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 measurements of δ13C of sedimented organic C(δ13CorgC) and non-apatite inorganic P (NAIP) in three cores. Inferred changes in lacustrine productivity were related to historic changes in phosphorus loading. P loading increased slowly after forest clearance and early settlement 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. Similarly, the sediment accumulation rate of NAIP increased rapidly during the 1940s and 1950s, peaked during the late 1960s and early 1970s, and then decreased after the mid-1970s. The signal of δ13CorgC normalized to account for the historic 1.4‰ depletion in δ13C of atmospheric CO2 mirrors that for accumulation of NAIP in sediments. Our results show that paleoproductivity peaked in the lower Great Lakes during the mid-1970s but has since declined as a result of P abatement programs. Inferred trends for NAIP and δ13CorgC 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.