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STUDY TITLE: Benthic-Pelagic Coupling in the Gulf of Mexico: Implications for the 

Fate of Organic Matter on the Continental Shelf 

REPORT TITLE: Ecosystem Analysis of the Louisiana Bight and Adjacent Shelf 

Environments, Volume I: The Fate of Organic Matter and Nutrients in the Sediments of 

the Louisiana Bight and Volume II: Bibliography of Ecological Studies 

CONTRACT NUMBER: 14-35-0001-30696 

SPONSORING OCS REGION: Gulf of Mexico Region 

APPLICABLE PLANNING AREA: Western and Central Gulf of Mexico 

FISCAL YEARS OF PROJECT FUNDING: 1994, 1995 

COMPLETION DATE OF REPORT: 1 June 1995 

COSTS: FY 94: $132,024: FY 95: $67,977: CUMULATIVE PROJECT COSTS: 

$200,000 

PROJECT MANAGER: R. Twilley 

AFFILIATION: Department of Biology 

ADDRESS: University of Southwestern Louisiana, Lafayette, LA 70504 

PRINCIPAL INVESTIGATOR*: B. McKee 

KEY WORDS: Western Gulf of Mexico; Central Gulf of Mexico; Texas-Louisiana Shelf; 

Mississippi River plume; Louisiana Bight; Mississippi Sound; delta; biogeochemistry; 

benthic-pelagic coupling; organic matter; nutrients; burial; sedimentation; carbon; 

nitrogen; benthos; ecosystem studies; petroleum impacts; benthic fluxes; nutrient 

regeneration; benthic respiration 

BACKGROUND: One of the major objectives of the ecosystem ecology program at 

University of Southwestern Louisiana is to understand the fate of nutrients and organic 

matter in the Mississippi River Plume and adjacent continental shelf. Individual studies 

indicate that the fate of this organic matter appears to vary, at times fluxing to the 

sediments and undergoing mineralization or transport, while at other times appearing to 

be efficiently grazed and remineralization in the water column. Previous studies od 

deposition suggest that the proximal region of the Mississippi River plume is a zone of 

extreme input of allochthonous particulate material to the seabed. The coupling of these 

benthic-pelagic processes is central to understanding the dynamics of the TexasLouisiana shelf ecosystem. Understanding the natural processes of this shelf 

ecosystem will lead to better attempts of evaluate human impacts in the region such as 

oil and gas activities. This is particularly complex given the additional effects of nutrient 
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enrichment to this OCS region due to inputs from the Mississippi and Atchafalaya 

Rivers. 

OBJECTIVES: One objective of this project was to provide an overview of existing 

information on the Louisiana-Texas continental shelf ecosystem by developing an 

annotated bibliography of existing ecological studies in this region of the Gulf of Mexico. 

In addition, the more complete analysis of existing sediment samples collected on 

previous studies of the Louisiana Bight provided new information on the role of benthic 

processes in affecting the fate of carbon and nitrogen in a river dominated shelf 

ecosystem. This information will contribute to findings from other studies in the central 

and western OCS regions of the Gulf of Mexico, to understand the coupling of the fifth 

largest river in the world to coastal margin ecosystems in the Louisiana-Texas region. 

DESCRIPTION: Box core samples were collected from 42 stations in Mississippi River 

plume region of the Louisiana Bight during the LASER 3 cruise in April 1989. Depths of 

water column ranged from 10 to 110 m and nearly 90 km from the mouth of Southwest 

Pass of the Mississippi River delta. Large diameter (16.5 cm) subcores were taken from 

box cores at each sampling station. Sediment subsamples were assayed for 210Pb and 

234Th to determine burial (100 yr time scale) and deposition (3 mo time scale) in the 

dispersal study area. Total carbon and nitrogen were assayed with a LECO Elemental 

Analyzer. Total phosphorus was determined by dissolving the ashed samples with Hcl 

and determining PO4 concentrations. The deposition and accumulation (burial) of 

carbon, nitrogen and phosphorus in bottom sediments was calculated from the following 

equation (Hatton et al. 1983): A = Cd x R x D x 104 (g m-2 yr-1); where A is the rate of 

nutrient deposition or burial, Cd is the dry mass nutrient concentration, D is bulk density, 

and R us the sedimentation rate (determined by 234Th or 210Pb). 

Intact sediment cores from stations in the shelf were incubated aboard ship under near 

ambient conditions to determine the exchange of inorganic nitrogen (ammonium, nitrate, 

nitrite), phosphate, silicate and dissolved oxygen across the sediment-water interface. 

A flow-through design in which an autoanalyzer pump (Cole Palmer) delivers the 

experimental solutions from reservoirs at controlled flow rates (typically 4 mL/min) 

through influent and effluent lines connected to the cores was used during each cruise 

(Koike and Hattori 1978, Miller-Way and Twilley in press). Pore water nitrogen, 

phosphate, and silicate concentrations were determined with standard colorimetric 

techniques (Strickland and Parsons) using an Alpkem RFA/2 autoanalyzer. Total 

nitrogen and phosphate on water and pore water samples were assayed using 

persulphate digestion techniques followed by assay of nitrate and phosphate using 

autoanalyzer techniques. 

SIGNIFICANT CONCLUSIONS: Results reveal that approximately 30% of the 

particulate organic carbon supplied by the Mississippi River to the adjacent shelf is 

remineralized within 4 months and an additional 40% is remineralized on a decadal time 

scale. Based on these results, combined with similar information from the Amazon, 

Changjiang and Huanghe Rivers, we conclude that only 30% of the particulate organic 

carbon supplied to the ocean by rivers is buried. Our area-weighted estimate of organic 
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carbon burial in deltaic-shelf sediments is 75 gC m-2 yr-1 with less than 50% of the 

organic carbon buried being of terrestrial origin. The resulting gobal estimate for 

terrigenous POC burial is 0.05 PgC yr-1, representing less than half the value currently 

used to constrain global CO2 budgets (Sarmiento and Sundquist 1992). 

There are distinct spatial and seasonal rates in benthic nutrient regeneration that are 

strongly linked to the discharge of Mississippi River. Zones of peak nutrient 

regeneration coincide with areas of high deposition of both terrigenous sediments and in 

situ primary production. Maximum rates of ammonium regeneration may provide up to 

40% of the nitrogen demand by primary producers. 

STUDY RESULTS: Chapters in this technical summary describe the accumulation and 

regeneration of sediment, carbon, nitrogen, and phosphate from the seabed in the 

plume region of the Louisiana Bight. Strong variation in sedimentation and nutrient 

accumulation rates in 5417 km2

 plume region of the Louisiana Bight indicate the 

problem of accurately determining reliable estimates for these processes in riverdominated shelf ecosystems. The area-weighted sedimentation rate was 4,442 g m-2

yr-1, carbon accumulation was 64, nitrogen 7.02 and phosphorus 2.89 g m-2 yr-1. 

Seasonal deposition of allochthonous organic matter during spring provides the 

predominant mechanism for sustaining peak rates of benthic regeneration in the plume 

region of the shelf. During April, ammonium regeneration in the plume region (station 

B50 and C50) may exceed 500 μmol m-2 h-1, compared to fluxes of less than 200 μmol 

m-2 h-1 during low river discharge in September and October. Further downfield 

(stations D50 and E50), ammonium regeneration is generally less than 200 μmol m-2 h1

. Seasonal differences in benthic regeneration are most evident nearest the mouth of 

the Mississippi River, while rates are more constant both temporally and spatially in 

more distant regions. The link between sediment deposition and benthic nutrient 

regeneration is clearly demonstrated in results of silicate flux at near and far - field 

stations on the Louisiana Bight. Fluxes of silicate across the sediment-water interface 

to the water column increase linearly with deposition rates from 0.3 to 2.5 cm /mo, 

reaching maximum silicate flux rates of 550 μmol m-2 h-1. Above a deposition rate of 2.5 

cm/mo, silicate flux was lower at less than half the maximum rates. However, there was 

no clear pattern in ammonium or phosphate regeneration with bulk deposition of 

sediments in the study area; suggesting that the quality of material must also be 

evaluated. 

Rates of benthic nutrient flux tended to decrease with distance from the riverine source, 

and exhibited contours of rates similar to patterns of sedimentation and nutrient 

accumulation. Modeling nutrient fluxes from pore water gradients showed little 

correlation to the actual measured nutrient flux; however these sediment characteristics 

do provide explanations for outliers in benthic regeneration rates. Benthic fluxes always 

showed a higher correlaiton to some qualitative index (input of carbon, nitrogen, 

phosphorus, or chlorophyll a) of deposition to the seabed than to the quantitative input 

of sediment. The combination of geochronologies on time scale of months and decades 

along with repeated measures of benthic nutrient regeneration provide spatial and 
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temporal resolution of these benthic processes in this river-dominated continental shelf 

ecosystem. 

STUDY PRODUCTS: Twilley, R.R. 1995. Ecosystem Analysis of the Louisiana Bight 

and Adjacent Shelf Environments. Vol. I. Bibliography of Ecological Studies. OCS 

study/MMS No. U.S. Dept. of the Interior, Minerals Mgmt. Service, Gulf of Mexico OCS 

Regional Office, New Orleans, La. 

Twilley, R.R. and B. McKee. 1995. Ecosystem Analysis of the Louisiana Bight and 

Adjacent Shelf Environments. Vol. II. The Fate of Organic Matter and Nutrients in the 

Louisiana Bight. OCS study/MMS No. U.S. Dept. of the Interior, Minerals Mgmt. 

Service, Gulf of Mexico OCS Regional Office, New Orleans, La. 

McKee, B. and R.R. Twilley. The burial and remineralization of terrigenous organic 

carbon in deltaic-shelf environments. Manuscript submitted to NATURE. 

Bourgeois, J. 1994. Patterns of Benthic Nutrient Regeneration on the Louisiana 

Continental Shelf. Master of Science Thesis, University of Southwestern Louisiana. 73 

pp. 

*P.I.’s affiliation may be different than that listed for Project Manager.