The second largest zone of coastal hypoxia (oxygen-depleted waters) in the world is found on the northern Gulf of Mexico continental shelf adjacent to the outflows of the Mississippi and Atchafalaya Rivers. The combination of high freshwater discharge, wind mixing, regional circulation, and summer warming controls the strength of stratification that goes through a well-defined seasonal cycle. The physical structure of the water column and high nutrient loads that enhance primary production lead to an annual formation of the hypoxic water mass that is dominant from spring through late summer. Paleoindicators in dated sediment cores indicate that hypoxic conditions likely began to appear around the turn of the last century and became more severe since the 1950s as the nitrate flux from the Mississippi River to the Gulf of Mexico tripled. Whereas increased nutrients enhance the production of some organisms, others are eliminated from water masses (they either emigrate from the area or die) where the...

Gulf of Mexico Hypoxia, A.K.A. “The Dead Zone”

Abstract The characteristics of mid-latitude shelf seas are primarily controlled by seasonal cyclicity in wind-driven mixing. Seasonal dysoxic-anoxic conditions may occur in summer in salinity-stratified estuarine or pro-delta settings, or more extensively in those open shelf areas where a total depth of ≤60 m and the seasonal thermocline result in a bottom water layer ≤10 m thick. When bottom circulation is limited the oxygen stored in this layer may become periodically exhausted if climatic factors extend the stratified period to seven or more months, or if there is additional organic loading (via flagellate blooms and/or pollution). This causes widespread mortalities and a shift to soft-bodied, non-fossilizing benthic faunas. Our review supports seasonal dysoxia-anoxia as being the best model to account for the key characteristics of many ancient epeiric sea black shales. It would appear that the latter rarely represent true continuous anoxia except locally in more-confined deeper sub-basins. We recommend that the following terms should be applied to environments, facies, or oxygenation levels: oxic (8.0–2.0 ml/l O2), dysoxic (2.0–0.2 ml/l), suboxic (0.2–0.0 ml/l) and anoxic (0.0 ml/l). The corresponding biofacies terms are: aerobic, dysaerobic, quasianaerobic (laminated, without macrofauna, but with in situ benthic microfauna) and anaerobic. Hypoxic and normoxic should only be used with regard to the physiological responses of living organisms.

Modern and ancient continental shelf anoxia: an overview

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Beyond Science into Policy: Gulf of Mexico Hypoxia and the Mississippi River

Ths report addresses the following two questions: 
1) What are the loads (flux) of nutrients transported from the Mississippi-Atchafalaya River Basin to the Gulf of Mexico, and where do they come from within the basin? 
2) What is the relative importance of specific human activities, such as agriculture, point-source discharges, and atmospheric deposition in contributing to these loads? 
These questions were addressed by first estimating the flux of nutrients from the Mississippi-Atchafalaya River Basin and about 50 interior basins in the Mississippi River system using measured historical streamflow and water quality data. Annual nutrient inputs and outputs to each basin were estimated using data from the National Agricultural Statistics Service, National Atmospheric Deposition Program, and point-source data provided by the USEPA. Next, a nitrogen mass balance was developed using agricultural statistics, estimates of nutrient cycling in agricultural systems, and a geographic information system. Finally, multiple regression models were developed to estimate the relative contributions of the major input sources to the flux of nitrogen and phosphorus to the Gulf 
of Mexico.

Flux and Sources of Nutrients in the Mississippi-Atchafalaya River Basin

Seasonally severe and persistent hypoxia, or low dissolved oxygen concentration, occurs on the inner- to mid-Louisiana continental shelf to the west of the Mississippi River and Atchafalaya River deltas. The estimated areal extent of bottom dissolved oxygen concentration less than 2 mg L-1 during mid-summer surveys of 1993-2000 reached as high as 16,000 to 20,000 km2. The distribution for a similar mapping grid for 1985 to 1992 averaged 8000 to 9000 km2. Hypoxia occurs below the pycnocline from as early as late February through early October, but is most widespread, persistent, and severe in June, July, and August. Spatial and temporal variability in the distribution of hypoxia exists and is, at least partially, related to the amplitude and phasing of the Mississippi and Atchafalaya discharges and their nutrient flux. Mississippi River nutrient concentrations and loadings to the adjacent continental shelf have changed dramatically this century, with an acceleration of these changes since the 1950s to 1960s. An analysis of diatoms, foraminiferans, and carbon accumulation in the sedimentary record provides evidence of increased eutrophication and hypoxia in the Mississippi River delta bight coincident with changes in nitrogen loading.

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Hypoxia in the Gulf of Mexico.

Measurement of a negatively buoyant plume in the coastal waters off freeport, Texas

A bucket wheel dredge system for offshore tin mining beyond the 50m water depth

Numerical Simulation On Dynamics of Suction Piles During Lowering Operations

Laboratory experiments were conducted in a wave flume to measure wave conditions (height, period, and kinematics) that caused movement of submerged ordnance and cylinders. Laboratory results were analyzed and two empirical theories were developed for predicting movement based on wave conditions and ordnance/cylinder characteristics: (1) modified Shields parameter approach comparing ordnance and cylinder parameters with a wave Shields parameter to identify critical regimes for ordnance/cylinder movement; and (2) Keulegan-Carpenter number (KC) approach comparing the ratio of maximum orbital velocity at the ordnance/cylinder surface and ordnance/cylinder diameter with relative water depth. Evaluation of these two approaches with the laboratory data indicated that the best predictive technique appeared to be the modified Shields parameter approach. A field deployment of two drogue cylinders instrumented with acoustic pingers was conducted to further investigate ordnance movement using acoustic tracking procedures. Measured wave conditions and cylinder characteristics were used in the modified Shields parameter approach and indicated that the cylinders would be mobile. The field deployment of two cylinders verified the prediction results showing net movements of 15 and 13 m, respectively.

Submerged Ordnance and Unrestrained Cylinder Movement in Coastal Zone

A nearly continuous discharge of a brine solution was begun in March 1980 through a submerged multiport diffuser to the Gulf of Mexico at a nominal rate of 3.7 \times 10^{4} m^{3} /day. The brine discharge is the result of leaching and filling of storage caverns in the salt dome at the Bryan Mound site of the Department of Energy's Strategic Petroleum Reserve Program. The purpose of this paper is to summarize the results of tracking the salinity plumes. The tracking procedure consists of towing a salinity sensor on the sea floor through the diffuser area. The data are used to construct isohaline contours which are used to evaluate the areal extent of the plume. Vertical salinity profiles are also measured to determine the vertical extent of the plume. The results show the largest areal extent of the +1\permil above ambient contour was 17.9 km2. The highest above ambient contour was +6\permil which covered an area of 0.04 km2. The average vertical extent was 5.4 m directly over the diffuser.

Robert E. Randall

Papers

Measurement of a negatively buoyant plume in the coastal waters off freeport, Texas

A bucket wheel dredge system for offshore tin mining beyond the 50m water depth

Numerical Simulation On Dynamics of Suction Piles During Lowering Operations

Submerged Ordnance and Unrestrained Cylinder Movement in Coastal Zone

Tracking of salinity plumes generated by brine discharges into the Northwestern Gulf of Mexico