Sedimentation, carbon export and food web structure in the Mississippi River plume described by inverse analysis

The Mississippi River stimulates the coastal marine ecosystem directly with dissolved organic matter and indirectly with inorganic nutrients that enhance primary production. To understand the river's effect, we need to track the fate of both sources of organic matter. Using readily available data, we investigated the planktonic ecosystem of the buoyant Mississippi River plume using an inverse analysis technique to describe the carbon flow for the complete planktonic system. For each season we divided the marine waters receiving Mississippi River discharge into 4 dilution regions connected by movement of river water. Our results show that during 3 seasons (spring, summer, and fall) mid-salinity waters (15 to 29 psu) exported organic matter (strongly net autotrophic), whereas the other regions imported it (net heterotrophic). More than 20% of total plume primary productivity was exported from the entire modeled region, as continued water movement carried organic carbon into surrounding waters. In contrast, the winter plume was net-heterotrophic everywhere, as high bacterial respiration overwhelmed relatively low primary production, and riverine dissolved organic carbon (DOC) and organic carbon from resuspended sediments were required to balance a carbon deficit. From the spring through fall, sedimentation of organic carbon was linked to primary production, with strongest sedimentation in mid-salinity regions. Sedimentation was enhanced beneath less productive, higher-salinity regions, by import of organic carbon moving out of mid-salinity regions. In contrast, winter organic carbon sedimentation rates were calculated to be zero in all model regions. The analysis showed a dynamic relationship between primary production and sedimentation and provides a good starting point for future development of mechanistic models that directly address the relationships between nutrient input, primary production, sedimentation and hypoxia on the Louisiana Shelf.