Recent changes in productivity and climate of Lake Ontario detected by isotopic analysis of sediments

Cores from Lake Ontario provide a record of lake responses to increased phosphorus loading that began after early forest clearance in the mid‐1800s. We measured 13 C/ 12 C ratios of organic and inorganic C (calcite) in the sediments to infer recent trends in primary production. The δ 13 C of organic C increased in response to historic increases in P enrichment until the mid‐1970s and then decreased in response to reduced P loading. The δ 13 C of calcite increased after 1940 but did not decrease in response to reduced P loading. We conclude that the results differ because δ 13 C of organic C is determined by primary production early in the seasonal cycle when flux of organic C to the sediments is greatest, whereas δ 13 C of calcite depends on the isotopic composition of dissolved inorganic C during late summer when calcite is precipitated. Calcite accumulation and its δ 13 C ratio oscillated after 1940 with peaks estimated at 1948, 1966, 1977, and 1983. These peaks correspond to minima in δ 18 O values, indicating calcite precipitation at higher temperature, and to years of early thermal stratification. We hypothesize that calcite accumulation and its stable isotopic ratios are controlled by interannual variability in onset of thermal stratification of the lake. Our results thus suggest a direct link from climate (i.e. thermal stratification) to interannual variation in lake productivity and to carbonate precipitation and its δ 13 C signature.