NUTRIENT SPECIATION AND MOBILITY GOVERNED BY REDOX CHANGES OF IRON OXYHYDROXIDE MINERALS CONTROLLED BY SEDIMENT MICROORGANISMS IN SHALLOW, EUTROPHIC, MISSISQUOI BAY, LAKE CHAMPLAIN

Nutrient cycling and speciation in sediments of shallow, eutrophic Missisquoi Bay, Lake Champlain, were examined during different stages of a cyanobacteria bloom across two consecutive summers (2007 & 2008). A bloom did not occur in 2007 for the first time in a decade but did in 2008, allowing for comparison of seasonal and diel sediment nutrient cycling in the presence and absence of a bloom. The relationship among P and N speciation, sediment microbe activity influenced by redox chemistry, and other sediment constituents were investigated. Sediment cores collected monthly throughout both summers were vertically profiled using in situ voltammetry (to determine porewater O₂, Fe(II), Fe(III), Mn(II), H₂S, and FeS_(aq)), and the top 10 cm were sectioned for determination of total and reactive P, Fe, Mn, Al, and Ca, as well as organic P (P_org, using ³¹P-NMR). Sediment extraction and porewater data shows that reactive P (RP) is strongly correlated to reactive Fe (RFe, ascorbic acid extractable; predominantly nanocrystalline ferrihydrite), comprises a large portion (30-40%) of the total sediment P pool, and is mobilized by changing redox conditions. In situ voltammetry was also performed over diel cycles at the sediment water interface (SWI) through summer 2007 and 2008. Data shows that the blooms themselves have a significant effect on redox chemistry at the SWI. In the absence of blooms RP accumulated in the oxic surface sediments throughout 2007, but when blooms reappeared the following year RP concentrations decreased and the surface sediments showed evidence of anaerobic microbial Fe and Mn reduction. P_org content ranged from 18-26% of the TP and was strongly correlated to porewater Fe(II), but not to RP, indicating that its mobility is partly associated with the microbial reduction of ferrihydrite, but independent from RP mobility. The relationship between bloom intensity and reducing conditions suggests the occurrence of a “feedback mechanism” in which redox influenced nutrient release from sediment provides more nutrients for the proliferation and sustainability of the bloom. Sediment nutrient flux also occurs on a diel scale, possibly contributing to the domination of vertically migrating cyanobacteria in some shallow eutrophic water bodies.