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.

Mathieu instability of a spar platform with mooring and risers

Hybrid wave model for unidirectional irregular waves—part I. Theory and numerical scheme

Nearshore hypoxia in the bottom water of the northwestern gulf of Mexico from 1981 to 1984

Hybrid wave model for unidirectional irregular waves—part II. Comparison with laboratory measurements

This report summarizes and presents design knowledge for both the functional and structural design of detached breakwaters for shoreline stabilization. Functional design of break:waters in the United States relies on a significant amount of engineering judgement, data from a few existing breakwater projects for comparison, and an understanding of basic coastal processes. The design process is an iterative one. Design guidance used to predict beach response to breakwaters is also presented in Dally and Pope (1986), Pope and Dean (1986), Rosati (1990), and Engineer Manual 1110-2- 1617. Guidance on the use of low-crested rubble-mound breakwaters for wetland development purposes is limited, and has been mostly based on experience from a few prototype sites. Ongoing research at WES under the Wetlands Research Program is further investigating and evaluating the use of breakwaters for these purposes.

Robert E. Randall

Papers

Mathieu instability of a spar platform with mooring and risers

Hybrid wave model for unidirectional irregular waves—part I. Theory and numerical scheme

Nearshore hypoxia in the bottom water of the northwestern gulf of Mexico from 1981 to 1984

Hybrid wave model for unidirectional irregular waves—part II. Comparison with laboratory measurements

Engineering Design Guidance for Detached Breakwaters as Shoreline Stabilization Structures