RELATING NUTRIENT MOBILITY TO SEDIMENT REDOX CHANGES IN MISSISQUOI BAY, LAKE CHAMPLAIN

Missisquoi Bay, Lake Champlain experiences toxic cyanobacterial blooms during the summer months as a result of eutrophication. Nutrients enter a water column via two general pathways – from the watershed (external loading) or from the release of nutrients tied up in sediments deposited in the bay (internal loading). This study focuses on the mechanisms that drive the latter pathway. Key parts of considering internal loading include consideration of changing redox chemistry and the role of organic vs. inorganic phosphorus, especially as related to P adsorption/release from FeOOH and MnOOH mineral surfaces.

Missisquoi Bay sediment redox fluctuations were monitored across diel and seasonal cycles over the course of two summers (May –October, 2007 and 2008) by using in-situ electric voltammetry with Au-Hg amalgam microelectrodes. Changing redox chemistry was monitored at the sediment-water interface (SWI) continuously over diel cycles, and the vertical profile of several key redox compounds obtained from cores collected at different times. The cores were then sectioned into segments and analyzed for Total P (TP), Reactive P (RP), Organic P (OP), Mn, Fe, Ca, Al, and N.

Missisquoi Bay did not experience a cyanobacteria bloom during the summer of 2007, but did in summer of 2008, providing an opportunity to compare P levels, P speciation, and redox fluctuations between non-bloom and bloom conditions. Redox monitoring at the SWI indicated that the sediments became increasingly anoxic over the course of summer 2008 with Mn reduction detected above the sediment surface in late July –October, whereas the sediments remained relatively oxic for the duration of summer 2007. The reactive components of Fe, Mn, and P in the sediments were more strongly correlated in 2008 compared to 2007. These results indicate that the conditions of the sediments in 2008 were more prone to P release into the water column. Monitoring of the SWI also indicate significant changes in redox chemistry over diel cycles and as affected by the presence of a bloom. Organic P species represent 20-26% of the P in sediments, but do not exhibit the same degree of mobility as inorganic P. Nitrogen speciation in sediment porewaters and the water column show significant spatial and temporal variation in the amount of ammonification affecting water column chemistry.