Sediment Record of Biogeochemical Responses to Anthropogenic Perturbations of Nutrient Cycles in Lake Ontario

Two sediment cores collected from the Rochester basin of Lake Ontario were dated with 210Pb and stratigraphic correlation and analyzed to determine whether nutrient accumulation with time was consistent with previous computer-simulated total phosphorus (TP) loadings. Relative increases in TP and nonapatite inorganic phosphorus (NAIP) accumulation were less than the fivefold increase in TP loading from 1800 to 1950 predicted independently from Chapra's simulation model. In addition, increases in TP accumulation occurred mainly after 1940 and the proportion of NAIP relative to TP increased in one core and decreased in the other. Of the nutrients studied, only increases in organic carbon (OC) paralleled the increases in modelled TP loadings. The relative increase in inorganic carbon (IC) was greatest, with accumulation increasing an order of magnitude after 1940 in one core. This large increase in IC, amounting to 20% calcite in recent sediments, was attributed to biologically induced calcite precipitation, a secondary consequence of increased planktonic photosynthetic removal of carbon dioxide that resulted from accelerated eutrophication after 1940 when modelled TP concentrations increased rapidly. Biogenic silica (BSi) accumulation, an indicator of increased diatom production, peaked between 1850 and 1870 when increases in TP and NAIP fluxes were minimal. Results provide evidence that historic biogeochemical responses inferred from OC, IC, and BSi accumulation in the sediment record provide stronger signals of phosphorus enrichment effects than can be inferred directly from changes in accumulation of different forms of phosphorus in the sediment record.