ACCESS NUMBER: 30851 

STUDY TITLE: Northeastern Gulf of Mexico Physical Oceanography Program: 

Chemical Oceanography and Hydrography Study 

REPORT TITLE: Northeastern Gulf of Mexico Chemical Oceanography and 

Hydrography Study: Synthesis Report 

CONTRACT NUMBER(S): 1435-01-97-CT-30851 

SPONSORING OCS REGION: Gulf of Mexico 

APPLICABLE PLANNING AREA(S): Central and Eastern Gulf of Mexico 

FISCAL YEAR(S) OF PROJECT FUNDING: 1998, 1999, 2000, and 2001 

COMPLETION DATE OF REPORT: January 2002 

COST(S): FY 1998 $654,969; FY 1999 $700,571; FY 2000 $733,077; FY 2001 

$224,761; CUMULATIVE PROJECT COST: $2,313,378 

PROJECT MANAGER(S): Worth D. Nowlin, Jr. 

AFFILIATION: Texas A&M University 

ADDRESS: Department of Oceanography, 3146 TAMU, College Station, TX 77843-

3146 

PRINCIPAL INVESTIGATOR(S)*: Douglas C. Biggs, Norman L. Guinasso, Jr., 

Matthew K. Howard, Ann E. Jochens, Mahlon C. Kennicutt II, Worth D. Nowlin, Jr., and 

Robert O. Reid 

KEY WORDS: Physical oceanography, Gulf of Mexico, Mississippi-Alabama shelf, west 

Florida shelf, circulation, hydrography, Loop Current, eddies, temperature, salinity, 

nutrients, dissolved oxygen, particulate matter, pigments. 

BACKGROUND: The Minerals Management Service (MMS) Gulf of Mexico Outer 

Continental Shelf Region sponsored a workshop on 5-7 April 1994 in Tallahassee, FL, 

to assess the knowledge of the circulation over the shelf and slope of the northeastern 

Gulf of Mexico and to provide recommendations on studies needed to fill gaps in 

knowledge. MMS then designed the Northeastern Gulf of Mexico Physical 

Oceanography Program (NEGOM). This summary presents the main results of the 

NEGOM Chemical Oceanography and Hydrography Study. 

OBJECTIVES: (1) To develop and conduct an effective and efficient oceanographic 

experimental design of research cruises over the NEGOM region at spatial and 

temporal scales sufficient to resolve seasonal variations of chemical oceanography and 

hydrographic properties. (2) To collect the ancillary data necessary to complement and 
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analyze the data collected under objective 1. (3) To analyze the data to describe the 

vertical and horizontal spatial distributions and temporal variations of properties and the 

chemical, physical, or biological processes that contribute to the observed distributions. 

DESCRIPTION: The study area is bounded by 89°W on the west, the 10-m isobath 

inshore, 27.5°N on the southeast, and the 1000-m isobath offshore. Nine 

oceanographic research cruises were conducted aboard the R/V Gyre over three field 

years from November 1997 to August 2000. During each field year, one cruise was 

conducted in spring (April/May), summer (July/August), and fall (November) seasons. 

Continuous profiles were made of temperature, salinity, pressure, light transmission, 

fluorescence, and downwelling irradiance using a Sea-Bird 911plus CTD system. A 

total of 883 CTD stations were occupied. Discrete water samples were drawn at each 

CTD station for analyses of dissolved oxygen, nutrients (nitrate, phosphate, silicate, 

nitrite, ammonium, and urea), pigments, particulate matter, and particulate organic 

carbon. Four to twelve discrete samples per station were taken, totaling in excess of 

8000 oxygen and nutrient samples, 1500 pigment and particulate matter (PM) samples, 

and 1000 particulate organic carbon (POC) samples. Over 800 expendable 

bathythermograph measurements were made. Acoustic Doppler current profiler 

(ADCP) observations, using a 150 kHz ADCP, and underway flow-through 

measurements of near-surface temperature, salinity, and fluorescence were made 

continuously along track. Nearly 1000 near-surface chlorophyll samples were taken to 

calibrate the underway, near-surface fluorescence data for determination of calculated 

chlorophyll. Data from a 38 kHz ADCP were collected on three surveys. Ancillary data 

sets, such as river discharge, sea surface height fields from satellite altimeter, and 

meteorological measurements, were assembled to support the data analysis and 

interpretation. 

SIGNIFICANT CONCLUSIONS: A general conclusion of this study is that interannual 

variability overshadows seasonal signals. This is due to a number of factors. First, the 

offshelf circulation, consisting of both cyclonic and anticyclonic eddies as well as the 

Loop Current and filaments thereof, is very energetic, highly variable, and greatly affects 

the shelf circulation, particularly over the Mississippi-Alabama shelf and near DeSoto 

Canyon. These features are not seasonal or deterministic in occurrence. Second, 

although there is a clear seasonal pattern in the climatological winds over the study 

area, the observations show there is much interannual variability. Moreover, the 

variability in direction from downcoast to upcoast occurs with greater frequency than in 

the northwestern Gulf shelves, leading to rapid shifts in direction of the wind-driven 

coastal current. Thus, the seasonal cycle of wind forcing over the inner shelf is neither 

so strong nor so regular as that needed to ensure a strongly seasonal coastal current 

regime. Finally, the discharge of rivers to this area follows a complicated pattern, 

particularly as compared to the northwestern Gulf shelves. The dominant river is the 

Mississippi and its discharge is often carried eastward along the outer continental shelf 

or over the slope, leading to a pattern of decreasing offshore salinity over the mid to 

outer shelf and consequent buoyancy effects. These factors also influence the 

distributions of water properties and chemical parameters, such as salinity, dissolved 
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oxygen, nutrients, light transmission, particulate matter, particulate organic carbon, 

chlorophyll a, and other pigments. 

STUDY RESULTS: Realizing the large interannual variability and the fact that only 

three realizations of the circulation for each season were available, space-time patterns 

over the area nevertheless were sought. Key results regarding the circulation are: (1) 

offshelf circulation features appear to account for about three-quarters of the variance of 

the near-surface circulation of the study area, (2) nearshore currents may be driven by 

local/regional winds. However, the wind regime in the study area is quite variable on 

both short and interannual time scales, rendering climatological predictions of 

diminished value. Only some ten percent of the variance of the near-surface circulation 

observed during the NEGOM study showed a seasonal pattern associated with the wind 

regime, (3) about 15% of the variability of near-surface circulation in the study region is 

associated mainly with currents over the inner shelf and may be attributed to the effects 

of local winds and river discharge, and (4) variability in both circulation and property 

distributions is judged considerably greater in the western than eastern study region. 

Dissolved oxygen patterns near surface reflected the relationship of enhanced oxygen 

solubility with decreasing temperature and salinity. The effect of river discharge, with its 

lower salinity, was to enhance the oxygen concentrations in the surface waters. At most 

stations with high chlorophyll a, water samples were highly supersaturated in oxygen, 

illustrating the potential effect of primary production on dissolved oxygen 

concentrations. Bottom dissolved oxygen concentrations were greatest in fall and 

lowest in summer. No hypoxic conditions were observed during the cruises, but very 

low concentrations were observed at a few stations over the inner shelf during two 

spring and two summer cruises. Results showed that, regardless of season, nutrient 

concentrations in the photic zone were elevated in areas of enhanced river water 

influence and of uplift of density surfaces by dynamic processes, such as the presence 

of cyclones near the shelf edge, divergence between cyclone-anticyclone pairs, and 

wind or bottom-induced upwelling. Oxygen and nutrient concentrations were examined 

on density surfaces corresponding to water masses found in the Gulf. No relative 

dissolved oxygen maximum associated with 18°C Sargasso Sea Water was found. A 

relative oxygen minimum associated with Tropical Atlantic Central Water near 425 m 

and a relative nutrient maxima (nitrate, phosphate) associated with Antarctic 

Intermediate Water near 750 m were observed. 

Cruise average water column PM mass was lower over the eastern shelf (east of 87°W) 

than the western shelf. Similarly, the cruise average percent of PM per unit surface 

area contained in bottom nepheloid layers over the shelf was much lower and less 

variable over the eastern than western shelf. This is evidence for the effects of the 

Mississippi and other rivers on PM and light transmission, as well as suggestive of more 

re-suspension on the western shelf. PM values showed lower mean near-surface 

concentrations in fall and most variability in spring. Near-bottom means were similar for 

the three seasons, although the smallest variability was observed in summer. Highest 

POC levels in all seasons generally were associated with high PM plumes near river 

mouths. Near-surface POCs were greater and evidenced more variability than sub-
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surface concentrations. Indicative of phytoplankton productivity in the photic zone and 

remineralization of organic carbon in the water column, the quantity of POC accounting 

for PM was more variable near surface than within the water column. Near-bottom 

particulates also may have a contribution from re-suspended sediments relatively poor 

in organic carbon. 

As determined from pigment data, the four major algal groups present in the NEGOM 

area were prymnesiophytes, prochlorophytes, pelagophytes, and cyanobacteria, 

although locally high abundances of other groups were detected. Prymnesiophytes 

were the dominant group on each cruise, accounting for between 31% and 46% of the 

chlorophyll present at the deep chlorophyll maximum (DCM). Prochlorophytes, 

pelagophytes, and cyanobacteria were the second most abundant groups in the DCM 

on four, three, and two cruises, respectively, accounting for 13-23%, 6-18%, and 0-21% 

of the chlorophyll a. High concentrations of chlorophyll a were found in areas influenced 

by discharges of the Mississippi and Apalachicola rivers. For the study region as a 

whole, the average surface and DCM chlorophyll concentrations were lower in boreal 

fall-winter (November–March) than boreal spring-summer (April–October); the opposite 

relationship would be expected for offshore Gulf waters. This was a consequence of (1) 

the fact that most stations were over the shelf rather than offshore, (2) summer 

entrainment and transport offshelf of low-salinity Mississippi River water with enhanced 

nutrient levels, and (3) the presence of various offshelf eddies that contributed 

enhanced nutrient levels to the photic zone during summer by uplift of density surfaces. 

As expected, however, average surface chlorophyll a values in waters of depth greater 

than 400 m were several times higher in fall than in spring. 

STUDY PRODUCT(S): Jochens, Ann E., Steven F. DiMarco, Worth D. Nowlin, Jr., 

Robert O. Reid, and Mahlon C. Kennicutt II. 2002. Northeastern Gulf of Mexico 

Chemical Oceanography and Hydrography Study: Synthesis Report. OCS Study MMS 

2002-055. U.S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico 

OCS Region, New Orleans, LA. 570 pp. 

Jochens, A. E., and W. D. Nowlin, Jr. 2000. Northeastern Gulf of Mexico Chemical 

Oceanography and Hydrography Study, Annual Report: Year 3. OCS Study MMS 2000-

078. U. S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region, New Orleans, LA. 89 pp. 

Jochens, A. E., and W. D. Nowlin, Jr. 1999. Northeastern Gulf of Mexico Chemical 

Oceanography and Hydrography Study, Annual Report: Year 2. OCS Study MMS 99-

0054. U. S. Dept. of the Interior, Minerals Management Service, Gulf of Mexico OCS 

Region, New Orleans, LA. 123 pp. 

Jochens, A. E., and W. D. Nowlin, Jr. 1998. Northeastern Gulf of Mexico Chemical 

Oceanography and Hydrography Study between the Mississippi Delta and Tampa Bay, 

Annual Report: Year 1. OCS Study MMS 98-0060. U. S. Dept. of the Interior, Minerals 

Management Service, Gulf of Mexico OCS Region, New Orleans, LA. 126 pp. 
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