Riverine Impact on Nutrients and Organic Matter in the Western Mississippi Bight

The Mississippi River is the largest freshwater source to the north-central Gulf of Mexico and can influence nutrient concentrations, phytoplankton production and food web structure in the region. However, recent studies have shown that smaller local rivers can exert a strong influence on biogeochemical characteristics of the Mississippi Bight (MSB). Analysis of five years of river flow data, salinity, nutrient concentrations and particulate organic matter (POM) in terms of C:N content and stable isotope values was conducted in the western MSB to assess how smaller rivers (the Pearl, Pascagoula and Mobile Rivers) impact these parameters compared to the Mississippi River. Multiple linear modeling was used to determine how different river flows with various lag times (one and three months) correlated with measured parameters across the area during the study period. Results were used to inform kreiging models that illustrate biogeochemical conditions under different freshwater flow regimes. Alterations were observed in nutrient concentration distribution as well as POM quantity,
$\delta^{13}\mathrm{C}$
values and carbon/nitrogen ratios to various extents across the region. Smaller regional rivers were shown to influence the western MSB as much as the Mississippi River, despite their lower freshwater outflows (∼10 times less). Overall, surface water nutrients and bottom water POM concentrations were most strongly impacted by the monthly averaged outflow from the Mississippi, while concentrations of bottom water nutrients and POM in the surface waters of the study area were most strongly impacted by the outflow from the Mobile River with a three month lag.