Relationship of Hypolimnetic Iron, Sediment Biogeochemistry, and Sulfate in the Promotion of Cyanobacterial Blooms

Abstract

Previous work suggests that a high rate of internal ferrous iron (Fe2+) loading from anoxic sediments into overlying waters favours cyanobacteria dominance over eukaryotic algae. This conceptual model was assessed in four embayments along the Georgian Bay coastline. Cyanobacteria dominated all embayments in the relatively warmer summer of 2012 but not in the much cooler summer of 2014; internal Fe2+loading was observed in both summers in all embayments. A large cyanobacteria bloom was observed only in warmer 2012 in the meso-eutrophic embayment. Results show that warm summer temperatures and internal Fe2+ loading are necessary preconditions for cyanobacteria dominance, while large blooms require high nutrient levels. Release of ferrous iron from sediments is restricted by sulfide production via the formation of insoluble ferrous sulfides, which reduces lake iron availability when they are buried in sediments. Increases in ferrous iron due to decreased sulfate levels may explain recent increases in bloom activity within Ontario and Quebec lakes. Sediment cores were collected from Lakes Erie and St. George and analyzed for historical trends in acid-volatile sulfides (AVS) and acid-extractable iron. Lake Erie seemed to closely follow the historical SO2 emission trends of the region, except for an anomaly occurring in the mid 1970s. Neither of the two cores demonstrated a clear historical relationship between acid extractable iron and AVS, although the patterns in Erie are similar. Surficial sediments from Hamilton Harbour and Lake Erie were processed to form sediment incubation cores, which had additions of sulfate and/or organic carbon to determine their effects on the rate of internal loading and production of iron sulfides. The overall results indicate that heterotrophic bacterial metabolism of organic matter is the primary driver of internal loading and subsequent redox-associated events such as sulfide deposition. Soluble iron was not sequestered with even relatively high levels of sulfate until the latest stages anoxia, and it therefore seems unlikely that ferrous iron will be controlled by sulfide production in these natural systems during the course of one season, but long-term sequestration may be more likely after large blooms.