A PRELIMINARY MASS-BALANCE MODEL OF PRIMARY PRODUCTIVITY AND DISSOLVED-OXYGEN IN THE MISSISSIPPI RIVER PLUME INNER GULF SHELF REGION

A deterministic, mass balance model for phytoplankton, nutrients, and dissolved oxygen was applied to the Mississippi River Plume/Inner Gulf Shelf (MRP/IGS) region. The model was calibrated to a comprehensive s et of field data collected during July 1990 at over 200 sampling stati ons in the northern Gulf of Mexico. The spatial domain of the model is represented by a three-dimensional, 21-segment water-column grid exte nding from the Mississippi River Delta west to the Louisiana-Texas bor der, and from the shoreline seaward to the 30-60 m bathymetric contour s. Diagnostic analyses and numerical experiments were conducted with t he calibrated model to better understand the environmental processes c ontrolling primary productivity and dissolved oxygen dynamics in the M RP/IGS region. Under water light attenuation appears relatively more i mportant than nutrient limitation in controlling rates of primary prod uctivity. Chemical-biological processes appear relatively more importa nt than advective-dispersive transport processes in controlling bottom -water dissolved oxygen dynamics. Oxidation of carbonaceous material i n the water column, phytoplankton respiration, and sediment oxygen dem and all appear to contribute significantly to total oxygen depletion r ates in bottom waters. The estimated contribution of sediment oxygen d emand to total oxygen-depletion rates in bottom waters ranges from 22% to 30%. Primary productivity appears to be an important source of dis solved oxygen to bottom waters in the region of the Atchafalaya River discharge and further west along the Louisiana inner Shelf. Dissolved oxygen concentrations appear very sensitive to changes in under water light attenuation due to strong coupling between dissolved oxygen and primary productivity in bottom waters. The Louisiana Inner Shelf in th e area of the Atchafalaya River discharge and further west to the Texa s border appears to be characterized by significantly different light attenuation-depth-primary productivity relationships than the area imm ediately west of the Mississippi Delta. Nutrient remineralization in t he water column appears to contribute significantly to maintaining chl orophyll concentrations on the Louisiana Inner Shelf.