An organic carbon budget for the Mississippi River turbidity plume and plume contributions to air-sea CO2 fluxes and bottom-water hypoxia

R.E. Green1,*, T.S. Bianchi1,a, M.J. Dagg2, N.D. Walker3 and G.A. Breed4

 

1Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA

2Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, Louisiana 70344, USA

3Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, USA

4Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada

 

*Corresponding author: rebecca.green@nrlssc.navy.mil; Current address: Ocean Sciences Division, Naval Research Laboratory, Stennis Space Center, Mississippi 39529

 

ABSTRACT: 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 budgets for labile (reactive on timescales of days to weeks) OC in each salinity subregion.  Autochthonous gross primary production (GPP) equaled 1.3x1012 g C yr-1 and dominated labile OC inputs (88% of the budget) because riverine OC was assumed mostly refractory (non-reactive).  For perspective, riverine OC inputs amounted to 3.9x1012 g C yr-1, such that physical inputs were ~3 times greater than biological inputs to the plume.  Annually, microbial respiration (R) accounted for ~65% of labile OC losses and net metabolism (GPP - R) for the entire plume was autotrophic, equaling 5.1x1011 g C yr-1.  Smaller losses of 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, indicating future improvements are required.  Application of our model to estimate air-sea CO2 fluxes indicated the plume was a net sink of 2.0x109 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 matm), and intermediate to high salinity waters were a sink of CO2 (pCO2 = 200-370 matm).  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.

REFERENCE:  Green, R.E., T.S. Bianchi, M.J. Dagg, N.D. Walker and G.A. Breed. 2006. An organic carbon budget for the Mississippi River turbidity plume and plume contributions to air-sea CO2 fluxes and bottom-water hypoxia. Estuaries 29: 579-597.

a Current address: Department of Oceanography, Texas A&M University, College Station, Texas 77843-3146, USA