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Production, Sedimentation, and Isotopic Composition of Organic Matter in Lake Ontario

David A. Hodell and Claire L. Schelske
Limnology and Oceanography
Vol. 43, No. 2 (Mar., 1998), pp. 200-214
Stable URL: http://www.jstor.org/stable/2839208
Page Count: 15
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Production, Sedimentation, and Isotopic Composition of Organic Matter in Lake Ontario
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Abstract

Organic matter and its carbon and nitrogen isotopic composition were measured in sequential sediment trap and core samples from the Rochester Basin of Lake Ontario to evaluate their usefulness in reconstructing historic changes in lake productivity. The greatest flux of organic matter from the epilimnion occurred during late summer and coincided with whiting events, indicating that calcite precipitation is an effective mechanism for sedimenting organic matter. Carbon isotopes of organic matter were low prior to the onset of stratification, increased to maximum values in late summer, and then decreased following fall overturn. This pattern is controlled mainly by the timing of stratification and primary productivity, which preferentially removes 12 CO2from the epilimnion. The physiological effect of decrease carbon isotopic fractionation with decreasing supplies of [CO2]aq may have also contributed to increased δ13 CorgC. Nitrogen isotopes showed a seasonal pattern opposite to that of carbon, whereby δ15 N values were low during the summer stratified period and high for the remainder of the year. Seasonal variability in δ15 NorgN probably reflects changes in the source of sedimented organic particles, which is dominated by isotopically depleted phytodetritus during the stratified period and isotopically enriched organic matter from heterotrophic or detrital sources during the mixed period. A comparison of organic carbon accumulation rates and δ13 CorgC between sediment cores collected in 1987 and 1993-1994 confirms earlier predictions that diagenetic processes reduce the mass accumulation of organic carbon in the zone of oxic pore waters, but will not change the δ13 CorgC values. All cores analyzed for δ13 CorgC display the reproducible pattern of a progressive increase in the 1900s, peaking in the early to mid-1970s, and then decreasing to the present. This pattern matches the historical trends of phosphorus loading to the basin, suggesting that δ13 C of organic carbon is a reliable proxy for paleoproductivity and responds to spring phosphorus supplies in the water column. The δ15 N of sedimentary organic matter increased linearly from 1840 to 1960 at a rate of 0.3%. per decade, and remained relatively constant thereafter except for an increase in the upper few centimeters of sediment. The increase in δ15 NorgN reflects a combination of factors, including early forest clearance by Europeans, increased sewering by municipalities after 1940, and increased nitrate utilization as productivity increased in the lower Great Lakes. Increased rates of denitrification in the central basin of upstream Lake Erie from the 1930s to the early 1970s may have also contributed to the rise in δ15 NorgN values.

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