Application of a physical-biological model for the Mississippi River plume to the determination of an organic carbon budget, air-sea CO2 fluxes, and contributions to botton-water hypoxia

Green, R.E., T.S. Bianchi, M.J. Dagg, Walker N.D

We investigated seasonal variability in organic carbon (OC) budgets using a physical-biological model for the Mississippi River turbidity plume. Plume volume was calculated from mixed layer depth and area in each of four salinity subregions based on an extensive set of cruise data and satellite-derived suspended sediment distributions. These physical measurements were coupled with an existing food web model to determine seasonally dependent budgetes for labile (reactive on time scales of days to weeks) OC in each salinity subregion. Autochthonous gross primary production (CPP) equaled 1.3 x 10^12 g C yr^-1 and dominated labile OC inputs (88% of the budget) because riverline OC was assumed mostly refractory (nonreactive). For perspective, riverine OC inputs amounted to 3.9 x 10^12 g C yr^-1, such that physical inputs were 3 times greater than biological inputs to the plume. Annually, microbial resporation (R) accounted for 65% of labile OC losses and net metabolism (CPP -R) for the entire plume was autotrophic, equaling 5.1 x 10^11 g C yr^-1. Smaller losses of a labile OC occurred via sedimentation (20%), advection (10%), and export to higher trophic levels (5%). In our present model, annual losses of labile OC are 10% higher than inputs, indication future improvements are required. Application of our model to estimate air-sea carbon dioxide (CO2) fluxes indicated the plume was a net sink of 2.0 x 10^2 mol CO2 yr^-1, of which 90% of the total drawdown was from biotic factors. In all seasons, low salinity waters were a source of CO2 (pCO2 = 560-890 uatm), and intermediate to high salinity waters were a sink of CO2 (pCO2 = 200-370 uatm). Our model was also used to calculate O2 demand for the development of regional hypoxia, and our spring and early summer budgets indicated that sedimentation of autochthonous OC from the immediate plume contributed 23% of the O2 demand necessary for establishment of hypoxia in the region.