Historic low-level phosphorus enrichment in the Great Lakes inferred from biogenic silica accumulation in sediments

Sedimentary biogenic silica (BSi) accumulation was used in conjunction with a hypothetical model of BSi accumulation to show that BSi is a sensitive proxy for low-level phosphorus enrichment in the Great Lakes. We hypothesize that historic nutrient-driven changes in diatom production altered silica biogeochemistry and induced biologically mediated silica depletion (BMSD) and that a record of the underlying mechanism, enhanced diatom production and BSi sedimentation stimulated by anthropogenic phosphorus enrichment, is preserved in the sediment record. Paleolimnological results support three hypotheses based on this model. First, BSi accumulation increased in Lake Superior and Lake Huron at total phosphorus (TP) concentrations (4 and 5 mu g TP L-1 or 0.13 and 0.16 mu mol L-1, respectively) too small to induce BMSD and with changes in TP concentration too small to be detected by routine water-column sampling. Second, a peak in BSi accumulation in Lake Michigan resulted from epilimnetic silica depletion that developed rapidly in the 1950s and 1960s when TP averaged 8 mu g L-1 (0.26 mu mol L-1). In addition, epilimnetic silica depletion in the late 1800s was inferred from BSi accumulation in Lake Erie and Lake Ontario when the TP concentration was < 10 mu g L-1 (0.32 mu mol L-1). Third, a secondary peak in BSi accumulation in the 1950s and 1960s signaled water-column silica depletion in Lake Ontario and the eastern basin of Lake Erie that developed as TP concentration increased to 27 mu g L-1 (0.87 mu mol L-1). Ratios of NAIP:TP, BSi:TP and BSi:NAIP also provide sensitive proxies for phosphorus enrichment. BSi accumulation is a sensitive proxy for phosphorus enrichment because BSi production by diatoms integrates silica utilization over an annual cycle, silica is recycled slowly (on annual time scales) compared with phosphorus, and sedimented BSi is focused into depositional zones.