Cores from Lake Ontario provide a record of lake :rcsponses to increased phosphorus loading that began after early forest clearance in the mid-1800s. We measured1XY2C ratios of organic and inorganic C (calcite) in the sediments to infer recent .trends in primary production. The 6% 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 6°C of calcite increased after 1940 but did not decrease in response to reduced P loading. We conclude that the results differ because 8°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 6°C of calcite depends on the isotopic composition of dissolved inorganic C during late summer when calcite is precipitated.Calcite accumulation and its 613C ratio oscillated after 1940 with peaks estimated at 1948, 1966, 1977, and 1983. These peaks correspond to minima in ,6'*0 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 6% signature. Lake Ontario is a large (19,700 km*), P-limited system in which long-term increases in production of organic matter were undoubtedly regulated by supplies of P (Schelske et al. 1988;Lean et al. 1987). Its sediments provide a historic record of lake productivity and exhibit a two-stage response to P loading (Schelske et al. 1988; Stoermer et al. 1985~). The first stage occurred after 1850, following early settlement and forest clearance by settlers, and is marked by an increase in biogenic silica and organic C flux to the sediments (Schelske et al. 1983(Schelske et al. , 1988. The second stage occurred AcknowledgmentsWe acknowledge Mark Brenner and the late E. S. Deevey for reading the manuscript and providing facilities and assistance with analyses of total carbon. We thank Jose Garrido for laboratory assistance with coulometric titration and Roger Bachmann for assistance with calculations of stable isotopic ratios of carbon resulting from changes in primary production. We also thank two anonymous reviewers for criticism of our interpretation of results and for their role in refining our model.