Nitrogen exchange at the continental margin: a numerical study of the Gulf of Mexico.
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A two-layered baroclinic circulation model and a 21-layered biochemical model are used to explore the consequences of Loop Current - induced upwelling and terrestrial eutrophication on new production within the Gulf of Mexico. During a quasi-annual penetration and eddy-shedding cycle of the Loop Current, the simulated seasonal changes of incident radiation, wind stress, and surface mixed layer depth induce an annual cycle of algal biomass that corresponds to in situ and satellite time series of chlorophyll. The simulated nitrate fields match those of shipboard surveys, while fallout of particulate matter approximates that caught in sediment traps and accumulating in bottom sediments. Assuming an f ratio of 0.06-0.12, the total primary production of the Gulf of Mexico might be 105-210g C/(m squared)(year) in the absence of anthropogenic nutrient loadings, i.e. 2-3 fold that of oligotrophic regions not impacted by western boundary currents. Less than 25% of the nitrogen effluent of the Mississippi River may be stored in bottom sediments, with most of this input dispersed in dissolved form beneath the pycnocline, after remineralization of particulate detritus within several production cycles derived from riverine loading. At a sinking rate of 3m/d, however, sufficient phytodetritus survives oxidation in the water column to balance estimates of bottom metabolism and burial at the margins.
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