The northern Gulf of Mexico is the recipient of a large volume of water exported primarily from the Mississippi River Delta (radar image from NASA)and Atchafalaya Bay. The watershed of this region is essentially the endpoint for drainage of an estimated one third of the continental United States. Both agricultural and urban development throughout this region contribute to elevated nutrient levels observed in the Mississippi River which ultimately empty into the coastal waters of the Louisiana Bight. This nutrient input is considered to be one of the major factors associated with significant alterations to the structure and functioning of the adjacent shelf ecosystem. In this regard, the formation of large hypoxic zones in nearshore waters during the warm summer months has been linked to nutrient-enhanced eutrophication processes. The NECOP project encompasses a diverse body of research related to the dynamics of physical, geological, chemical, and bilogical processes as influenced by Mississippi River and Atchafalaya Bay runoff. The main objectives of the study were to describe:
The availability of inorganic nutrients and photosynthetically active irradiance essentially regulate the distribution and productivity of phytoplankton biomass. In the euphotic zone, phytoplankton rapidly deplete nutrients to undetectable levels via assimilation and subsequent incorporation into organic compounds. It is for this reason that primary production is often curtailed or limited by the relative concentrations of required nutrients. Nitrate, ammonia, phosphate, and silicate are normally supplied through the natural process of microbial mineralization of decomposing organic matter. The regenerated forms sustain a steady-state type ecosystem based on recycling processes characteristic of oligotrophic, "blue-water" regions.
"New" sources of these major nutrients may be supplied to coastal marine phytoplankton communities through various physical processes such as river run-off, upwelling, storm events, etc. In coastal waters of the northern Gulf of Mexico, new nutrients are continuously being supplied from the Mississippi River. Furthermore, river-water concentrations appear to be increasing as a result of anthropogenic inputs. The hypothesized sequence of events is as follows: i) new nutrients stimulate new primary production, ii) a certain proportion of this biomass remains un-grazed by primary consumers and eventually sinks out of the photic zone to reside in the bottom waters of the Louisiana and adjacent shelf regions, iii) microbial respiration of this biomass removes a substantial amount of the ambient dissolved oxygen from the bottom water layers, and iv) conditions of hypoxia and anoxia ensue.
The development of oxygen deficits is exacerbated by temporal changes in the physical conditions of shelf water masses. Thermal stratification of the water column during the warm summer months effectively maintains a barrier to prevent water column turnover which would regenerate oxygen supplies. The effects of this phenomenon include increased physiological stress (and eventual mortality) in organisms living within or on top of the bottom sediment or in the adjacent lower layers of the water column. Additionally, alterations in the concentrations and relative ratios of nutrients generally result in significant changes in phytoplankton community structure. Both species diversity and richness tend to decrease - often at the expense of those phytoplankton populations which make good food for higher organisms with corresponding blooms of "undesirable" ones. These shifts in phytoplankton species compositions are eventually translated up the food chain with potentially significant impacts on the natural balance in the structure and functioning of the coastal food web.
The following is a list of the NECOP research cruises by date, cruise designation, and research vessel:
* Atwood, D.K., A. Bratkovich, M. Gallagher, G.L. Hitchcock. 1994. Introduction to the dedicated issue. Estuaries. Vol. 17(4): 729-721.