USING CARBON ISOTOPES OF BULK SEDIMENTARY ORGANIC-MATTER TO RECONSTRUCT THE HISTORY OF NUTRIENT LOADING AND EUTROPHICATION IN LAKE ERIE

Lacustrine productivity in Lake Erie was reconstructed using measureme nts of delta(13)C of sedimented organic C (delta(13)C(orgC)) and non-a patite inorganic P (NAIP) in three cores. inferred changes in lacustri ne productivity were related to historic changes in phosphorus loading . P loading increased slowly after forest clearance and early settleme nt in the late 1800s, then increased exponentially from the late 1940s to early 1970s, and finally decreased after the mid-1970s in response to management practices implemented to improve water quality. Similar ly, the sediment accumulation rate of NAIP increased rapidly during th e 1940s and 1950s, peaked during the late 1960s and early 1970s, and t hen decreased after the mid-1970s. The signal of delta(13)C(orgC) norm alized to account for the historic 1.4 parts per thousand depletion in delta(13)C of atmospheric CO, mirrors that for accumulation of NAIP i n sediments. Our results show that paleoproductivity peaked in the low er Great Lakes during the mid-1970s but has since declined as a result of P abatement programs. Inferred trends for NAIP and delta(13)C(orgC ) are those predicted in P-limited lacustrine systems and demonstrate that the carbon isotopic ratio of organic C can be used as a proxy for paleoproductivity in large, deep lakes.