PRODUCTION, SEDIMENTATION, AND ISOTOPIC COMPOSITION OF ORGANIC-MATTERIN LAKE-ONTARIO

Organic matter and its carbon and nitrogen isotopic composition were m easured in sequential sediment trap and core samples from the Rocheste r Basin of Lake Ontario to evaluate their usefulness in reconstructing historic changes in lake productivity. The greatest flux of organic m atter from the epilimnion occurred during late summer and coincided wi th whiting events, indicating that calcite precipitation is an effecti ve mechanism for sedimenting organic matter. Carbon isotopes of organi c matter were low prior to the onset of stratification, increased to m aximum values in late summer, and then decreased following fall overtu rn. This pattern is controlled mainly by the timing of stratification and primary productivity, which preferentially removes (CO2)-C-12 from the epilimnion. The physiological effect of decreased carbon isotopic fractionation with decreasing supplies of [CO2](aq) may have also con tributed to increased delta(13)C(orgC). Nitrogen isotopes showed a sea sonal pattern opposite to that of carbon, whereby delta(15)N values we re low during the summer stratified period and high for the remainder of the year. Seasonal variability in delta(15)N(orgN) probably reflect s changes in the source of sedimented organic particles, which is domi nated by isotopically depleted phytodetritus during the stratified per iod and isotopically enriched organic matter from heterotrophic or det rital sources during the mixed period. A comparison of organic carbon accumulation rates and delta(13)C(orgC) between sediment cores collect ed in 1987 and 1993-1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone o f oxic pore waters, but will not change the delta(13)C(orgC) values. A ll cores analyzed for delta(13)C(orgC) display the reproducible patter n of a progressive increase in the 1900s, peaking in the early to mid- 1970s, and then decreasing to the present. This pattern matches the hi storical trends of phosphorus loading to the basin, suggesting that de lta(13)C of organic carbon is a reliable proxy for paleoproductivity a nd responds to spring phosphorus supplies in the water column. The del ta(15)N of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0.3 parts per thousand per decade, and remained rela tively constant thereafter except for an increase in the upper few cen timeters of sediment. The increase in delta(15)N(orgN) reflects a comb ination of factors, including early forest clearance by Europeans, inc reased sewering by municipalities after 1940, and increased nitrate ut ilization as productivity increased in the lower Great Lakes. Increase d rates of denitrification in the central basin of upstream Lake Erie from the 1930s to the early 1970s may have also contributed to the ris e in delta(15)N(orgN) values.