Relating sediment phosphorus mobility to seasonal and diel redox fluctuations at the sediment-water interface in a eutrophic freshwater lake

Relationships between phosphorus cycling and redox conditions in the sediments of eutrophic Missisquoi Bay, Lake Champlain were investigated over diel and seasonal timescales in three consecutive summers (2007-2009), one of which (2007) did not experience a cyanobacteria bloom for the first time in a decade. Sediment extraction data showed that reactive phosphorus (RP) is strongly correlated to reactive iron (RFe), suggesting that the mobility of a large portion (30-40%) of the total sediment phosphorus pool is associated with nanocrystalline iron oxide minerals that may be influenced by redox conditions. RP concentrations in the top sediments increased throughout 2007 but decreased throughout 2008; calculations indicate that similar to 1 mm of sediment could account for the increased total dissolved phosphorus observed in overlying water. Redox conditions were measured over 24 h in situ within sediment cores and at the sediment-water interface (SWI) at different stages of each season using voltammetry. SWI redox conditions became progressively more reduced across the season and overnight and were significantly more reducing in the presence of a bloom. Soluble RP and cyanobacteria cell counts measured at five depths through the water column strongly correlated with the most reducing conditions at the SWI. Observations suggest that redox controlled nutrient flux between the sediments and the water column is variable over diel and seasonal cycles. Cyanobacteria blooms most significantly affect SWI redox conditions, suggesting that blooms may enhance RP flux from sediments, setting up a positive feedback loop that can propagate and sustain blooms in shallow freshwater systems.