OCS Study

BOEM 2012-071

U.S. Department of the Interior

Bureau of Ocean Energy Management

Gulf of Mexico OCS Region

Cooperative Agreement

Coastal Marine Institute

Louisiana State University

Coastal Marine Institute

Long-term Trends in Environmental

Parameters along the LouisianaMississippi Outer Continental Shelf

Using Remote-Sensing Data
OCS Study

BOEM 2012-071

Published by

U.S. Department of the Interior

Bureau of Ocean Energy Management

Gulf of Mexico OCS Region

Cooperative Agreement

Coastal Marine Institute

Louisiana State University

Coastal Marine Institute

Long-term Trends in Environmental

Parameters along the LouisianaMississippi Outer Continental Shelf

Using Remote-Sensing Data

Author:

Eurico J. D’Sa

December 2012

Prepared under BOEM Cooperative Agreement

M08AX12685

by

Louisiana State University

Coastal Marine Institute

Baton Rouge, Louisiana 70803
iii

DISCLAIMER

This report was prepared under contract between the Bureau of Ocean Energy Management 

(BOEM) and Louisiana State University. This report has been technically reviewed by BOEM, 

and it has been approved for publication. Approval does not necessarily signify that the 

contents reflect the views and policies of BOEM, nor does mention of trade names or 

commercial products constitute endorsement or recommendation for use. 

REPORT AVAILABILITY

To download a PDF file of this Gulf of Mexico OCS Region report, go to the U.S. 

Department of the Interior, Bureau of Ocean Energy Management, Environmental Studies 

Program Information System website and search on OCS Study BOEM 2012-071. 

This report can be viewed at select Federal Depository Libraries. It can also be obtained 

from the National Technical Information Service; the contact information is below. 

U.S. Department of Commerce

National Technical Information Service

5301 Shawnee Rd. 

Springfield, Virginia 22312 

Phone: (703) 605-6000, 1(800)553-6847 

Fax: (703) 605-6900 

Website: http://www.ntis.gov/

CITATION

D’Sa, E.J. 2012. Long-term trends in environmental parameters along the Louisiana-Mississippi 

outer continental shelf using ocean color remote-sensing data. U.S. Dept. of the Interior, 

Bureau of Ocean Energy Management, Gulf of Mexico OCS Region, New Orleans, LA. 

OCS Study BOEM 2012-071. 35 pp. 

ABOUT THE COVER 

The cover image depicts SeaWiFS ocean color sensor-derived chlorophyll estimates during 

August 2008 along the Mississippi-Louisiana-Texas coast and the Outer Continental Shelf. 

QuikSCAT derived wind stress vectors (N m-2) are superimposed on the SeaWiFS imagery.
v 

ABSTRACT

The Mississippi River is the primary riverine source of freshwater, nutrients, 

organic/inorganic particulate and dissolved matter to the Louisiana continental shelf and 

strongly influences biogeochemical processes in the northern Gulf of Mexico (NGOM). The 

region is also an important center of economic activity associated with a well-developed 

fisheries and oil and gas industry. Ocean color remote sensing potentially provides the 

information on regional scales to address the issue of monitoring both short-term and long-term 

seawater particulate and dissolved matter distributions and an assessment of the effects of both 

anthropogenic and natural influences on coastal ecosystems. Sea-viewing Wide Field-of-view 

Sensor (SeaWiFS) satellite ocean color data together with field observations were used to 

examine the short-term, seasonal and long-term distribution of colored dissolved organic matter 

(CDOM) and chlorophyll along the coastal and shelf waters of the Mississippi, Louisiana, and 

part of Texas coast. A previously derived CDOM empirical algorithm was validated and used to 

derive estimates of surface CDOM absorption distribution at 412 nm from the SeaWiFS 

imagery. Spatial and temporal CDOM absorption distributions from satellite for the NGOM

indicated strong seasonal influence associated with river discharge and effects of storms. 

Multiyear time-series satellite derived chlorophyll (Chl) estimates were used to examine the 

dominant scales of chlorophyll variability in the river-dominated coastal and oceanic waters. 

Wavelet analysis was applied to almost 10 years (1998-2007) of data and interpreted in 

conjunction with wind data from QuikSCAT satellite sensor to provide additional insights on 

the Chl variability in the region. Near the river deltas, seasonal maxima in peak variance were 

associated with seasonal peaks in river discharge, while in offshore waters similar peaks 

occurred with less regularity and appeared to be influenced mainly by the offshore wind stress 

that transported river plume waters with elevated Chl offshore. Although similar to the trends in 

the Mississippi River, Chl variability was lower in a zone off the Atchafalaya River while west 

of the Atchafalaya River, significant peaks in Chl variance appeared to be associated with the 

effects of Hurricane Rita in 2005. 
vii 

CONTENTS

LIST OF FIGURES .................................................................................................................... ix

ABBREVIATIONS AND ACRONYMS .................................................................................. xi

1.0 INTRODUCTION .................................................................................................................. 1

2.0 DATA AND METHODS........................................................................................................ 3

2.1 Study Area 3

2.2 Field Bio-Optical Measurements .................................................................................... 3

2.3 Satellite Remote-Sensing Data Processing ..................................................................... 3

2.4 Time-Series Ocean Color Data Processing .................................................................... 4

3.0 RESULTS ................................................................................................................................ 5

3.1 CDOM Optical Properties along the Louisiana Coast .................................................. 5

3.2 Seasonal and Storm Effects on CDOM in NGOM ........................................................ 5

3.3 Long-Term Trends in Chlorophyll Distribution Using Wavelet Analysis .................. 6

3.3.1 Chlorophyll Variability West of Mississippi River Delta (Zone 4) ................ 8

3.3.2 Chlorophyll Variability off the Atchafalaya Delta (Zone 3) ......................... 10

3.3.3 Chlorophyll Variability West of the Atchafalaya River Delta 

(Zone 1) .......................................................................................................... 10

4.0 SUMMARY AND CONCLUSIONS ................................................................................... 13

5.0 REFERENCES ..................................................................................................................... 15

APPENDIX A ............................................................................................................................. 19
ix

LIST OF FIGURES

Figure 1. The study region in the northern Gulf of Mexico, showing the study domain 

extending from 27 to 30.5°N latitude, and 88.2 to 95.5°W longitude. Labels 

show the Mississippi River (MR), the Atchafalaya Rivers (AR), and southwest 

pass. ............................................................................................................................. 3

Figure 2. Seasonal surface CDOM absorption at 412 nm derived from SeaWiFS imagery 

for (a) winter (b) spring, (c) summer and (d) fall of 2005. ......................................... 6

Figure 3. Study area in the northern Gulf of Mexico showing 5 zones and bathymetric 

contours at 20, 50, 100, 500, and 1000 m depth. AR and MR represent the 

locations of the Atchafalaya and Mississippi Rivers. The study region is divided 

into five zones: zone 4 is located immediately west of the Mississippi delta, 

zone 3 is off the Atchafalaya delta, and zone 1 is west of the Atchafalaya delta. ...... 8

Figure 4. Mississippi River discharge measured at Tarbert Landing, Mississippi. Obtained 

from the U.S. Army Corps of Engineers Web site. ..................................................... 8

Figure 5. Figures a and b show wavelet power spectra (mg m-3)

2 of Chl averaged along 

the 20 and 500 m isobaths in Zone 4 immediately west of the Mississippi River 

delta. Figures c and d show corresponding scale-averaged time series for the 

period band 30-100 days; the green line shows the 95% confidence interval. ........... 9

Figure 6. QuikSCAT satellite derived wind stress vectors (N m-2) superimposed on 

SeaWiFS derived Chl (mg m-3) for the months of (a) February 2004 and (b) 

March 2003. ................................................................................................................ 9

Figure 7. Figures a and b show local wavelet power spectra (mg m-3)

2 of the time series 

Chl at 20 and 500 m isobaths in Zone 3 off the Atchafalaya River delta. 

Figures c and d show corresponding scale-averaged time series for the period 

band 30-100 days; the green line shows the 95% confidence interval...................... 10

Figure 8. (a) Local wavelet power spectra (mg m-3)

2 of the time series chlorophyll at 100 

m isobath in zone 1 west of the Atchafalaya River delta. (b) Corresponding 

scale-averaged time series for the period band 30-100 days; the green line 

shows the 95% confidence interval. .......................................................................... 11

Figure 9. QuikSCAT satellite derived wind stress vectors (N m-2) superimposed on 

SeaWiFS derived chlorophyll (mg m-3) for the month of September 2005. ............. 12
xi

ABBREVIATIONS AND ACRONYMS

CDOM Colored Dissolved Organic Matter

Chl chlorophyll concentration (mg m-3) 

DAAC NASA Goddard Distributed Active Archive Center 

JPL Jet Propulsion Laboratory 

LA Louisiana 

LSU Louisiana State University 

MS Mississippi 

NASA National Aeronautics and Space Administration 

nm nanometer 

NGOM Northern Gulf of Mexico 

OCS Outer Continental Shelf 

SeaWiFS Seawing Wide Field of View Sensor 

SPM Suspended Particulate Matter (mg L-1) 
1 

1.0 INTRODUCTION

The Mississippi River is the primary riverine source of freshwater, nutrients, organic and 

inorganic particulate, and dissolved matter to the Louisiana-Mississippi continental shelf and to 

the Gulf of Mexico (Lohrenz et al. 1997; 1999; 2008). The river-influenced coastal and offshore 

region is also of great ecological and economic importance to the region and the country. 

However, these regions are under the combined threats of subsidence, wetland loss, energetic 

meteorological events, and human-induced changes to the ecosystem (Rabalais et al. 2002). 

Many of these coastal regions are undergoing further population pressures, development and 

resource exploitation, while, at the same time, threats due to natural (storms, hurricanes, algal 

blooms) and anthropogenic hazards (oil spills, pollutant transport) are increasing. Information 

related to water masses and water quality indicators are important to support strategies for 

response, recovery, mitigation, and coastal restoration. Following Hurricanes Katrina and Rita, 

for example, there was a clear need to monitor water quality and transport of pollutants (e.g., oil 

spills) along the near-shore Gulf waters. Ocean color remote sensing potentially provides the 

information on regional scales to address the issue of monitoring both short-term and long-term 

seawater particulate and dissolved matter distributions and to assess the effects of both 

anthropogenic and natural influences on coastal ecosystems. 

Satellite remote sensing makes it possible to conduct surveillance on large areas of the 

world’s oceans for natural conditions and changes related to human activities. Plankton 

biomass, chlorophyll concentrations, colored dissolved organic material (CDOM), and other 

ocean color products are used in various ways to describe the water quality conditions and their 

positive or negative trends. In river-dominated areas, such as coastal Louisiana, the use of 

satellite imagery to monitor changes in water quality parameters is complicated by the 

variations due to characteristics of the river inflow and its contents (D’Sa et al. 2006; Walker 

and Rabalais 2006; D’Sa 2008; Green et al. 2008). 

In this project, satellite ocean color data products, such as chlorophyll concentration and 

CDOM, were examined using a standard NASA and a regionally-developed CDOM absorption 

algorithm to improve our understanding and use of ocean color data for short- and long-term 

trends in their variability in the northern Gulf of Mexico (NGOM). A regional suspended 

particulate matter (SPM) algorithm (D’Sa et al. 2007) was also applied to SeaWiFS imagery to 

examine short-term (e.g., storms and hurricanes) (D’Sa and Ko 2008; D’Sa et al. 2011a) and 

long-term (D’Sa et al. 2011b) influences of river discharge and storms on SPM distribution in 

the NGOM. 

With an annual discharge rate of approximately 19,000 m3

 s-1, approximately 70% of the 

Mississippi River discharges into the Gulf of Mexico through the birdfoot delta and into deeper 

shelf waters, and the remaining 30% of the discharge is delivered by the Atchafalaya River into 

the broad Atchafalaya shelf, creating a major region of freshwater influence along the 

Louisiana, Mississippi, and Texas coast (Dinnel and Wiseman 1986). In coastal waters 

dissolved organic matter contribution to seawater comes from rivers and other terrestrial inputs. 

A significant proportion of dissolved organic matter in these waters is present as CDOM that 

influences ocean color, water quality, and remotely sensed signals (Carder et al. 1989). CDOM 

thus represents a fraction that can be determined through measurements of optical absorption of 
2 

filtered seawater and also through remote sensing. Light absorption by CDOM in the ultraviolet 

(UV) and visible wavelength range exhibits an exponential decay with increasing wavelength. 

Absorption of sunlight by CDOM can reduce light availability for primary production and also 

reduce the damaging effects of solar radiation in the ultraviolet. In the process CDOM can 

undergo photo-oxidation or photo-bleaching (Vodacek et al. 1997; D’Sa 2008; D’Sa and 

DiMarco 2009). 

Because CDOM is an important component of dissolved organic matter in large river 

dominated margins, both field and ocean color remote sensing studies have been conducted in 

the coastal waters influenced by the Mississippi River system (Chen et al. 2004; Conmy et al. 

2004; D’Sa et al. 2006; Del Castillo and Miller 2008; D’Sa 2008; D’Sa and DiMarco 2009). 

Studies of CDOM optical properties in the NGOM have indicated a highly variable distribution 

influenced mainly by the seasonal discharge from the Mississippi River (Chen et al. 2004; D’Sa 

and DiMarco 2009). However, other factors, such as in situ CDOM production associated with 

phytoplankton primary production or its bacterial decomposition and CDOM photo-bleaching, 

have also been found to influence CDOM variability in the study region (Chen et al. 2004; D’Sa 

and DiMarco 2009). Empirical ocean color remote sensing algorithms have been developed for 

various coastal regions, including the NGOM, for the determination of CDOM using remote 

sensing reflectance ratios and coefficients that have been regionally optimized (D’Sa and Miller 

2003; D’Sa et al. 2006; Mannino et al. 2008). In this study we have examined CDOM and 

particulate optical properties along the Louisiana coast (D’Sa 2008; D’Sa and DiMarco 2009; 

Naik et al. in press) and further used a regional CDOM algorithm for SeaWiFS satellite sensor 

to examine the short-term and seasonal variability of CDOM absorption in coastal and oceanic 

waters extending from the Mississippi delta to Galveston, Texas. A seasonal increase in 

hurricane activity (e.g., Hurricane Rita) in the Gulf of Mexico in 2005 allowed for the 

assessment of hurricane and storm activity on CDOM distribution in the NGOM. 

Another important water quality parameter in the NGOM is chlorophyll biomass, an 

indicator of productivity in the Gulf (Lohrenz et al. 1997; 1999). In addition to river discharge, 

chlorophyll concentrations are also influenced by cold fronts, wind-induced currents, and 

hurricanes that often impact the region (Walker et al. 2005; Walker and Rabalais 2006; D’Sa et 

al. 2006). Satellite ocean color chlorophyll estimates have provided a synoptic pattern of 

distribution along the Louisiana coast (D’Sa et al. 2006). The SeaWiFS satellite sensor has 

provided the longest record of ocean color data that has been used to examine trends in optical 

properties and phytoplankton chlorophyll distributions (Lohrenz et al. 2008; Greene and Gould 

2008). In this study we use a 10-year record of SeaWiFS chlorophyll estimates in the NGOM to 

examine chlorophyll variability occurring at different temporal scales at different locations in 

the study area. We use wavelet analysis on time series data (D’Sa and Korobkin 2008a) to study 

the temporal variance in SeaWiFS derived chlorophyll by examining the dominant frequencies 

and their interannual variability in different regions of the NGOM. 
3 

2.0 DATA AND METHODS

2.1 STUDY AREA

The study area located in the Gulf of Mexico extends from 27-30.5° N latitude, 88.2-95.5° 

W longitude and includes coastal and OCS waters of the states of Mississippi, Louisiana, and 

part of Texas (Figure 1). Seasonal discharge from the Mississippi and Atchafalaya Rivers 

through the birdsfoot and Atchafalaya deltas results in a major region of freshwater influence 

along the coasts of Mississippi, Louisiana, and Texas. Additional outflows from the bays and 

smaller rivers can also contribute biogenic material and sediments to the shelf waters.

Figure 1. The study region in the northern Gulf of Mexico, showing the study domain extending 

from 27 to 30.5°N latitude, and 88.2 to 95.5°W longitude. Labels show the Mississippi 

River (MR), the Atchafalaya Rivers (AR), and Southwest Pass (SW Pass). 

2.2 FIELD BIO-OPTICAL MEASUREMENTS

Field data processed and analyzed as part of this study were obtained during cruises in 2005, 

2007, 2008, and 2009, along the Louisiana coast. They include field absorption measurements 

of CDOM and other constituents, such as phytoplankton and non-algal particles and are 

described in detail in D’Sa and DiMarco (2009) and Naik et al. (2011). 

2.3 SATELLITE REMOTE-SENSING DATA PROCESSING

Satellite remote sensing data used in this study include mainly ocean color data from 

SeaWiFS satellite sensor. SeaWiFS Level L1A data was downloaded from NASA DAAC and 

processed using NASA’s SeaDAS 5.1 processing software using a regional CDOM and SPM 
4 

algorithm (D’Sa et al. 2006 and 2007). SeaWiFS estimates of chlorophyll concentrations were 

obtained using the standard OC4 algorithm (O’Reilly et al. 1998). In addition, winds from 

QuikSCAT satellite sensor were downloaded from NASA JPL DAAC and processed using 

methods described in Sharma and D’Sa 2008. 

2.4 TIME-SERIES OCEAN COLOR DATA PROCESSING

Level 1A SeaWiFS data for the NGOM from 1998 to 2007 were downloaded from NASA 

DAAC and processed to Level 2 Chl product at 2-km resolution for the study region. NASA 

default coefficients and the standard atmospheric correction algorithm and the standard OC4 

algorithm were used to generate the Level 2 Chl, which were then composited into 15-day, 

monthly, or seasonal means to generate the time series data used in this study. Wavelet 

transform was applied to 15-day composite image data along bathymetric contour lines to 

examine long-term trends in their variability. 
5 

3.0 RESULTS

3.1 CDOM OPTICAL PROPERTIES ALONG THE LOUISIANA COAST 

Absorption properties of particulate and dissolved constituents of water along the Louisiana 

coast have been studied from data acquired during various field campaigns conducted both 

before and during the project (D’Sa and DiMarco 2009; Singh et al. 2010a, b; Naik et al. in 

press). Some details on CDOM absorption properties along the Louisiana coast are shown in 

Appendix A (D’Sa and DiMarco 2009); those of phytoplankton and non-algal absorption 

properties have been described in Naik et al. (in press). Briefly, a conservative relationship was 

observed between CDOM and salinity; also, seasonal changes in CDOM concentrations were 

observed that were attributed to sources (algal biomass) and sinks (photo-bleaching) of CDOM 

(D’Sa and DiMarco 2009). 

3.2 SEASONAL AND STORM EFFECTS ON CDOM IN NGOM

Short-term and seasonal estimates of CDOM absorption at 412 nm and surface salinity were 

derived from the SeaWiFS satellite data for the Louisiana-Texas coast during 2005 using an 

empirical CDOM algorithm and a conservative CDOM-salinity relationship (D’Sa et al. 2006; 

D’Sa and Korobkin 2008b; D’Sa and DiMarco 2009). Field measurements obtained during 

various seasons in 2005 indicated high correlations between field and satellite estimates of 

CDOM, suggesting satellite estimates to be a good representation of the surface CDOM and 

salinity fields. Discharge from the Mississippi and Atchafalaya Rivers strongly influenced the 

seasonal surface CDOM distribution. During fall and winter, frontal passages also influenced 

CDOM distribution over the shelf waters. Clear satellite imagery obtained before and after the 

passage of a cold front in March 2005 indicated a general decrease in surface CDOM and an 

offshore increase of elevated CDOM suggesting that frequent frontal passages contribute to 

mixing of riverine CDOM and its offshore transport (D’Sa and Korobkin 2008b). An 

examination of the spatial and temporal CDOM absorption distributions from SeaWiFS satellite 

data indicated strong seasonal influence associated with discharge from the MississippiAtchafalaya River system (Figure 2). A seasonal increase in storms and hurricanes allowed an 

assessment of the effects of Hurricane Rita in September 2005 (D’Sa and Korobkin 2008b). 
6 

Figure 2. Seasonal surface CDOM absorption at 412 nm derived 

from SeaWiFS imagery for (a) winter (b) spring, (c) 

summer and (d) fall of 2005. 

3.3 LONG-TERM TRENDS IN CHLOROPHYLL DISTRIBUTION USING WAVELET 

ANALYSIS

River discharge, cold fronts, wind induced currents, and hurricanes often influence the 

chlorophyll distribution in the NGOM (Walker et al. 2005; Walker and Rabalais 2006; D’Sa et 
7 

al. 2006; D’Sa and Korobkin 2009). Satellite ocean color Chl estimates have provided a 

synoptic pattern of distribution along the Louisiana coast (Walker and Rabalais 2006; Lohrenz 

et al. 2008). The SeaWiFS satellite sensor has provided the longest record of ocean color data 

that has been used to examine long-term trends in phytoplankton Chl distributions (Henson and 

Thomas 2007). In this study, we use a 10-year record of SeaWiFS Chl estimates in the NGOM

to examine Chl variability occurring at different temporal scales at different locations in the 

study area. We use wavelet analysis on time series data (Morlet 1983; Machu et al. 1999; 

Henson and Thomas 2007) to study the temporal variance in SeaWiFS derived Chl by 

examining the dominant frequencies and their interannual variability along different 

bathymetric contours. To further examine the physical linkages we use QuikSCAT satellite 

derived monthly wind stress vectors superimposed on SeaWiFS Chl. 

The study area shown in Figure 3 comprises the coastal states of Mississippi, Louisiana, and 

part of Texas extending between 88.2° - 95.5° W and into the Gulf of Mexico to 27.0° N 

latitude. Two main rivers discharging into the study area are the Mississippi and Atchafalaya 

Rivers. The Mississippi River discharge over a period from 1998 to 2008 is shown in Figure 2. 

Discharge from the Atchafalaya River (not shown) has the same seasonal variability but lower 

volume than the Mississippi River. 

Level 1A SeaWiFS data for the NGOM between 1998 and 2007 were downloaded from 

NASA DAAC and processed to Level 2 Chl product at 2 km resolution for the study region. 

NASA default coefficients and the standard atmospheric correction algorithm and the standard 

OC4 algorithm were used to generate the Level 2 Chl which were then composited into 15-day 

means to generate the time series data used in this study. The study area was divided into five 

zones (Figure 3) and time series Chl anomalies were determined in zone 1, 3, and 4 along the 

20, 100, and 500 m isobaths. Wavelet transform, a method that decomposes a signal into timefrequency space (Morlet 1983; Torrence and Compo 1998) was applied to the mean Chl 

anomaly data. The output of the wavelet transform is a local wavelet power spectrum (Figure 

5a) that depicts the variance in Chl ((mg m-3)

2

) and can be interpreted as a “map” of the time 

variability of dominant frequencies (Henson and Thomas 2007). Because the time series has 

finite length, errors increase at the edges of the wavelet spectrum and should be interpreted with 

caution (Torrence and Compo 1988). The normalized scale-averaged time series (Figure 5c) 

denotes the mean variance in the 30-100 period band and is obtained by normalizing the 

wavelet power by N/2σ2 (where N is the number of data points and σ2 is its variance); the green 

line denotes the 95% confidence level. 
8 

Figure 3. Study area in the northern Gulf of Mexico showing 5 zones and bathymetric 

contours at 20, 50, 100, 500, and 1000 m depth. AR and MR represent the 

locations of the Atchafalaya and Mississippi Rivers. The study region is divided 

into five zones: zone 4 is located immediately west of the Mississippi delta, 

zone 3 is off the Atchafalaya delta, and zone 1 is west of the Atchafalaya delta.

3.3.1 Chlorophyll Variability West of Mississippi River Delta (Zone 4)

Figure 4 shows the discharge from the Mississippi River for the period the time series 

SeaWiFS Chl data were analyzed using the wavelet transform (Figure 5). Seasonal variability 

with peaks in river discharge generally occurred in winter or spring, and a minimum in late 

summer. Highest discharge occurred in 1999, 2001, 2002, and 2005, and lowest discharge 

occurred in 2000 and 2006. 

Figure 4. Mississippi River discharge measured at Tarbert Landing, Mississippi. Obtained from the U.S. 

Army Corps of Engineers Web site.

Figure 5a shows the time variability in the wavelet power peaks at the 20 m isobath in zone 

4 (Figure 3), which occurs mainly in spring and occasionally in winter, that appears to coincide 

with peaks in river discharge. The dominant period of most of the peaks is approximately 

between 30 and 60 days. The strongest peak of wavelet power at the 20 m isobath occurred in 
9 

February 2004 that was related to a winter peak in river discharge and southerly wind stress 

over the region (Figure 6a). The normalized scale-averaged time series (averaged in the 30-100 

day band) at the 20 m isobath (Figure 5c) further revealed the regular seasonal variability and 

the interannual differences in the wavelet power. At the 500 m isobath the wavelet power is 

weak most of the years; significant chlorophyll variability occurs in the spring of 1998, 2003, 

and 2007. As shown in Figure 6b, the increases in wavelet power during these events were 

related mainly to the peaks in river discharge and southerly wind stress. However, in 

comparison with the 20 m isobath, the dominant periods of some of the peaks in Chl variance 

were of longer duration at the 500 m isobath (Figure 5b). 

Figure 5. Figures a and b show wavelet power spectra (mg m-3)2 of Chl averaged along the 20 and 

500 m isobaths in Zone 4 immediately west of the Mississippi River delta. Figures c and d 

show corresponding scale-averaged time series for the period band 30-100 days; the green 

line shows the 95% confidence interval.

Figure 6. QuikSCAT satellite derived wind stress vectors (N m-2) superimposed on SeaWiFS

derived Chl (mg m-3) for the months of (a) February 2004 and (b) March 2003.

days

(

) (

b)
10 

3.3.2 Chlorophyll Variability off the Atchafalaya Delta (Zone 3)

At the 20 m isobath off the Atchafalaya River (Figure 3) seasonal peaks in wavelet power 

spectra were observed with interannual variability that appeared reduced during some years 

(Figures 7a, 7c) in comparison with those off the Mississippi River. The large peaks in January 

1998 and February 2003 at the 20 m isobath were associated with northerly wind stress (not 

shown). 

Figure 7. Figures a and b show local wavelet power spectra (mg m-3)2 of the time series Chl at 20 and 

500 m isobaths in Zone 3 off the Atchafalaya River delta. Figures c and d show 

corresponding scale-averaged time series for the period band 30-100 days; the green line 

shows the 95% confidence interval. 

A strong peak variance observed in January 1999 at the 500 m isobath (Figure 7b, d) 

appeared to be due to the offshore transport of a large Chl plume just east of the Atchafalaya 

Bay (not shown). The role of wind stress in driving these high Chl waters offshore could not be 

ascertained because QuikSCAT wind data was not available during this period. 

3.3.3 Chlorophyll Variability West of the Atchafalaya River Delta (Zone 1)

A peak in wavelet power in June 2002 (Figure 8a and b) appeared to be linked to a peak in 

river discharge in early summer. West of the Atchafalaya River (zone 1) at the 100 m isobath, 

the strongest peak in wavelet power with a period of about 60 days appeared in September 2005 

(Figure 8a and b). Hurricane Rita, which made landfall in zone 1, appeared to have a longer 

duration impact on the chlorophyll variability in the region; this is indicated in Figure 9, which 

shows the surface chlorophyll distribution for September 2005. A large patch of elevated 

chlorophyll observed in the region of zone 1 is unusual in this region and may have been due to 

nutrient discharged from the ponding of flooded waters due to the hurricane.
11 

Figure 8. (a) Local wavelet power spectra (mg m-3)

2 of the time series chlorophyll at 100 m 

isobath in zone 1 west of the Atchafalaya River delta. (b) Corresponding scaleaveraged time series for the period band 30-100 days; the green line shows the 95% 

confidence interval.
12 

Figure 9. QuikSCAT satellite derived wind stress vectors (N m-2) superimposed on 

SeaWiFS derived chlorophyll (mg m-3) for the month of September 2005. 
13 

4.0 SUMMARY AND CONCLUSIONS

The Mississippi-Atchafalaya River system strongly influences the biogeochemical 

properties of the NGOM due to the discharge of freshwater and delivery of particulate and 

dissolved organic matter to the coastal waters. In this study we examined the spatial and 

temporal variation of CDOM absorption properties and chlorophyll concentrations along the 

Louisiana coast and the NGOM shelf waters. CDOM optical properties were found to be 

strongly influenced by riverine discharge. Additional factors, such as photobleaching and in situ 

biological production, also influenced the CDOM absorption properties. Estimates of seasonal 

distribution of CDOM obtained from SeaWiFS satellite data indicated highest levels of CDOM 

in the nearshore waters in winter; these gradually decreased in spring, summer, and fall, and are

potentially associated with a contraction of the nearshore region of freshwater influence. The 

seasonal impact of storms and hurricanes on CDOM distribution were studied using SeaWiFS 

CDOM imagery obtained before and after Hurricane Rita made landfall near the TexasLouisiana border. A comparison of the CDOM imagery indicated a decrease in CDOM and an 

increase in salinity of the nearshore waters, suggesting an intrusion of marine waters due to 

Hurricane Rita. The application of wavelet analysis to the time series SeaWiFS data revealed 

different scales of variability along the Louisiana coast, a region that is strongly influenced by 

the discharge from the Atchafalaya and Mississippi Rivers. The seasonal river discharge 

appears to strongly influence the timing of the peak variance in waters that are directly 

influenced by the two rivers. The use of monthly QuikSCAT derived wind stress vectors 

superimposed on monthly Chl maps provided some insights into the Chl variance determined by 

wavelet analysis. Away from the river influence the interannual peaks in Chl variance were 

weak with strongest wavelet power peak associated with hurricane activity in 2005. 
15 

5.0 REFERENCES

Carder, K.L., R.G. Steward, and G.R. Ortner. 1989. Marine and fulvic acids: their effect on 

remote sensing of ocean chlorophyll. Limnology and Oceanography 34:68-81. 

Chen, R.F., P. Bissett, P.G. Coble, R. Conmy, G.B. Gardner, M.A. Moran, X. Wang, M.L. 

Wells, P. Whelan, and R.G. Zepp. 2004. Chromophoric dissolved organic matter (CDOM) 

source characterization in the Louisiana Bight. Marine Chemistry 89:257-272. 

Conmy, R.N., P.G. Coble, R.F. Chen, and G.B. Gardner. 2004. Optical properties of colored 

dissolved organic matter in the Northern Gulf of Mexico. Marine Chemistry 89:127-144. 

Dinnel, S.P. and W.J. Wiseman. 1986. Freshwater on the Louisiana and Texas Shelf. 

Continental Shelf Research 6:765–784. 

Del Castillo, C.E. and R.L. Miller. 2008. On the use of ocean color remote sensing to measure 

the transport of dissolved organic carbon by the Mississippi River Plume. Remote Sensing of 

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APPENDIX A
21 

A peer reviewed, open-source article on CDOM absorption properties along the Louisiana 

coast was published with support from this funded study. This paper is included in its entirety 

beginning on the next page. 

Citation: D’Sa, E.J., and S.F. DiMarco. 2009. Seasonal variability and controls on 

chromophoric dissolved organic matter in a large river-dominated coastal margin. Limnology 

and Oceanography, 54(6), pp. 2233–2242. 

The spring to summer seasonal transition in the spatial and vertical distribution of 

chromophoric dissolved organic matter (CDOM) optical properties were examined during 

surveys conducted in March, May, July, and August of 2005 in the waters influenced by the 

Mississippi-Atchafalaya River system of the NGOM. Strong seasonal river influences and a 

well-correlated inverse relationship between CDOM absorption at 412 nm and salinity during 

spring of 2005 suggest conservative mixing in the water column between the riverine freshwater 

sources and the oceanic end members. Trends in the CDOM absorption and the spectral 

absorption slope (S) and deviations from the end member conservative mixing line in surface 

and bottom waters during the summer suggest apparent sources and sinks of CDOM. Elevated 

CDOM in lower salinity surface waters at many stations appear linked to high dissolved oxygen 

(DO) concentrations (> 10 mg L-1) and elevated chlorophyll concentrations. Effects of 

photooxidation in surface waters were observed as increasing S and enhanced CDOM loss in the 

~27-32 salinity range and attributed to seasonal stratification and the increased solar radiation 

during the summer. In the bottom waters, elevated CDOM in the ~32-36 salinity range were 

associated with low DO and relatively higher bacterial abundance; this suggests a potential 

CDOM source due to microbial remineralization of organic matter in the bottom hypoxic zone. 

Seasonal dynamics associated with CDOM loss in surface waters and gains in bottom waters 

may be another pathway influencing hypoxia in coastal waters. 
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