Abstract
Biological productivity in the northern Gulf is significantly affected by the Mississippi River. The freshwater discharge (577 km3 yr-1, approx 10% of the volume of water on the shelf) contains high concentrations of dissolved nutrients (100-150 μmol N03 l-1). Flow is primarily constrained by prevailing winds to the continental shelf west of the Mississippi Delta. River plumes are regions of high phytoplankton stock (>30 g Chi l-1) and production (5 g C m-2 d-1), high copepod stocks (nauplius concentrations >1000 l-1) and high ichthyoplankton stocks (larval concentrations >50 m-3). The high temperature of shelf waters assures high physiological rates, implying high rates of trophic transfer and high turnover rates. The primary fate of phytoplankton production is grazing by macrozooplankton and microzooplankton. However, sinking of phytoplankton and other organic material fuels the annual development of a band of hypoxic water along the Louisiana coast. Fisheries production is high; the northern Gulf supports the largest volume fishery in the United States, the Gulf menhaden, Brevoortia patronus. The Loop Current in its northernmost position affects shelf processes to the east of the Delta. Anticyclonic rings derived from the Loop Current occasionally impact on the Louisiana shelf west of the Delta but usually drift over to the western Gulf resulting in exchange of oceanic and shelf water off Texas.
The northern Gulf of Mexico is highly productive biologically. Portions of the shelf affected by the Mississippi River discharge have long been noted as regions of high phytoplankton stocks and productivity. This chapter presents an initial characterization of the shelf environment of the northern Gulf of Mexico, and to indicate some of the processes that support the high biological production of this region. The northern Gulf of Mexico is characterized by strong frontal boundaries, at the interface between river discharge plumes and receiving shelf waters, at the interface between the coastal boundary layer and mid-shelf waters, and at the interface between the outer-shelf waters and the open Gulf. Fishes of the northern Gulf of Mexico are generally considered by zoogeographers to be warm temperate, with close affinities to the ichthyofauna along the U. S. east coast from Cape Hatteras to about Cape Canaveral. Biological productivity in the northern Gulf of Mexico is dominated by discharge from the Mississippi River system.