NUTRIENT CHANGES IN THE MISSISSIPPI RIVER AND SYSTEM RESPONSES ON THEADJACENT CONTINENTAL-SHELF

The Mississippi River system ranks among the world's top 10 rivers in freshwater and sediment inputs to the coastal ocean. The river contrib utes 90% of the freshwater loading to the Gulf of Mexico, and terminat es amidst one of the United States' most productive fisheries regions and the location of the largest zone of hypoxia in the western Atlanti c Ocean. Significant increases in riverine nutrient concentrations and loadings of nib-are and phosphorus and decreases in silicate have occ urred this century, and have accelerated since 1950. Consequently, maj or alterations have occurred in the probable nutrient limitation and o verall stoichiometric nutrient balance in the adjacent continental she lf system. Changes in the nutrient balances and reduction in riverine silica loading to the continental shelf appear to have led to phytopla nkton species shifts offshore and to an increase in primary production . The phytoplankton community response, as indicated by long-term chan ges in biological uptake of silicate and accumulation of biologically bound silica in sediments, has shown how the system has responded to c hanges in riverine nutrient loadings. Indeed, the accumulation of biol ogically bound silica in sediments beneath the Mississippi River plume increased during the past two decades, presumably in response to incr eased nitrogen loading. The duration, size, and severity of hypoxia ha s probably increased as a consequence of the increased primary product ion. Management alternatives directed at water pollution issues within the Mississippi River watershed may have unintended and contrasting i mpacts on the coastal waters of the northern Gulf of Mexico.