Diverted Mississippi River sediment as a potential phosphorus source affecting coastal Louisiana water quality

Abstract

Mississippi River water and associated sediment are seasonally diverted into Louisiana coastal basins to restore historic hydrologic and salinity regimes and to slow or reverse the widespread wetland loss. More river diversions are planned; however, very little research has been conducted on the potential source of phosphorous (P) of these sediments transported by the Mississippi River into the coastal areas through these diversions. In addition, the sediments have the potential to undergo changes in redox status when these significant flooding events occur. Therefore, we examined the effect of sediment redox condition on P release from newly deposited Mississippi River sediment. Sediment was collected from Big Mar Lake, the receiving water body of the Caernarvon diversion structure, through which river water is diverted into the Breton sound estuary. Sediment suspensions were incubated in the laboratory under reducing conditions, and the relation of redox conditions to P release was determined. Soluble reactive P (SRP), dissolved organic P (DOP), Fe, and Mn concentrations increased with decreasing sediment redox potential. The SRP and DOP concentrations increased 32 - and 8-fold during 20 days of anaerobic conditions. The increase in SRP and DOP availability was significantly correlated to an increase in ferrous iron with no significant relationship with Al, Ca, or Mg. While the vast majority of studies on nutrients associated with the Mississippi River diversions have focused on excess inorganic nitrogen forms (primarily NO3) in the river water, this study is one of the first to demonstrate that upon reduction associated with prolonged flooding, the diverted Mississippi River suspended sediment can be a significant source of available P to the coastal basin.