An observational study of the Mississippi-Atchafalaya coastal plume: Final Report, OCS Study MMS 98-0040.

Murray, S.P., 1998.

The Mississippi River Plume Hydrography study (LATEX B) is part of a larger Louisiana-Texas Physical Oceanography Program, which conducted field observations from 1992-1994. The objective of this component of the program was to characterize the hydrographic and the velocity structure, the pollutant chemistry, and biological properties of the coastal plume arising from the discharge of the Mississippi and Atchafalaya Rivers. A general geographic map of the area is presented in Figure 1.

This final report of the project presents new data from our cruises in 1994, including the one cruise designed to sample the near-field plume of the Mississippi River. More importantly, this report synthesizes the data from all six cruises over a three-year period. Considering the paucity of knowledge prior to our study, the original objectives were laid out without the benefit of extensive supporting data and reflected the physical processes identified at an MMS-sponsored workshop (Symposium on the Physical Oceanography of the Louisiana/Texas (LA/TX) Shelf, Galveston, Texas, May 24-26, 1989) as important in this area. After five years of cruises and data analyses, the complexity of detail and the extreme spatial and temporal variability in the velocity and property fields of the Mississippi-Atchafalaya coastal plume has now come to light. Prior to this project, detailed knowledge of this coastal plume was sorely lacking.

The Mississippi-Atchafalaya River system typically has peak discharge in April in excess of 30,000 m3/s and a low in September to October of about 10,000 m3/s. Satellite images and scattered observations of the hydrographic and current regimes of the coastal waters from the Mississippi Delta west and south to the Texas-Mexico border indicate the presence of brackish water, turbid plume emanating from this discharge. This plume is strongly modulated and even reversed by the annual cycle of the winds from Louisiana to south Texas. Intense northernly wind events associated with frontal passages in late fall, winter, and early spring apparently completely disrupt the Atchafalaya source of the coastal current and its dissolves and suspended sediment particulate load. We expected the inner shlef distribution of waters and sediments to result from the interaction of momentum, buoyancy forces, winds, waves, longshore currents, and pressure gradients (sea surface slope and density gradients).

Fresh water distribution has heretofore been the most valuable tracer of large-scale water motion in the LATEX region. The fresh water content in the coastal plume and on the shelf is clearly an annual cycle triggered by the spring flood on the Mississippi-Atchafalaya Rivers (Dinnel and Wiseman, 1986). The Mississippi-Atchafalaya discharge is advected westerly and southerly along the LATEX coastline even as far as Mexico by downcoast wind components from the time of spring flood until early summer. The onset of strong southerly and southeasterly winds in early summer off Mexico and south Texas then exert upcoast wind stress components on the low salinity layer near the coast, reversing the flow, causing a convergence in the coastal currents, and advecting low salinity water offshore as Ekman transport in the surface layer. The result fo this activity is that nearly the entire LATEX shelf east of Galveston Bay out to the 200 m isobath is covered with low salinity water by late July. With the slackening of the strong southerly and southeasterly winds in late summer, downcast flow returns to the LATEX region and downwelling favorable winds re-establish the low salinity coastal current (Cochrane and Kelly, 1986). Salinity in the south Texas coastal current decreases as water of northerly origin advects back into the region (Smith, 1980). Salinities gradually increase in October all along the LATEX coastal plume region as the major rivers approach low river discharge.

The link between the dnamics of the coastal plume and its chemistry and biology is nowhere more apparent than off central Louisiana. The Mississippi and Atchafalaya Rivers (a third of the total flow of the Mississippi River system enters the Gulf of Mexico via the Atchafalaya River) are the major source of "new" nutrients to phytoplankton on the inner and mid-shelf. Since the 1950s, water quality in the Mississippi River has changed dramatically (Meade and Parker, 1985; Smith et al., 1987; Turner et al., 1987). Suspended sediments and silicate concentrations have decreased since the 1950s, wheras nitrogen and phosphorous loading have increased. These large changes, reulting from human activities, provide a basis for concern that this loading, combined with increased water clarity, have resulted in a "eutrophication" effect on the shelf. The area impacted by, and the duration of, hypoxia on the Louisiana shelf is of considerable concern since Louisiana fisheries are 28% of the U.S. Total. Fish, shrimp, and benthic annual densities are severely commercially imporant species (Renaud, 1986) may be affected. Of more recent concern is the content of chemical pollutants as they are transported via fresh water into the open Gulf waters.

Accordingly, this project has focused on the structure and dynamics of the coastal plume, its nutrient and pollutant chemistry, its biological characteristics--phytoplankton, zooplankton and ichthyology--and the characteristics of its suspended sediment. The extensive use of satellite-acquired remotely sensed data added a unifying element to the interpretation of the physical, biological, and chemical data.

The following chapters present our first real attempt to explore these links. The impressive sum of data gathered during our project and the conclusions reached from its analysis will allow for an increasingly more detailed understanding of the coastal plume system in the future.

Ref: U.S. Dept. of the Interior, Minerals Mgmt. Service, Jan. 1, 1998

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