

TX 

v 

S7 S6 ss 6L s3 ~ 

S8 

S2 81 

N = 52 

96 95 94 93 

Longitude 

92 91 90 

Figure 90. LATEX B biological stations during Cruise N, July 1993. 

104 
30 

29 

m 

-a 

.. 

28 

27 

1 

I^ 

" < 30 mg/m3 

30 - 60 mg/m3 

" 61 - 105 mg/m3 

~ > 105 mg/m3 

96 95 94 93 92 91 

Longitude 

Figure 91. Plankton biomass (mg/m3) for 153-gym samples collected during Cruise N. 

105 

90 
30 

29 

v 

28 

27 

LA 

TX 

< 10 mg/m3 

10 - 20 mg/m3 

" 21 - 30 mg/m3 

> 30 mg/m3 

96 95 94 93 

Longitude 

92 91 90 

Figure 92. Plankton biomass (mg/m3) for 335-dun mesh samples collected during Cruise N. 

106 
30 

29 

m 

.. 

28 

27 

v L/'1 

1 x 

e 

< 1 .5 mg/m3 

1 .5 - 3.0 mg/m3 

" 3.1 - 4.5 mg/m3 

~ > 4.5 mg/m3 

96 95 94 93 92 91 

Longitude 

Figure 93. Total larval fish density (#/m3) for Cruise N zooplankton samples. 

107 

90 
,U. Again, no trend was observed for the density estimates. The 153-Em fraction ranged from 

50.99-105.56 mg/m3 and the 335-jixn samples ranged from 11 .77-39 .96 mg/m3. Larval fish 

densities ranged from 0.74-4.081arvae/m3. 

&. The 153-pin biomass estimates were highest in the central portion of the transect, ranging 

from 39.90-176.22 mg/m3. The 335-tan samples tended to increase with increasing distance from 

shore, ranging from 14.85-79 .38 mg/m3. Ichthyoplankton densities were fairly high throughout 

the transect and ranged from 2.85-8.52 larvae/M3. 

,U. Density estimates for the 153-Nrn fraction and for fish larvae increased with distance from 

shore. The 153-p.m biomass increased from 16.19-45.87 mg/m3, while the 335-tun fraction 

ranged from 8.15-19.10 mg/m3. Larval fish densities ranged from 0.82-6.691arvaelm3. 

a. The 153-N.m densities increased offshore, ranging from 28.00-80.44 mg/m3. Density 

estimates were highest at the central station for both the 335-jixn samples and fish larvae . Dry 

weight densities ranged from 10.09-23 .94 mg/m3 for the 335-tan samples, while larval fish 

densities ranged from 3.03-10 .27 larvae/M3. 

108 
IV. FIELD MEASUREMENTS OF BOTTOM BOUNDARY LAYER 

PROCESSES AND SEDIMENT SUSPENSION 

ON THE LOUISIANA INNER CONTINENTAL SHELF 

L.D. Wright, R W. Sternberg, and C. Sherwood 

As a small component of the LATEX-B study, we obtained direct field measurements of nearbottom flows, bed stresses, and sediment transporting processes by means of an instrumented 

tetrapod system deployed at a depth of 15.5 m in spring of 1992 and 20.5 m in spring and summer 

of 1993. In all three deployments, the tenapod supported five Marsh McBimey electromagnetic 

current meters (EMCM's; 3.8-cm probe diameter) situated within the lower 100-113 cm of the 

water column; two Paroscientific pressure sensors; a flux-gate compass; an array of 5 Downing 

optical backscatterance (OBS) turbidity sensors, located within the lower 100-113 cm of the water 

column; and, a Datasonics digital sonar altimeter (DSA; 300 kHz) to measure changes in bed 

elevation. All sensors were interfaced to Onset data loggers. Table 11 summarizes the information 

concerning sensor elevations, burst duration, sampling frequency, burst interval and total length of 

"good data" records for each deployment 

Consistent with the results of others, our measurements show that near-bed flows were very 

weak under non-storm conditions. Bed stresses were typically too low to resuspend bed 

sediments. However, the advection of high-turbidity layers or plumes past the instrumentation 

apparently caused a sustained period of high suspended sediment concentration throughout the log 

layer in spring of 1993. In the absence of wave activity or high suspended-sediment 

concentrations, boundary layer profiles showed the bed to have been hydraulically very smooth 

with zo < 0.1 cm. However, wave agitation, combined with increased suspended sediment 

concentration caused hydraulically rough conditions with zo >_1.0 cm. 

Time series of smoothed near-bottom current vectors measured during the spring 1993 

deployment are shown in Figure 94; notably, the near-bottom flows set persistently to the northerly 

quadrants throughout the period . Figure 95 summarizes the mean-current and bottom orbital 

speeds for the same period. An episode of moderate wave activity produced orbital speeds that 

remained above 10 cm s-1 over a 30-hour period centered around hour 216. Examples of 

logarithmic near-bottom velocity profiles obtained during the spring 1993 deployment are shown in 

Figure 96. During the early phase of the record, when wave activity was negligible, hydraulic 

roughness was extremely low with zo values less than 0.01 (drag coefficients, CD(100) 

2 x 10-3). However, with the onset of moderate wave activity, zo increased by up to two orders 

of magnitude to over 1 cm episodically . Figure 96b shows one hour of high value for zo; 

corresponding CD(100) - 7 - 10 x 10-3). In summer of 1993, zo remained < 0.01 cm. 

No local resuspensions were apparent during the spring 1992 or summer 1993 deployments. 

However, the moderate wave event of spring 1993 initiated a prolonged period of high suspended 

sediment concentration as shown in Figure 97. Following the onset of the moderate wave energy 

"event," which began at hour 216 (Figure 95), concentrations increased dramatically at z.=23 cm 

and continued to increase after the event subsided and skin friction stresses dropped below 0.05 

Pa. Eventually, the high turbidity spread throughout the lower 114 cm of the water column. 

Notably, the time of maximum concentration, centered around hour 504 (Figure 97), coincided 

with a time of minimal bed stress. Additionally, the peak concentrations were similar at all 

elevations . This suggests that the high turbidities were more likely related to a wide-spread turbid 

water mass adverted past the tetrapod rather than to local resuspension . 

Our data are limited to normal faiiweather or non-storm conditions. For such conditions we 

conclude: (1) benthic flows over the inner shelf are weak; 

(2) bed stresses induced by mean currents are too weak to suspend sediments; 

109 
Table 11. Data summary. 

Spring 1992 28'56.29 N 90'28.81 W April 22-30-1992 

Elevation Samples 

b b d b t 

Sampling 

j I 

Bursts Number 

of bursts 

Total 

Senso Duration 

635 

r 

press 

a ove e p 

250 cm 

er urs 

2048 

pLBpC~ e 

0.5 sec 

very 

2 hr 37 72 hr 

626 veil 2 2048 0.5 2 35 68 

vel2 33 2048 0.5 2 35 68 

vela 63 2048 0.5 2 35 68 

vel4 93 2048 0.5 2 35 68 

OBS C1 2 2048 0.5 2 41 80 

C2 35 2048 0.5 2 41 80 

C3 60 2048 0.5 2 41 80 

C4 93 2048 0.5 2 41 80 

C5 118 2048 0.5 2 41 80 

Spring 1993 28'56.22 N 90'27.72 W May16-June 15, 1993 

635 press 245cm 4096 0.2 sec 4 hr 180 716 hr 

626 vel 53 4096 0.2 4 180 716 

626 press 245 1024 1 .0 4 180 716 

626 vel1 18 1024 1 .0 4 180 716 

ve12 48 1024 1 .0 4 180 716 

vela 78 1024 1 .0 4 180 716 

vel4 113 1024 1 .0 4 180 716 

OBS C1 23 1024 1 .0 4 180 716 

C2 49 1024 1 .0 4 180 716 

C3 49 1024 1 .0 4 180 716 

C4 49 1024 1 .0 4 180 716 

C5 114 1024 1 .0 4 180 716 

Summer 199 3 28'52.20 N 90'27.72 W May16-J une 15, 1993 

635 press 228cm 4096 1 .0 sec 4 hr 53 208 hr 

626 vel 65 4096 1 .0 4 53 208 

626 press 228 2048 0 .5 4 53 208 

626 veil 4 2048 0 .5 4 53 208 

ve12 34 2048 0 .5 4 53 208 

vela 64 2048 0.5 4 53 208 

ve14 100 2048 0.5 4 53 208 

OBS C1 5 2048 1 .0 4 53 208 

C2 34 2048 1 .0 4 53 208 

C3 64 2048 1 .0 4 53 208 

C4 100 2048 1 .0 4 53 208 

C5 126 2048 1 .0 4 53 208 

110 
LATEX Spring 1993 

(Smoothed by 4 Point Averaging) 

10 

0 

-1o 

10 

0 

-1o 

lc 

-1 

cm/sec 

10 

0 

-10 

0 72 144 216 288 360 432 504 

Hour (from 1800 GMT, 16 May 1993) 

North 

L East 

~o ~m~c~c 

576 648 720 

Elevations above bed 

4 113 cm 

3 78 

2 48 

1 18 

4 

3 

2 

Figure 94. Vector diagram of near-bottom flows measured in spring 1993 at 20.5 m. The series 

has been smoothed over time. 
20 

15 

N 

v 

.a 10 

m m 

a 

5 

0 

LATEX, Spring 1993 ;Depth = 20.5 meters 

Benthic flows (z = 113 cm) 

R.M .S . Orbital speed (Ub) 

Mean current speed {Uc} 

I . . Y = Y 

`5 a 

1 t l 

-i 

1 i ~Y V 

0 72 144 216 288 360 432 504 576 646 12u 

Hour (from 1800 GMT, 16 May,1993) 

Figure 95. Time series of mean current and bottom orbital speeds near the bed at 20.5 m in spring 

1993. 

112 
1000 

100 

v 10 

0 

m' 1 

w 

0.1 

0.01 

Hour 48_, 

~Z'o=0.009 cm 

Hour 52 LATEX, Spring, 1993 

Z'o = 0.003 cm 'Velocity profiles during 

negligible wave activity 

0 5 10 15 20 

Velocity (cm/s) 

1000 

100 

v 10 

0 

m 1 

w 

0.1 

0.01 

LATEX, Spring, 1993 

Velocity profiles during 

moderate wave activity 

B 

Hour 292-, 

=0.88 cm 

Hour 308 

Z'o =0.29 cm 

0 5 10 

Velocity (cm/s) 

15 20 

Figure 96. Near-bottom velocity profiles from the 20.5 m site in spring 1993. 

113 
.. 3~ 

c 

0 

L 

v 

0 

v 

c 

m 

VI 

C 

B. 

0 

0 72 

LATEX, 16 May- 15 June, 1993 

Depth = 20.5 meters 

Suspended sediment z = 49 cm :" 

concentration at 

four elevations v 

y 

' y ~ tt 23 cm r 

77 

.

: . 

v 

z = 114 cm 1 

144 216 288 360 432 504 576 648 720 

Hour (from 1800 GMT, 1 s May,1993) 

Figure 97 . Time series of burst-averaged suspended sediment concentration measured at four 

elevations above the bed at the 20.5 m site in spring 1993. 

114 
(3) stresses induced by combined wave-induced and mean (tidal or wind-driven) boundary 

layer flows probably resuspend sediments at times of moderate wave activity ; 

(4) under low wave activity and in the absence of high near-bed turbidity, the bottom 

boundary layer is hydraulically smooth with z < 0.1 cm; and, 

(5) hydraulic roughness increases by one or more orders of magnitude when wave 

agitation is significant and when near-bed concentrations of sediment are high. 

115 
V. HYPOXIA 

Nancy N. Rabalais 

A. Introduction 

Hypoxia (5 2 mg/1 dissolved oxygen) is a seasonally dominant feature of the inner- to midcontinental shelf of Louisiana, most widespread and severe during June-August, but occurring as 

early as February and as late as October (Rabalais et al. 1991, 1992). Hypoxia forms as a result of 

and is maintained by a combination of physically controlled density structures (mediated by the 

high freshwater inputs of the Mississippi and Atchafalaya Rivers) and biological processes that 

contribute to productive coastal water masses and oxygen depletion of the lower water column. 

The configuration of hypoxic water masses can be related to large-scale circulation patterns, river 

flow, density structure, and biological features (in particular patterns of phytoplankton biomass, 

the flux of surface-produced organic matter to the lower water column, and decomposition of 

organic material). 

B. Methods 

Dissolved oxygen concentrations were recorded electronically by the LUMCON SeaBird CTD 

unit The precision of measurements for the SeaBird oxygen probe is 0.1 ml/l. The upper and 

lower dissolved oxygen limits of the probe were determined by the LiJMCON Marine Technician 

prior to each cruise and incorporated into the configuration files for the SeaBird software . 

Calibration of the oxygen probe was accomplished by Winkler titrations on water collected in 5-1 

Niskin bottles from the CTDhosette across the range of values observed. Replicate titrations from 

replicate 300-m1 BOD bottles were made on a Mettler DL21 automatic oxygen titrator . Mean 

chemically-derived values were compared to the processed SeaBird oxygen data for the depth at 

which the water sample was collected. A regression equation was developed by which the SeaBird 

oxygen data were corrected or not, depending on the goodness of fit of the regression equation and 

its deviation from a 1 :1 relationship . 

A protocol was developed for extracting usable oxygen data from the SeaBird CTD unit This 

methodology was used to delete oxygen data from the upper parts of CTD casts where the oxygen 

probe had not stabilized adequately prior to the downcast, where the oxygen probe had not 

stabilized adequately prior to the collection of a near-bottom water sample, and where the oxygen 

data for the entire cast were suspect. The remaining oxygen data were processed by the same 

protocols for the other SeaBird parameters. Near-bottom oxygen values for the SeaBird CTD unit 

are those recorded at the end of the downcast, after the oxygen probe stabilized, and just prior to 

the tripping of Niskin bottles or just prior to the beginning of the upcast These values are usually 

1-1.5 m above the bottom, but may be as much as 2-3 m above the seabed in rough sea conditions. 

A Hydrolab Surveyor 3 mufti-parameter CTD unit was used on Cruise N in July 1993 to 

obtain dissolved oxygen measurements closer to the seabed than available from the SeaBird CTD 

unit The Hydrolab was deployed on every station, unless the depth exceeded the capabilities of 

the 50-m cable. Hydrolab readings were made at the bottom, for 0.5-m increments to a depth of 2 

m above the bottom, and at the surface. The precision of measurements for the Hydrolab oxygen 

probe is 0.2 mg/1. The Hydrolab oxygen probe is calibrated before and after each cruise . 

Comparative hydrographic data were available from the NOAH NECOP Hypolcia Monitoring 

and Related Process Studies of Rabalais, Turner and Wiseman (e.g ., Rabalais et al . 1992, 1994). 

These data were collected with a Hydrolab Survey 3. Transect C of Rabalais et al. parallels the 

nearshore end of the LATEX-B S 1 line, then turns to the southeast Similar station depths are 

occupied along the two transects and provide suitable data to examine temporal and/or small-scale 

117 
spatial variability. LATEX-B stations 505 and 104 were comparable in location and depth to 

station C6B of the NOAH NECOP study. 

C. Results 

1. Cruise III, April 1993 

A broad range of dissolved oxygen concentrations (1 .9-10.2 mg/1) was obtained for the 

development of a regression equation of CTD oxygen values versus Winkler titrations. The 

relationship (data not shown) was strong with an intercept near zero (y = -0.143 + 1 .017x, r2 = 

0.995). The SeaBird C'TD data were not corrected for April 1993. 

Near-bottom waters were generally well-aerated with no values falling below 2 mgll dissolved 

oxygen, the upper limit for hypoxic waters (Figure 98). Near-bottom waters low in oxygen, but 

not hypoxic (2.0 < x _< 4.0 mg/1) were located along the S1, S2 and S3 lines, from offshore 

Terrebonne Bay to offshore Atchafalaya Bay. Lower oxygen concentrations (< 4.0 mg/1) were 

present along most of the length of S 1 in the lowest 2-3 m of the water column (Figure 99). 

Similar low values were present on the S2 line between 13 and 26 m on the outer end of the 

transect (Figure 99). 

Several distinct patches of hypoxic bottom waters were observed during the NOAH NECOP 

cruise (April 26-30, 1993) that were not evident during the LATEX B cruise the previous week 

(April 12-22, 1993). The distribution of a low oxygen area off Terrebonne Bay (NECOP cruise) 

was consistent with the lower concentrations of dissolved oxygen along S 1 in mid-April (LATEX 

B). 

2. Cruise IV, July 1993 

A broad range of dissolved oxygen concentrations (0.1-6 .8 mg/1) was obtained for the 

development of a regression equation of CTD oxygen values versus Winkler titrations . The 

relationship (data not shown) was strong with an intercept near zero (y = -0.019 + 0.940x, r2 = 

0.994). The SeaBird CTD data were not corrected for July 1993. 

There were extensive areas of near-bottom and bottom water hypoxia during Cruise N in July 

1993 (Figure 100). Hypoxia extended from the transects off Terrebonne and Timbalier Bays on 

the east to the western end of the study area off Freeport, Texas (95.5°W). The importance of 

obtaining oxygen concentration measurements as close to the bottom as possible, especially during 

expected conditions of hypoxia, is revealed in the comparative extent of near-bottom water hypoxia 

documented with the SeaBird C"TD and the bottom water measurements made with the Hydrolab 

Surveyor 3 CTD (Figure 101). The areal extent of hypoxia is much greater based on the Hydrolab 

measurements. 

During Cruise IV, the hypoxic water mass was confined close to the seabed (Figures 102 and 

103), except along the inshore end of S1 where values less than 2 mg/1 extended well up into the 

water column. Lower oxygen concentrations, but not hypoxic (2.0 < x S 4.0 mg/1), were located 

well up into the water column along S 1 and S2 (Figure 102), but not along S4 and S6 (Figure 

103). 

The areal extent of hypoxia documented during late July 1993 (NOAH NECOP cruise) was 

twice the size documented over the same station design for the previous eight years (Rabalais et al ., 

1994). During the NECOP cruise (July 24-28),the hypoxic water mass did not extend as far to the 

west as during the previous mid-July LATEX B cruise. Persistent winds from southerly and 

southwesterly directions had displaced much of the surface waters to the east (along with lowered 

118 
29. 

m 29. v 

J 

28. 

28 

27 

Near Bottom Dissolved Oxygen (mg/1) 

LATEX April 1993 

~o 

rP 

109 '0 ~< 

8.0 

o 

1 -f C 

j 4.0 0 v 

~~ 6.0 ~ 

Longitude 

Figure 98 . Distribution of near-bottom dissolved oxygen (mg/1) for Cruise III ; SeaBird CTD data. 

0 
Transect S1 Dissolved Oxygen (mg/L) 

LATEX April, 1993 

Station 

E 

s 

a (D 

D 

Station 

E 

r 

a a) 

D 

t 

Figure 99. Cross-shelf profiles of dissolved oxygen (mg/1) for the S 1 line (upper 

panel) and S2 line (lower panel) in April 1993, Cruise IH. 

120 

0 10 20 30 40 50 60 

Distance (km) 

Transect S2 Dissolved Oxygen (mg/L) 

LATEX April, 1993 

0 5 10 15 20 25 30 35 40 45 50 

Distance (km) 
29.5 -

m 

-p 

29.0 -

J 

110 ~ 

28 .5 

3: 1 110j 

o zo ,~ o 

If D1,/ 

a 

D 

0, 

,\ 10 ~le\3.0 01 ',, v 

O 

o J> 

-yb .b -yb.U -y4 .b -94 .0 -93.5 -93.0 -92 .5 -92 .0 -91 .5 -91 .0 

Longitude 

-90.5 -90 .0 

Figure 100. Distribution of near-bottom dissolved oxygen (mg/1) for Cruise N; 

Hydrolab's Surveyor 3 data. 
Hydrolab Bottom Dissolved Oxygen 

LATEX July 1993 

0 

_j 29 .0- 

O 

i -95 .5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -92 .5 -92 .0 91 .5 -91 .0 -90 .5 -90 .0 

Longitude 

CTD Bottom Dissolved Oxygen 

LATEX July 1993 

m 

J 

~--~ 

p --- 

1 

~~% :.l:- .. 

-, 

-95 .5 -95 .0 -94 .5 -94 .0 -93 .5 -93 .0 -92.5 -92 .0 -91 .5 -91 .0 -90 .5 .9C 

Longitude 

Figure 101 . Extent of near-bottom water hypoxia (stippled area) documented with 

the SeaBird CTD (upper panel) and the bottom water measurements 

made with the Hydrolab Surveyor 3 CTD (lower panel) . 

122 
Transect S1 Dissolved Oxygen 

LATEX July,1993 

Station 

E 

r 

n 

m 

D 

E 

z 

n 

N 

501 502 503 1504 505 i 104 1 105 1 106 107 1 ~ 708 

-_ 

Distance (km) 

110 111 

50 55 

Figure 102 . Cross-shelf profiles of dissolved oxygen (mg/1) for the S 1 line (upper 

panel) and S2 line (lower panel) in July 1993, Cruise N. 

123 

5 10 15 20 25 30 35 40 45 

Transect S2 Dissolved Oxygen (mg/I) 

LATEX July, 1993 

Station 

124 123 122 121 120 119 t,e 117 116 117 
Transect S4 Dissolved Oxygen (mg/L) 

LATEX July, 1993 

Station 

144 145 146 14 149 150 1151 752 

_7---- 

-~_ so ' eo 

-10- Zo 3 

..0 

40---~ 

D l 

-2o0 10 20 30 40 50 

Distance (km) 

Transect S6 Dissolved Oxygen (mg/L) 

LATEX July, 1993 

Station 

170 171 172 173 77 175 

o 

E 6,p . 6.0 

a 

~~ 

-1 3.0 0 

O S.p\ 

0.00 10.04 20.00 30 .00 40 .00 

Distance (km) 

1 

4.0 

so 

154 

176 , 177 

T f 

- o I 

f 

~~ a o 

I~O~ 

50.00 60.00 

Figure 103 . Cross-shelf profiles of dissolved oxygen (mg/1) for the S4 line (upper 

panel) and S6 line (lower panel) in July 1993, Cruise N. 

124 
salinities and higher nutrients), as well as some of the bottom water hypoxia (Rabalais et al., 

1994). While hypoxia was extensive during Cruise III, it was located near the seabed and was 

somewhat patchy in distribution. One week later during the NECOP cruise, hypoxia had become 

well-developed and was present in much of the lower water column, was severe (well below 0.5 

mg/1 at many stations), and was a continuous water mass. As freshwater inflow and nutrient flux 

to the coastal system from flooding of the Mississippi River extended well into August and 

September, the persistence, severity, and extent of bottom water hypoxia continued as well 

(Rabalais et al ., 1994). The progression of hypoxia development is illustrated in Figure 104. 

125 
7/13/93 7114/93 Temperature (deg . C) Temperature (deg . C) 

25 26 27 78 29 M 91 32 25 26 27 211 29 30 31 92 

p D 

02 --0 -Safwily 0 

'S o t -5 

'b 06 o* 

o0 

N 

-to ~ ~ _o 

00 

o 

o 

o 

o 

0 

~ Salinity (ppt) 

10 ~ ,S 2n 25 30 35 

.p 

0 1 2 9 4 S 6 ) B 

C\ssd%.ed Oxygen (mg/L) 

7/25/93 

Temperature (deg . C) 

25 26 zr 28 - 29 30 31 32 0 

-5 

E 

L -10 

a 

" IS 

4p 

Dissdked Oxygen (mcyL) 

E 

L ~10 

-15 

to 

'Zp I ~T~ 

15 M 25 M 35 

2

3 4 

5 'T~T7 

Dissdu."c1 Oxygen (mcyL) 

7/30/93 

Temperature (deg . C) 

25 26 27 20 r) M 31 32 

0 

.5 

E 

~10 

o. 

-15 

.p 

Oissohed Oxygen (myl) 

Figure 104. CTD profiles from Station C6B during LATEX-B Cruise N (July 13-14, 1993) and 

during the NECOP cruise (July 25 and 30, 1993) . 
VI. REMOTE SENSING AND IMAGING 

Nan D. Walker 

A. Methodology and Image Analysis Synthesis 

NOAH Advanced Very High Resolution Radiometer (AVHRR) satellite date coverage of the 

Gulf of Mexico is acquired several times each day at the Earth Scan Lab, Coastal studies Institute, 

Louisiana State University . These data are captured and archived suing a SUN Sparc II computer 

running Terascan [TM] software from Seaspace, San Diego, California. After capture, the thermal 

and visible channels are calibrated from count values to science units. The thermal infrared 

channels are converted to brightness temperatures in Celsius, and the visible channels are calibrated 

to percent albedo (Kidwell, 1991). The Gulf of Mexico portion of the image is automatically cut 

out of the raw data and placed on the Earth Scan Lab's newest computer, the SGI Crimson server 

with 85 mips and 144 mb RAM. All further processing and analysis of the satellite data is 

performed on this computer, accessed by Tektronix X-11 terminals. The data is screened for cloud 

cover and general image quality. A determination is made at this time whether further processing is 

to be performed. If an image is sufficiently free of cloud cover over an area of interest, the dataset 

is run through additional processing procedures, which include calculation of sea surface 

temperature (SST), calculation of reflectance (to reveal surface suspended sediment), and 

navigation/registration of the image to a standard rectangular map projection. Sea surface 

temperatures are computed using the mold-channel algorithms of McClain et al . (1985) . 

Reflectance is obtained from the bias correction technique of Stumpf (1988, 1992). Estimates of 

suspended sediment concentrations are sometimes made using a calibration equation developed 

specifically for Mississippi River plume waters (Walker and Rouse, 1993). The satellite data 

processing procedures are summarized in Figure 105. Frontal analyses are routinely performed on 

the satellite usage data with the assistance of the sobel filtering method and digitizing software, 

available within the Terascan image processing package. Information on coastal and river plume 

fronts, shelf fronts, squirts and jets, Loop eddies, and the Loop Current were digitized from the 

satellite image data. 

Calibration of V'kible and Thermal Infrared Channel Data 

from count values to science units 

Initiial Screening of the Data for Cloud Cover 

Calculation of Reflectance 

Calculation of Sea Surface Temperatures 

Navigation/Registration ofd Images to Map Projection 

Frontal Analyses 

Production of GIF images 

Archival of products 

(4mm tape, 35 mm slide film, and color ptirds) 

Figure 105. Image processing procedure summary for NOAH AVHRR data. 

Further processing of the frontal information for provision to the other LATEX tasks via 

GULF.MEX was performed on a PC using the OPCPLOT software (Dr. Murray Brown, pers. 

comm.) . GIF images were routinely provided (corresponding to the GULF.MEX frontal 

127 
analyses) covering either the LATEX area or the entire Gulf of Mexico. They are made available 

via anonymous ftp on "nevado.srcc.lsu .edu." In support of the LATEX cruises or overflights, 

more detailed information was provided in either GIF image format, as line drawings (faxable) or 

as color prints. In 1993, 55 frontal analyses were posted to GULF.MEX (Table 12) and GIF 

images corresponding to these analyses were made available via anonymous ftp. In addition, 

support was provided for the LATEX B April 1993 and July 1993 cruises. 

An additional component of the satellite data analysis is to analyze a representative sample of 

the historical NOAH AVHRR data sets within the Earth Scan Lab archive, which begins in July 

1988. During 1992, approximately 2 images/day were uploaded from the archive tapes between 

July 1988 and January 1992, totaling 3000 images. The images were screened for image quality 

and the best images were processed for further analysis by the investigators. In the next section of 

this report analysis of historical data, as well as data collected during the LATEX program, is 

presented. 

B. Historical Analysis: Annual and Interannual Variability of Surface 

Temperatures and Fronts 

1. Introduction 

Surface temperature structure across the Louisiana and Texas continental shelves varies 

considerably throughout the year. During winter, surface temperature variability is maximized as 

winter storms move over the northern Gulf of Mexico and remove heat from shelf water masses. 

Inner shelf waters cool most rapidly as a result of their depth-limited heat storage capacity (Huh et 

al ., 1984). This results in a pronounced temperature structure across the shelf with coolest waters 

close to the coast and warmest waters offshore . In contrast, during summer, almost no surface 

temperature structure is discernible over the continental shelves of Louisiana and Texas. An 

exception is the southeast Texas coastline where relatively cool upwelled waters contrast in 

temperature to ambient Gulf of Mexico waters during the summer months. 

In this section, the seasonal and interannual variabilities of surface temperatures and surface 

temperature fronts will be discussed using satellite image data for three years; 1989/90, 1992193 

and 1993/4. These three years were chosen as they demonstrate the range of meteorological 

conditions experienced by the continental shelf of the northwestern Gulf of Mexico. 

Climatological air temperature information from Lake Charles, Louisiana, coinciding with the 

satellite image archive within the Earth Scan Laboratory (beginning in June 1988) was used to 

select the years for investigation Figure 106 illustrates the monthly mean air temperatures from 

Lake Charles from June 1988 through June 1994, plotted against the 30-year (1961-1990) 

climatological monthly means. These data weal that the 1989/90 winter season was much colder 

than average. The most anomalous air temperatures were recorded in December 1989 when the 

monthly mean was about 5° C lower than the climatological mean. The 1992/93 winter season was 

warmer than normal, and the 1993/94 winter season was near normal. 

For this study, surface temperatures were extracted from 5 profile lines corresponding to the 

LATEX-A mooring lines (Locations, Figure 107). The year is defined here as beginning in 

September and ending in August to focus on the winter season and to facilitate the investigation of 

the autumn cooling and spring warming on the shelf. An attempt was made to locate at least two 

cloud-free images across the entire LATEX region for each month 

It is relevant to point out at this stage that although the satellite radiometer senses the ocean's 

skin temperature, this measurement is generally representative of the surface muted layer, 

particularly in the presence of wind and/or at night In addition, several comparisons of LATEXB cruise data with contemporaneous satellite frontal information have demonstrated that the 

satellite-observed fronts were indicative of boundaries between water masses of different densities . 

128 
Table 12. Frontal Analyses and GIF files provided to LATEX personnel in 1993. 

1 Jan 93 LATEX fronts/ squirt 

9 Jan 93 LATEX fronts/ squirt 

16 Jan 93 LATEX fronts/Eddy V 

17 Jan 93 Eddy U/ Loop Current 

26 Jan 93 LATEX fronts/ squirt 

31 Jan 93 LATEX fronts 

8 Feb 93 LATEX fronts 

12 Feb 93 LATEX fronts/ Eddy V and W 

21 Feb 93 LATEX fronts/ Loop Current 

23 Feb 93 Eddy V/ Loop Current 

4 Mar 93 LATEX fronts/ Eddy V and squirt/ Loop 

7 Mar 93 LATEX fronts/ Eddy V and squirt/ Loop 

9 Mar 93 LATEX fronts/ Eddy V/ cyclonic eddy/ Loop 

14 Mar 93 LATEX fronts/ squirt 

22 Mar 93 Eddy V 

25 Mar 93 Mississipppi plume front 

27 Mar 93 Eddy V/ Loop Current 

1 Apr 93 Eddy V/ Mississippi plume/ Loop Current 

10 Apr 93 Eddy V/ cyclonic eddy/ Loop Current 

18 App 93 Eddy V / Loop Current 

3 May 1993 Eddy V/ squint Loop Current 

21 May 1993 Loop Current 

23 May 1993 Loop Current and filaments 

30 May 1993 Loop Current 

31 May 1993 Formation of Eddy W 

6 June 1993 Eddy W 

8 June 1993 Eddy W 

24 June 1993 Eddy W 

25 June 1993 Eddy W / Loop Current 

7 July 1993 Texas upwelling/ River plume fronts 

28 July 1993 LATEX fronts/ Loop Current 

1 August 1993 Eddy V and W/ River plumes 

5 August 1993 Eddy W/ Texas upwelling 

10 August 1993 River plumes 

12 August 1993 Eddy W 

17 August 1993 LATEX fronts/ Gyclonic eddy 

18 August 1993 Texas upwelling 

22 August 1993 Loop Current 

29 August 1993 Loop Current and Eddy X 

2 September 1993 Texas upwelling 

21 September 1993 LATEX fronts 

22 September 1993 Loop Current 

29 September 1993 LATEX fronts/Eddy X 

5 October 1993 LATEX fronts/ Eddy X 

18 October 1993 Loop Current/Eddy X 

1 November 1993 LATEX fronts 

7 November 1993 LATEX fronts 

11 November 1993 LATEX fronts/ Eddy W/Eddy X 

14 November 1993 Loop Current 

17 November 1993 LATEX fronts/ Eddy W/ squirt 

21 November 1993 LATEX fronts 

28 November 1993 LATEX fronts/Eddy V/W and X/ Loop 

5 December 1993 LATEX fronts/ Eddy X/ squirt/ Loop Current 

15 December 1993 LATEX fronts/ Eddy V/W and squirt 

20 December 1993 Loop Current 

129 
w 0 

Lake Charles Air Temperatures 

30 

25 

U 

20 

~. 

~ 15 

H 

10 

Y 

5 

Jul-88 Jul-89 Jul-90 Jul-91 Jul-92 Jul-93 Jul-94 

Time (months) 

- Monthly mean Long-term mean 

Figure 106. Monthly mean air temperatures compared with long-term monthly mean air 

temperatures recorded at Lake Charles, Louisiana, from July 1988 through June 

1994. 
Figure 107. Map depicting the location of LATEX A mooring lines along which SSTs were 

extracted in this study. 

131 
Comparisons of the ADCP near-surface currents obtained during the October 1992 LATEX-B 

cruise indicate that the main SST front may mark the outer boundary of the coastal current. The 

strongest currents within the westward-flowing Louisiana coastal current were found in close 

proximity and shoreward of the strongest SST front on the LATEX shelf (Walker et al. in Murray, 

1994) . 

2. Shelf Temperature Changes, Frontal Characteristics and Circulation 

Observations during 1989-90 

The episodically cold winter of 1989-90 was investigated to reveal surface temperature 

structure across the shelf, frontal intensities and locations, as well as associated circulation 

patterns. Although SST information was extracted from 23 images during the 1989-90 year, only 

the most informative SST profiles will be referred to in this discussion. For the September 

through December 1989 period, SST information, extracted from five images, is presented for 

profile lines A and D (Figure 108a,b) . The image dates shown are September 18 (1900 LTTC), 

October 20 (2000 UTC), November 10 (2000 UTC), December 14 (0700 LJ'TC), and December 26 

(2000 iJTC). 

The September 18, 1989, image data revealed very little temperature structure as SSTs were 

27° to 28° C across the entire LATEX region, a situation typical of summer. The first significant 

cold-air outbreak of the autumn season moved southeastwards over the study region on October 18 

when coastal air temperatures measured at Grand Isle dropped from 24' C on October 18° to 8° C 

on October 20. SST information extracted from the October 20 image demonstrated that 

substantial cooling of continental shelf waters resulted from passage of this winter storm. Cooling 

across the inner shelf ranged from 4' C to 9' C, whereas cooling of 2° C to 3' C was observed 

across the outer portions of the shelf. The greatest temperature changes were observed on the 

inner shelf seaward of Atchafalaya Bay where river water and cool bay water were forced onto the 

shelf by westerly and northwesterly (offshore) winds during this winter storm. SST gradients 

appeared across the shelf as the deeper offshore waters chilled less rapidly than shallower waters. 

On October 20, relatively weak SST fronts were observed on profile lines A, B, and D. Along line 

A, an SST front of 3' C over 20 km was observed mid-shelf (Figure 108). Along line B, an SST 

front of 1 ° C over 5 km was observed about 40 km offshore (not shown). The frontal intensities 

were greatest along line D whew Atchafalaya River and Atchafalaya Bay waters had been 

discharged onto the shelf (Figure 3b). Multiple fronts were observed along line D approximately 

15, 35, and SS km from the coast (see F1, F2, F3, on Figure 108). The inner shelf front was the 

strongest, approximately 3' C over 81an. 

Between October 20 and November 10, only one weak winter storm passed over the area and, 

as a result, little temperature change was observed over the continental shelf. Although coastal 

waters were 1' C to 4° C cooler than offshore waters, no distinct SST fronts were observed, 

indicating that the fronts had weakened between October 20 to November 10. 

Six winter cold air outbreaks were experienced between the November 10 and the December 14 

images. The temperature changes that resulted were impressive. The inner shelf waters responded 

most rapidly with temperature changes ranging from 6° C on lines A and B (in the west) to 8 to 10° 

C on lines C, D, and E (in the east) . The lowest shelf temperatures (8° C) were again observed 

seaward of Atchafalaya Bay (Figure 108). On line D, multiple fronts wee again observed. Near 

the coast, an SST front of 3° C over 6 km was observed (Figure 108, see F4). SST fronts were 

also observed 35 km and 80 km from the coast (Figure 3, See FS and F6). Along line E, a front of 

3° C in 10 km was observed near the coast and a sharper front (2° C in 4 km) was observed about 

80 km from the coast (not shown). Along the other lines, surface temperatures increased gradually 

in an offshore direction with no major SST gradient features. 

132 
Profile A 

30 

25 

-20 

F- 

~' 15 

10 

5 

`A 

- c 

D~ 

E 

0 20 40 60 80 100 120 140 

Distance (km) 

A 9/18/89 B 10/20/89 C 11/10/89 D 12/14/89 E 12/26/89 

Profile D 

30 

25 

v 20 

U' 15 

10 

5 

0 

A "V 

- 

-------------- - F2 - ---- 

-B~ ;' 

_, ra 

v 

I D 1~F 

F4 

E 7 ~ i 

20 40 60 80 100 120 140 

Distance (km) 

A 9/18/89 B 10/20/89 C 11/10/89 D 12/14/89 E (2/26/89 

Figure 108. SST information extracted from satellite image data for line A (upper 

panel) and line D (lower panel) between September 1989 and 

December 1989. F1-F6 are fronts referred to in the text 

133 
Between December 14 and December 26, an episodic winter storm moved through the study 

region. Air temperatures at Grand Isle fell below freezing from December 22 through December 

25 (Figure 109) and accompanying offshore northwesterly winds exceeded 10 m/s for 2 days. 

Along lines A and B, cooling from this cold air outbreak was similar to that of the six minor winter 

storms which occurred between November 10 and December 14. Along the inner shelf of south 

Texas, cooling of 5° C to 6° C was experienced. (Figure 108). In contrast, comparatively little 

temperature change occurred offshore. Distinct SST fronts were observed after this winter storm. 

Along line A (Figure 3a), two SST fronts were observed within 40 km of the coast The first front 

measured 3° C in 15 km and the second stronger front was 6' C in 20 km. . Temperatures seaward 

of Atchafalaya Bay fell below 6° C (Figure 108b), and inner shelf waters along line C were below 

10° C 60 km from the coast (not shown). During this event, ice several inches thick formed in 

shallow bays and in nearshore coastal areas of Louisiana (Edward Weeks, pens. comm.) . The midshelf temperature fronts previously observed on December 14 along line D (Figure 108, F5, F6) 

increased in strength and moved seaward between December 14 and 26. Analysis of the usage 

data reveals that line D was located west of the strongest SST front associated with the Atchafalaya 

plume. The strong northwesterly winds had forced the coldest portion of the plume towards the 

southeast where an SST front of 14° C over 20 km was observed (not shown). 

Three minor winter storms occurred between the December 26 and January 10 images. It is of 

interest that the inner shelf waters of line A experienced further cooling, whereas the inner shelf 

waters of C, D, and E warmed considerably over the two week period between images (Figure 

110). The most likely explanation is that the cooler water masses on the inner along lines A and B 

resulted from advection of cooler water from the northeast, transported by the Louisiana-Texas 

coastal current This seems the most likely source of cold water since atmospheric forcing was 

relatively weak and, therefore, local heat loss could not have explained the cooling which took 

place on the inner shelf. On January 10 along line A, a very sharp SST front (6° C over a few km) 

was observed 35 km from the coast (Figure 110), replacing the somewhat weaker multiple front 

structure observed on December 26. Along line D (Figure 110b), the inner shelf waters had 

warmed by 2° C to 3' C, while the mid-shelf waters remained essentially unchanged. In contrast, 

the outer shelf waters (80-140 km) had cooled substantially (3° C to 4° C). The profile data 

extracted from line D also show that the SST fronts were located further seaward on January 10. 

To investigate changes in frontal structure across the entire LATEX shelf between December 26 

and January 10, the maul SST fronts were digitized and are displayed in figure 111 . The outer 

Atchafalaya plume front moved seaward as much as 35 km over the 2 week period . Further west, 

the wavy front observed south of Galveston on December 26 had become further deformed such 

that several "mini-squirts" were observed with length scales of 35 km and separation distances of 

50-70krn. Further south, an elongated cool feature was observed extending seaward to the 200 m 

isobath. This region is a preferential area for squirt formation as it is influenced by strong currents 

associated with warm core eddies that often reside in close proximity to the edge of the shelf. 

Between January 10 and February 23, a warming trend was observed over most of the 

continental shelf. Temperature change was most rapid in shallower regions of the shelf, as would 

be expected. Warming of the inner shelf was most rapid on line A (furthest south) where 10° C 

water on January 10 was replaced by 16' C water by February 23. In addition, the strong SST 

fronts observed between December 26 and January 26 had disappeared Subsequently, warming 

of the shelf waters continued and by April 20 the shelf was nearly isothermal with temperatures 

across the shelf near 22° C. Between April 20 and August 5, surface temperatures increased by 

about 10° C on all parts of the shelf, which remained virtually isothermal. 

The analysis of temperature data during the 1989/90 year has shown that the inner shelf waters 

of the LATEX region cool rapidly in response to the passage of winter storms. The differential 

cooling of shelf waters (as a result of depth differences) results in a transition from coldest waters 

near the coast to warmer waters offshore. The lowest temperatures were observed seaward of 

134 
w 

30 

25 

20 

15 

U `r 

10 

CV 

a 5 

u E-+ 

-5 

-10 

-15 

12/01 12/11 12/21 12/31 01/10 01/20 01/30 

Days 

Figure 109. Hourly air temperatures recorded at Grand Isle, Louisiana, during December 1989 

and January 1990. The astericks depict times of satellite image acquisition, 

discussed in the text. 
Profile A 

30 

25 

U f . . . . . . ... ... . . . . . . .. . . . . . . . . . . . . . . 2o 

I 

I 

~ 15 I I 

~A I g. / 

10 

5 

0 20 40 60 80 100 120 140 

Distance (km) 

a 12/26/89 8 1/10/90 C 1/26/90 v 2/23/90 E d/20/90 F 8/5/90 

Profile D 

30 

25 

-- . 

------ 

E'-- --------- -- ------------ -- . . . ... . . . . . . ... . .. . . . . . . . ... . ----

a2p 

--- , -- -- -- - - --- - -

------ 

D 

__- 

--- 

--'------- 

r 

C 15 . . . . . . . 

-- ---- 1111 1 

10 

A 5 ~T i 

0 20 40 60 80 100 120 140 

Distance (km) 

A 12/26/89 8 1/10/90 C 1/26/90 D 2/23/90 E 4/20/90 F 8/5/90 I 

Figure 110. SST information extracted from satellite image data for line A (upper 

panel) and line D (lower panel) between December 1989 and August 

1990. 

136 
Figure 111. Schematic depicting the location of major SST fronts in December 26, 1989, and 

January 10, 1990, in relation to the shelf break and the LATEX A mooring lines. 

137 
Atchafalaya Bay where inner shelf waters reached 6° C. Along line A on the south Texas shelf, 

temperatures reached 10° C. Minimum temperatures on the outer shelf were about 20° C. 

Strongest fronts were observed along lines A, B and D. A series of strong fronts was observed 

seaward of Atchafalaya Bay where the cool plume had been forced offshore by northerly winds 

associated with the winter storm episodes. These fronts may be generated by seaward pulsing of 

the cool bay water by consecutive frontal passages. Further west, a continuous SST front was 

observed between lines C and A. The frontal structure in this area was not straight, but rather 

exhibited a wavy structure with "wavelengths" of 30-40 km. After repeated cold front passages 

the undulations grew in the offshore direction measuring approximately 35 km in length with 

wavelengths of about 50 km. A major site for offshelf movement of cool shelf water was noted 

between lines A and B, where long thin squirts were observed extending offshore to the 200 m 

isobath . This site of squirt formation corresponds with the well-known "graveyard" eddy zone in 

the northwest GOM. The anticyclonic motion associated with eddies in this region appear to 

enhance the movement of inner shelf waters previously initiated by winds associated with cold 

front passages. 

The strongest SST fronts were observed subsequent to the most intense winter storms . Under 

conditions of repeated westerly or northwesterly wind forcing, the main SST fronts moved 

seaward. However, relaxation of the wind forcing or a wind shift to the more frequent easterly 

wind regime resulted in frontal repeat towards the coast 

3. Interannual Variability of Temperature Structure and Fronts 

The interannual variability in temperatures and frontal structure was investigated by comparing 

the temperature profile data along lines A and D, which exhibited the lowest inner shelf 

temperatures for the 1989/90, 1992!93 and 1993/94 winters. It is apparent from this analysis that 

temperatures on the inner shelf were abnormally cold during the 1989/90 winter. Along line A 

inner shelf temperatures were 4° C to 5° C lower during 1989/90 for a distance of 30 km (Figure 

112, upper panel) . Along line D, shelf temperatures were several degrees colder out to 80 km 

(Figure 112, lower panel) . The same pattern was not observed on the outer portions of the shelf 

where the temperauu+es during 1989/90 were not necessarily colder than the other years studied. 

There is an indication from this analysis that the SST fronts generated along line A were 

stronger during 1989/90 and located further offshore than during the other years (Figure 112, 

upper panel). This was not, however, obviously the case along line D. In contrast to line A, 

which exhibited one main front, line D exhibited a series of strong multiple fronts during the 

"average" winter of 1993!94, as well as the cold winter of 1989/90 (Figure 112, lower panel). 

4. Frontal Summary Based on 1989-90, 1992-93 and 1993-1994 Satellite Data 

In order to synthesize frontal information, it was necessary to define front For this study, a 

front had to meet the following criteria. Fast, the SST gradient in an offshore direction had to 

exceed 1' G10 km. Secondly, the total temperature change across the front had to exceed 1° C. 

This analysis is based on extracting frontal information from the lines A-E. An attempt was made 

to choose satellite imagery which was clear over the entire LATEX region . If this was not possible, 

imagery as closely spaced in time was used Approximately two images/ month were used in the 

analysis. The total images used varied from a low of 23 in 1989-90 to a high of 30 in 1992-93. 

The average frontal intensity along each line for each year is shown in Table 13. This data 

compilation suggests that the average frontal intensity was somewhat greater on lines AB, and D. 

Line C exhibited the weakest fronts. There was no indication that the average front across the shelf 

changed from year to year, as the averages only varied from 1 .9° C and 2.0' C. Table 14 shows 

the maximum frontal intensities along each line for each year and their averages. The strongest 

SST fronts occur along lines A, B, and D, while the weakest fronts occur along Line °C. Line 

138 
Profile A 

25 

20 - -- " 

B i~ 

r--- -r"1 F 

F: 15 - - 

10 - -

5 

0 20 40 60 80 100 120 140 

Distance (km) 

a 1/10/90 s 2/8/93 C _2/18/94 

Profile D 

25 

20 

U 

F: 15 

10 

54 

0 20 40 60 80 100 120 140 

Distance (km) 

12/26/89 s 1/31/93 c 1/14/94 

Figure 112. Surface temperatures extracted from line A (upper panel) and line D 

(lower panel), which exemplify the lowest temperatures on the inner 

shelf for each of the years I989-90, 1992-93 and 1993-94. 

139 
Table 13. Average frontal intensity (°C) during 1989-90, 1992-93 and 1993-94. 

Average Frontal Intensity (°C) 

Yom[ A ]1 Sc 12 E Mean 

89-90 2.5 2.1 1 .4 2.2 1 .7 2.0 

92-93 2.4 2.0 1 .6 2.2 2 .0 2.0 

93-94 2.1 1 .9 1 .7 2.0 1 .8 1 .9 

Mean 2.3 2.0 1 .6 2.1 1 .8 

Table 14. Maximum frontal intensity (°C) during 1989-90, 1992-93 and 1993-94. 

Maximum Frontal Intensity (°C) 

Yom[ A ]I S Q JE Mean 

89-90 8.3 5.8 2 .5 4 .0 3 .3 4.8 

92-93 5.1 4.8 2 .8 4 .5 4 .0 4.2 

93-94 4.3 5.9 3 .5 5.3 3 .5 4.5 

Mean 5.9 5.5 2.9 4.6 3.6 

E does not really represent the Atchafalaya or Mississippi River plumes because it lies in the zone 

of mixing between them. There was some indication that the maximum frontal intensity was 

correlated with cold-front intensity (at least in terms of air temperature) as the relatively cold 1989-

90 winter exhibited the strongest fronts and the relatively warm 1992-93 winter exhibited the 

weakest fronts. Table 15 gives information on frontal distance from the coast The fronts along 

line A were closest to the coast and exhibited the least variability in location, whereas those along 

line C were furthest from the coast and exhibited the most variability in location. There was no 

obvious relationship between severity of the winter and average frontal location across the entire 

LATEX shelf. 

Table 15. Average frontal distance from the coast (km) during 1989-90, 1992-93 and 1993-94. 

Average Frontal Dista nce from Coast 

Yom[ A a S J2 E Mean 

89-90 45 56 78 60 50 58 

92-93 41 58 54 60 62 55 

93-94 33 58 86 49 41 49 

Mean 40 57 66 56 51 

s(X) 20 29 39 36 34 

140 
A schematic summarizing the average position of shelf fronts along each line and position 

variability (one standard deviation from the mean) are depicted in Figure 113. It is interesting to 

note that although Line A fronts are closest to the coast, they lie in deeper water than those of Lines 

C, D, and E. The mean location of fronts along Line C were furthest from the coast; however, in 

terms of bathymetry the fronts were in the shallower water than those across the rest of the shelf. 

What is noteworthy is that the mean frontal position along Line B is in considerably deeper water 

than that of Line C, perhaps indicating a preferential site for seaward movement in this region, 

either caused by convergences or wind-forced flows. Indeed, the 1989-90 image data showed this 

region to be a preferential site of mini-squirt formation. The fronts along line A may lie closer to 

the coast as a result of the frequent occurrence of anticyclonic eddy activity along the seaward end 

of this line. Warm eddy filaments are often observed spilling onto the shelf in this region, a 

process that would inhibit the offshore movement of coastal waters. However, just north of line A 

where the depth contours curve sharply to the east, a preferential region for squirt formation has 

been observed. This region lies on the northern flank of the detached anticyclonic eddy zone and 

thus eastward currents associated with the north side of the eddy would enhance offshore 

movement of shelf water. 

It is important to note that the standard deviations of the frontal positions are very high for all 

lines except line A. The frequent occurrence of multiple fronts along these lines explains a large 

part of this observed variability. Scattergrams of frontal locations and intensities along each line 

are shown in Figures 114 through 118. 

5. Conclusions 

The analyses of annual and interannual variability of shelf fronts and temperature structure on 

the LATEX shelf have demonstrated the following: 

1 . Shelf fronts result from differential cooling of shelf water caused by winter storms and 

subsequent convergence of water masses of contrasting temperature. 

2. Shelf fronts exceeding 1 ° C are generally observed on the shelf between October and 

March. 

3. The strongest shelf fronts occur from December through February, coinciding with the 

most sever winter storm events. 

4. The frontal locations and intensities across the LATEX shelf are complex and change 

rapidly. 

5. The Atchafalaya plume front responds rapidly to wind forcing. It moves seaward under 

conditions of westerly and northwesterly wind forcing and landward under conditions 

of southeasterly wind forcing. 

6. The strongest SST fronts along the middle portion of the Texas shelf often exhibit a 

wavy character. This region, between lines A and C, has been observed to be a 

preferential region for offshelf flow during winter. 

141 
Figure 113. Summary figure depicting the average position of fronts along lines A through E as 

well as one standard deviation for 1989-90, 1992-93 and 1993-94. 

142 
Profile A 

9 

8 

7 

v6 

a 

`' S 

04 

E~ 3 

2 

1 

0 

0 10 20 30 40 50 60 70 80 90 100 

Distance (km) 

Figure 114. Scattergram showing positions of fronts and their intensities based on 

all data from 1989-90, 1992-93 and 1993-94 for line A. 

143 
9 

8 

7 

U 

~, ~ 5 

¢, 4 

H3 

2 

1 

0 

Profile B 

- - - - - - - - - - - -

:. -----~-- 

-- -- . ; . - : ~ . 

i 0~`~ ~ ~ i t '0 0 

. I, 

0 10 20 30 40 50 60 70 80 90 100 

Distance (km) 

Figure 115 . Scattergram showing positions of fronts and their intensities based on 

all data from 1989-90, 1992-93 and 1993-94 for line B. 

144 
Profile C 

9 

8 

7 

c~ 6 

~5 

10.4 

H3 

2 

1 

0 

0 10 20 30 40 50 60 70 80 90 100 

Distance (km) 

Figure 116. Scattergram showing positions of fronts and their intensities based on 

all data from 1989-9n, 1992-93 and 1993-94 for line C. 

145 
Profile D 

9 

8 

7 

U6 

~5 

4 

H3 

2 

1 

O 

00 

a~-- 4W-" --- 0 

000 

' t 

.-~. 

N 

--000000 !_. ; 0 ~ " 

I ~ 

Op 0 

1 

10 "

0 0 ! 

0 10 20 30 40 50 60 70 80 90 100 

Distance (km) 

Figure 117. Scattergram showing positions of fronts and their intensities based on 

all data from 1989-9n, 1992-93 and 1993-94 for line D. 

146 
Profile E 

9 

8 

7 

U 6 

'~ 5 

°' a 4 

H3 

2 

1 

0 

0 10 20 30 40 50 60 70 80 90 100 

Distance (km) 

Figure 118. Scattergram showing positions of fronts and their intensities based on 

all data from 1989-90, 1992-93 and 1993-94 for line E. 

147 
VII. HIGHER RESOLUTION SURVEY OF THE GALVESTON PLUME AREA 

AND ADJACENT AREA OF CONVERGENCE 

Curtis C. Ebbesmeyer, Eddie Weeks, Walter Johnson, Peter Niiler 

A. Objective 

Plumes exiting the bays, estuaries, and rivers along the coast contribute significant quantities of 

sediments, organic material, and chemicals to the coastal plume of the Louisiana-Texas shelf. To 

investigate these plumes, surveys of the position and characteristics of several plumes were 

planned as a part of the LATEX B program. The Calcasieu Plume was the first of these to be 

surveyed and has been reported in the first annual report (Cox and Ebbesmeyer in Murray, 1994). 

The second to be sampled was that in the approached to the Galveston ship channel. 

Because the Convergence Survey was conducted close to the Plume Survey, and because water 

exiting Galveston Bay appears to feed the convergence zone, analysis of the two surveys has been 

combined. 

B. Parameters Measured 

The object of the Plume and Convergence Surveys was to characterize the plume by sampling 

the plume's water structure, sediments, biology, and circulation . 

To monitor the plume, four approximately north-south transects were conducted off the mouth 

of the Galveston ship channel during approximately twenty hours on July 19 and 20, 1993. 

Immediately after completing the transects the Convergence Survey was completed with a sawtooth 

pattern spaced between Sections SS and S6 during approximately 17 hours on July 20-21, 1993. 

The combined Plume and Convergence Surveys lasted approximately 37 hours on July 19-21. 

Because of their proximity in time and space, presentations of data for the Plume and Convergence 

surveys are combined in this report. 

While underway, surface water temperature, conductivity, light transmission, and 

chlorofluorescence were sampled with LUMCONs Mufti-Interface Data Acquisition System 

(MIDAS), while currents were measured with the 1200 kHz ADCP. Salinity and density (sigmatheta) were calculated from the temperature and conductivity data. 

For additional description of the flow field, drifter trajectories were superposed on maps of the 

water properties. 

C. Plume Survey Results 

1. Water Properties 

The near-surface characteristics of the Galveston plume may be distinguished using 

temperature, salinity, and ADCP velocities. Plan view contours of the MIDAS data are presented 

in Figure 119 and 120; ADCP vectors at the shallowest depth sampled by the ADCP (4.5 meter 

depth), superimposed with pre-SCULP trajectories, are presented in Figure 121 . 

Effluent from the Galveston Ship Channel was distinguished by temperatures warmer than 

approximately 30.8° C and salinities between 28-29 psu. Using these contours to define the 

plume, the MIDAS temperatures indicate that the plume can be traced approximately 20 kilometers 

to the east of the Ship Channel's mouth. 

149 
30.0 

29.5 

.c 

-' 29.0 

28.5 

28.0 

a 

C 

-95.5 -95.0 -94.5 -94.0 -93.5 -93.0 -92.5 -92.0 -91.5 -91 .0 -9U.5 

Longitude 

Figure 119. Plume and convergence surveys: map of sea surface temperature from MIDAS, 

Cruise N. 

30.0 

29.5 

29 .0 

28.5 

28 .0 

-95.5 -95.0 -94.5 -94.0 -93 .5 -93.0 -92.5 -92.0 -91 .5 -91 .0 -90.5 

Longitude 

Figure 120. Plume and convergence surveys : map of sea surface salinity from MIDAS, Cruise 

IV. 

150 
2).G 

14 

29.2 

28.8 

ti r f, 

200 

rt ~~Y:" ~ 199^ , . . . Q-'0-'- 197 

196 

197 

19 9 

~3 195 

195 

a------- D Buoy 20017 

0----0 Buoy 20021 

P93-2, ADCP, 4 .5 meters, 2 km, (W =. 00 .4632 Amp= 1 .0229), Iterative Method 

205 

Scale = 20 cads o--# 

-94.6 -9h .2 

Longitude 

-93 .8 -93 .4 

Figure 121 . Plume and convergence surveys : ADCP current vectors at 4 .5 meter depth and preSCULP trajectories, CCLl1SC IV. 

206 

211 

210 

208 

e4kl 

206 

204 ~" 

,_ . 

v+i 

,p ~, 

202 

204._%-

. .., 

~"y,, Ow ,l f 

"nh I 
The salinity field indicates that the Galveston plume is sandwiched between two other water 

masses, one to the north and the other to the south. To the south, much saltier water is being 

adverted northeastward along the coast under the prevailing southerly winds. To the north the 

salinities less than 29 psu indicate a plume of water from another coastal source of freshwater . 

2. Circulation 

Various aspects of the flow field were resolved by the water properties, ADCP, and preSCULP drifters. The near-surface fields of temperature and salinity suggest that Galveston Plume 

water flowed offshore in an "S" pattern into the area sampled in the Convergence Survey. South 

of the plume, the ADCP vectors at 4.5 meters depth showed the northward flowing, saline water. 

Two pre-SCULP buoys (Nos. 20017 and 20021) were transported from the south, skirted the 

plume, and entered the convergence area. 

In the convergence area the ADCP vectors indicate regions of offshore flow. However, the 

two pre-SCULP buoys appear to have passed north of this offshore flow and continued drifting to 

the east of the convergence area. 

3. Phytoplankton Pigments and Associated Environmental Variables (Nancy N. 

Rabialais) 

During Cruise N, Stations 401 through 423 were occupied to document the biological 

characteristics of the Galveston plume and their associated water quality parameters. Surface water 

temperature was uniformly high across the area with a slight cooling towards the seaward edge of 

the plume study area (2° C) (data not shown). Salinities ranged from 2431.5 ppt and were lower 

in the northeast quadrant of the study area and were higher to the southwesk Light transmission 

paralleled the salinity distributions, with the most turbid waters being associated with the lower 

salinides in the northeast quadrant of the study area. Scaled in vivo fluorescence from the MIDAS 

flowthrough system differed from the salinity/turbidity trends. The highest fluorescence was 

located centrally in the station grid, was fairly high in the northwest quadrant and decreased 

seaward in all directions (data not shown). For discrete water samples, the lowest chlorophyll a 

biomass was associated with the highest salinity (Figure 122), and higher chlorophyll a with the 

intermediate salinities. A similar relationship was observed with the MIDAS data in that the high 

peaks of scaled in vivo fluorometer voltage were associated with the lower salinities (Figure 122). 

4. Zooplankton Characteristics (Richard F. Shaw) 

Dry weight density for the 153-Em samples showed no trend associated with Galveston 

Bay's plume, which ranged from 17.89-143.35 mg/m3. The 335-pin biomass estimates were 

higher outside of the plume's influence, ranging from 6.60-63 .56 mg/m3. Larval fish density 

increased as the distance from the bay increased and ranged from Q7.371arvae/m3. 

D. Convergence Survey Results 

1. Water Properties 

The temperature and salinity contours of the Plume Survey appear to blend with those the 

Convergence Survey. As the saltier waters from the south collided with the fresher waters to the 

north, Galveston Plume water stained the union Near the shore the two water masses met during 

Cruise N near the mouth of the Galveston Ship Channel. The shapes of the salinity contours and 

the pre-SCULP drifter trajectories indicate that water in plumes from farther north continues to hug 

the coast under the influence of the earth's rotation (Coriolis acceleration). The saline water from 

152 
Surface Salinity and Chlorophyll a for Discrete Water Samples 

Galveston Plume 

LATEX July, 1993 -- 

5.00 M 

4 .50 f t- 32 

4 00 

31 

ISO 

M 

'-4 

E 30 

3.00 

+r .a . z' 

h \ ~ °.c 29 T 

a 

2.50 

'~ "jr . . ` / 

T 

n 

D 

.2 2 00 

z' \ 

. 

\ 

28 

4 w ~`~y 

S ~`~~ , \ 

1 .50 

~ 4 ' 

+yq x ',~ \ 27 

1 .00 

26 

050 

r~ ~rp~ 

,. 9 0 .00 5 

401 403 405 407 408 409 415 420 423 

Station 

Factored Midas Data 

Galvestan Plume 

LATEX July, 1993 

70 

60 

50 

r I 1-_ 

40 . 

Temp. (dag. C)'2 

I Y.Llght7rans/2 

Salinity (ppq 

Cond(Slm)'S 

30 . . A C -Scaled Fhwr.Volu'20 

20 NOTE: 

Midas dale values have 

been multiplied a divided 

by lectors (sae legend 

10 in order to emphasize 

features and plot on a 

single scale 

0

17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 B 9 10 11 12 13 13 14 16 17 

Hour 

Figure 122. Comparison of chlorophyll a biomass to salinity for discrete water 

samples (upper panel) and of MIDAS scaled in vivo fluorescence and 

salinity (lower panel) for the Galveston plume survey . 

153 
the south, also under the effect of Coriolis acceleration, turns to the south, riding around the 

coastal plume water. Galveston plume water "stains" the junction of the two water masses. 

The Galveston plume water appears to snake its way into the convergence area between 

Sections SS and S6. This may be seen by following the band of water between salinities of 28-29 

psu in Figure 123, and by water having temperatures exceeding 30.6° C. 

2. Circulation 

Both the ADCP vectors suggest the transport of Galveston Plume water into the convergence 

zone between Sections SS and S6. The ADCP vectors (Figure 3) indicate a squirt-like region 

approximately 20 Ian wide between approximately 93.7° and 93.9° W. To the west of the offshore 

low, the saline waters from the south are carried offshore, and to the east, less saline coastal waters 

are also carried offshore . Therefore, the convergence area is a zone of offshore transport of three 

water types: saline water from the south, Galveston Plume water, and coastal brackish water from 

the coast. 

As pre-SCULP buoy 20021 drifted northward, it intersected the Galveston plume on July 20. 

Upon meeting the plume, the buoy turned eastward, tracing with its trajectory the southern outline 

of the plume. Assuming that the buoys continue tracing the southern edge, it appears that the 

Galveston Bay water was adverted to the east, north of the region of the convergence survey . This 

implies that the Galveston water lay north of the SCULP trajectory and offshore of the plume 

nearest the shore. It does not appear that the Galveston Bay water was ejected farther offshore in 

the squirt-like feature seen in the ADCP data. 

3. Phytoplankton Pigments and Associated Environmental Variables (Nancy N. 

Rabrzlais) 

Surface water temperature was uniformly high across the area with a slight cooling towards the 

eastern side of the study area (not diel related) (data not shown). Salinities and percent light 

transmission were also fairly uniform across the study area. The lowest percent light transmission 

was found in the northwest quadrant and associated with the highest in vivo scaled fluorometer 

voltage. This indicates that the turbidity was phytoplankton biomass. Similar lows in percent light 

transmission on the continuous MIDAS data were associated with peaks of in vivo scaled 

fluorometer voltage (Figure 123). There was no clear relationship between salinity and measured 

chlorophyll a biomass or in vivo scaled fluorometer voltage (Figure 123). 

4. Zooplankton Characteristics (Richard F. Shaw) 

For the 153- and 335-N.m biomass estimates, no trend was associated with the convergence 

zone; however, higher densities were noted on the shoreward side of both water masses. Dry 

weight densities ranged from 30.33-171.45 mg/m3 for the 153-Em fractions and from 8.62-88 .21 

mg/m3 for the 335-gym fractions. Larval fish density was higher on the Louisiana side of the 

convergence. This contrasts the overall picture of ichthyoplankton densities, where the highest 

densities were found along the Texas shelf. Larval fish densities ranged from 0.57-6.73 

larvae/m3. 

154 
Surface Salinity and Chlorophyll a for Discrete Water Samples 

The Convergence ._ 

LATEX July, 1993 

1 .60 

1 .40 

1 .20 

1 .00 

m 

w 

t 0.80 

0 

` 

to 

u 0.60 

0.40 

020 

0.00 

70 

60 

50 

40 -

30 . 

20 --

10 

29 

29 

28 

28 

ze v 

-i-Chbr . 

c -~ Sal . 

28 a 

N 

28 

27 

27 

27 

Figure 123. Comparison of chlorophyll a biomass to salinity for discrete water 

samples (upper panel) and of MIDAS scaled in vivo fluorescence and 

salinty (lower panel) for the convergence survey . 

155 

601 604 605 606 610 613 

Station 

Factored Midas Data 

The Convergence 

Latex July, 1993 

0 

17 18 19 20 21 22 23 00 01 02 03 04 06 07 08 09 10 

Hour 
VIII. LITERATURE CITED 

Butler, J.L. 1992. Collection and preservation of material for otolith analysis. pp. 13-17. 

In: Stevenson, D.K. and S.E. Campana (eds.) . Otolith Microstructure Examination 

and Analysis. Can. Spec. Publ. Fish. Aquat Sci. 117. 

Cox, J. and C. Ebbesmeyer. 1994. MIDAS survey of the Calcasieu River plume. In: 

Murray, S.P. (ed.) . Mississippi River plume hydrography: annual report. OCS Study 

MMS 940028. U.S. Dept of the Interior, Minerals Mgmt. Service, Gulf of Mexico 

OCS Region, New Orleans, LA, pp. 211-219. 

Crepon, M. and C. Richez. 1982. Transient upwelling generated by two-dimensional 

atmospheric forcing and variability in the coastline. JPO, 12, pp. 1437-1457 . 

Csanady, G.T. 1971. On the equilibrium shape of the thennocline in the shore zone. 

JPO, 1, pp. 263-270. 

Csanady, G.T. 1974. Spring thermocline behavior in Lake Ontario during IFYGL, JPO, 

4, pp. 425-445. 

Csanady, G.T. 1977. The coastal jet conceptual model in the dynamics of shallow seas. 

In: Goldberg, E.D., I.N. McCave, J.J . O'Brien, and J.H. Steele (eds.) . The Sea, 6, John 

Wiley, NY, pp. 117-144. 

Dortch, Q. 1994x. Phytoplankton Survey. 1W. Murray, S.P. (ed.) . Mississippi River 

plume hydrography: annual report . OCS Study MMS 94-0028. U.S. Dept of the 

Interior, Minerals Mgmt Service, Gulf of Mexico OCS Region, New Orleans, LA, 

pp. 124142. 

Dortch, Q. 1994b. Changes in phytoplankton numbers and species composition. Im 

Coastal oceanographic effects of 1993 Mississippi River flooding. Special NOAH 

Report. NOAH Coastal Ocean Office/National Weather Service, Silver Spring, MD, 

pp. 46-49. 

Dortch, Q., R. Robichawc, S. Pool, D. Milstead, G. Mire, N.N. Rabalais, T. M. Soniat, 

G.A. Fryxell, R.E. Turner, and M.L. Parsons. Submitted. Abundance and vertical 

flux of Pseudo-nitzschia in the northern Gulf of Mexico. Marine Ecology Progress 

Series . 

Fryxell, G.Q., M.E. Reap, and D.L. Valencic. 1990. Nitzschia pungens Grunow f. 

multiseries Hasle: observations of a known neurotoxic diatom. Beiheft zur Nova 

Hedwigia 100:171-188. 

Hallegraeff, G.M. 1993. A review of harmful algal blooms and their apparent global 

increase. Phycologia 32:79-99. 

Huh, O.K., L.J . Rouse, Jr ., and N.D. Walker. 1984. Cold air outbreaks over the 

northwest Florida continental shelf: heat flux processes and hydrographic changes. J. 

of Geophysical Res. 89(C1) :717-726. 

Kidwell, K.B. 1991. NOAH polar orbiter data users guide. U.S. Dept of Commerce, 

Satellite Data Service Division. 

157 
Lawrence, G.C. 1976. Caloric value of some north Atlantic calanoid copepods. Fish. 

Bull. 74:218-220. 

Lovegrove, T. 1966. The determination of the dry weight of plankton and the effect of 

various factors on the values obtained. pp. 429-467. In: Barnes, H. (ed.) . Some 

Contemporary Studies in Marine Science. George Allen and Unwin Ltd., London. 

McClain, E.P., W.G. Pichel, and C.C. Richardson. 1985. Comparative performance of 

AVHRR-based multichannel sea surface temperatures. J. Geophys. Res. 90:11,587-

11,601 . 

McEwen, G.F., M.W. Johnson, and T.R. Folsom. 1954. A statistical analysis of the 

performance of the Folsom plankton sample sputter, based upon test observations . 

Arch. Met Geophys. Bioklim, Series a 7:502-527. 

Meade, RH. and RS. Parker. 1985. Sediment in rivers of the United States. National 

Water Supply-Summary 1984. U.S . Geological Survey Water-Supply Paper No. 

2275, U.S. Dept of the Interior, Geological Survey, Washington, DC. 

Methot, Jr., RD. and D. Kramer. 1979. Growth of northern anchovy, Engraulis mordax, 

larvae in the sea. Fish. Bull. 77(2):413-423. 

Murphy, L.S. and E.M. Haugen. 1985. The distribution and abundance of phototrophic 

ultraplankton in the North Atlantic . Limnol. Oceanogr. 30:47-58. 

Murray, S.P. 1994. Mississippi River plume hydrography: annual report . OCS Study 

MMS 940028. U.S. Dept of the Interior, Minerals Mgmt. Service, Gulf of Mexico 

OCS Region, New Orleans, LA, 229 pp. 

NOAH. 1993. Climate Diagnostics Bulletins. U.S. Department of Commerce, National 

Weather Service, National Meteorological Center. 

Olson, R.J ., S.W. Chisholm, E.R Zettler, and E.V. Armbrust. 1988. Analysis of 

Synechococcus pigment types in the sea using single and dual beam flow cytometry. 

Deep-Sea Res. 35:425-440. 

Parsons, T.R, Y. Maita, and M. Lalli. 1984. A manual of chemical and biological methods for 

seawater analysis. Pergamon Press, NY, 173 pp. 

Pettigrew, N.R. 1981. The dynamics and kinematics of the coastal boundary layer off 

Long Island. Ph.D. Dissertation, WHOUNIIT, 262 pp. 

Pettigrew, N.R. and S.P. Murray. 1986. The coastal boundary layer and inner shelf. In: 

Mooers, C.N.K. (ed.) . Baroclinic processes on continental shelves. Coastal and 

Estuarine Sciences, 3, AGU, pp. 95-108. 

Pick, F.R. 1991 . The abundance and composition of freshwater picocyanobacteria in 

relation to light penetration . Limnol. Oceanogr. 36:14570-1462. 

Rabalais, N.N. 1994. Phytoplankton Pigements. In: Murray, S.P. (ed.). Mississippi 

River plume hydrography: annual report. OCS Study MMS 940028. U.S . Dept of 

the Interior, Minerals Mgmt Service, Gulf of Mexico OCS Region, New Orleans, 

LA, pp. 124142. 

158 
Rabalais, N.N., R.E. Turner, W.J . Wiseman, Jr., and D.F. Boesch. 1991. A brief 

summary of hypoxia on the northern Gulf of Mexico continental shelf: 1985-1988. 

In Tyson, R.V. and T.H. Pearson (eds.) . Modem and ancient continental shelf anoxia. 

Geological Society Special Publication No. 58, The Geological Society, London, pp. 

35-47. 

Rabalais, N.N., R.E. Turner, and W.J. Wiseman, Jr. 1992. Distribution and 

characteristics of hypoxia on the Louisiana shelf in 1990 and 1991. Ia: Nutrient 

enhanced coastal ocean productivity . Publ. No. TAMU-SG-92-109, Texas Sea Grant 

College Program, Galveston, TX, pp. 62-67. 

Rabalais, N.N., R.E. Turner, and W.J. Wiseman, Jr. 1994. Hypoxic conditions in bottom 

waters on the Louisiana-Texas shelf. ja: Coastal oceanographic effects of 1993 

Mississippi River flooding. Special NOAH Report. NOAH Coastal Ocean 

Office/National Weather Service, Silver Spring, MD, pp. 50-54. 

Renaud, M.L. 1986. Hypoxia in Louisiana coastal waters during 1983: implications for 

fisheries. Fish. Bull. 84(1):19-26 . 

Robichaux, RJ. and Q. Dortch. Submitted. Occurrence of Gymnodinium sanguineum in 

Louisiana and Texas coastal waters, 1989-1994. Contr. Mar. Sci. 

SAS Institute Inc. 1985. SAS user's guide: basics, version 5. Cary, NC. 1,290 pp. 

Shapiro, L.P. and E.M. Haugen. 1988. Seasonal distribution and temperature tolerance 

of Synechococcus in Boothbay Harbor, ME. Esc Coast Shelf Sci. 26: 517-526. 

Shapiro, L.P., E.M. Haugen, and E.J. Carpenter. 1989. Occurrence and abundance of 

green fluorescing dinoflagellates in surface waters of the Northwest Atlantic and 

Northeast Pacific Oceans. J. Phycol. 25:189-191. 

Stumpf, R.P. 1988. Remote sensing of suspended sediments in estuaries using 

atmospheric and compsitional corrections to AVHRR data. In: Proceedings of the 

21st Intl. Symp. on Remote Sensing of Environment Ann Arbor, MI, pp. 205-222. 

Stumpf, R.P. 1992. Remote sensing of water clarity and suspended in coastal waters. Ia: 

Proceedings of the First Thermatic Conference on Remote Sensing for Marine and 

Coastal Environments. 13 pp. 

Siegel, S. and N.J. Castellan, Jr. 1988. Nonparametric statistics for the behavioral 

sciences, 2nd edition. McGraw-Hill, Inc., NY, 399 pp. 

Smith, N.P. 1980. On the hydrography of shelf waters off the central Texas Gulf Coast. 

J . Physical Oceanogr. 10:806-813. 

Smith, R.A., RB. Alexander, and M.G. Wolman. 1987. Water-quality trends in the 

nation's rivers. Science 235 :1607-1615. 

Taylor, F.J .R 1976. Appendix: Treatment of micro- and nanoplankton samples. Ia: 

Steedman, H.F. (ed.) . Zooplankton fixation and preservation . Unesco Press, Paris, 

pp. 32-33. 

159 
Tranter, D.J. and P.E. Smith. 1968. Filtration performance. In: Zooplankton Sampling. 

Unesco Press, Paris, pp. 27-56. 

Turner, RE., R Kaswadji, N.N. Rabalais, and D.F. Boesch. 1987. Long-term changes in 

the Mississippi River water quality and its relationship w hypoxic continental shelf 

waters . I1: Fstuarine and coastal management tools of the trade. In: Proceedings, 

Tenth National Conference, The Coastal Society, October 12-15, 1986, New Orleans, 

LA, pp. 261-266. 

Vaulot, D. and X. Ning. 1988. Abundance and cellular characteristics of marine 

Synechococcus spp. in the dilution zone of the Changjiang (Yangtze) River, China. 

Cony Shelf Res. 8:1171-1186. 

Walker, N.D. and L.7. Rouse, Jr. 1993. Satellite assessment of Mississippi River 

discharge plume variability . OCS Study MMS 93-0044. U.S. Dept of the Interior, 

Minerals Mgmt Service, Gulf of Mexico OCS Region, New Orleans, LA, SO pp. 

Walker, N.D., O.K. Huh, and L. J. Rouse, Jr. 1993. Warm-core eddy discovered in the 

Gulf of Mexico. EOS, Transactions, American Geophysical Union, 74(30) :337-338. 

Walker, N.D., L.J . Rouse, Jr., and O. K. Huh. 1994. Remote sensing and imaging. In: 

Murray, S. P. (ed.) . Mississippi River plume hydrography: annual report. OCS 

Study MMS 940028. U.S. Dept of the Interior, Minerals Mgmt Service, Gulf of 

Mexico OCS Region, New Orleans, LA, pp. 191-209. 

Walker, N.D., O.K. Huh, L.J. Rouse, and S.P. Murray. In press. Evolution and structure 

of a coastal squirt off the Mississippi River delta: northern Gulf of Mexico. J. 

Geophys. Res. 

Waterbury, J.B.S., S.W. Watson, F.W. Valois, and D.G. Franks. 1986. Biological and 

ecological characterization of the marine unicellular cyanobacterium Synechococcus. 

In: Platt, T. and W.K.W. Li (eds.). Photosynthetic picoplankton . Can. Bull. Fish. 

Aquat. Sci. 214, pp. 71-120. 

Wood, A.M., P.K. Horan, K. Muirhead, D.A Phinney, C.M. Yentsch, and J.B. 

Waterbury. 1985. Discrimination between types of pigments in marine 

Synechococcus spp. by scanning spectroscropy, epifluorescence microscopy, and flow 

cytometry. LimnoL Oceanogr. 30:1303-1316. 

Yentsch, C.S. and J.F. Hebard. 1957. A gauge for determining plankton volume by the 

mercury immersion method. J. Conseil . 22(2):184190. 

160 
NT OFTy 

' 

~y The Department of the Interior Mission 

a `' , 3 ~ ~ p As the Nation's principal conservation agency, the Department of the Interior has responsibility for 

~ most of our nationally owned public lands and natural resources. This includes fostering sound use 

of our land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

4 9 

environmental and cultural values of our national parks and historical places; and providing for the 

ACH 9 ~ enjoyment of life through outdoor recreation . The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U .S . administration . 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior , the Minerals Management Service's (MMS) primary 

res p onsibilities are to mana g e the mineral resources located on the Nation's O u te r C on ti nen t a l Sh e lf 

s i Y (OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

s~4 a those revenues . '~ ~ 

~. , ~Ewv~` Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program 

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S . Treasury . 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection.