RESPONSE OF FOUR NEW YORK FINGER LAKES TO MULTIPLE STRESSORS OVER THE LAST CENTURY

We used a multi-indicator approach to infer a continuous history of cultural eutrophication in four New York Finger Lakes over the last century using sediment box cores. On the basis of the carbon (%C) and nitrogen concentrations, atomic C/N ratios, and the stable carbon (δ¹³C_org) and nitrogen isotopic composition of bulk organic matter as well as % calcite and % terrigenous composition of the cores, three stages of eutrophication were recognized in each lake: (1) a gradual eutrophication pre-1950 due to agricultural deforestation and land use conversion; (2) rapid eutrophication between ~1950 and ~1990 due to shifts in agricultural practices, a post-Word War II population boom and sewage discharge into the lakes; and (3) a return to gradual eutrophication post-1990 following establishment of filter-feeding Desissena spp. (zebra and quagga mussels). By combining our analysis of sediment core records of cultural eutrophication with limnological monitoring records obtained during a literature review, we were able to verify our paleoproductivity interpretations. Because of the temporal overlap of in-lake measurements of water quality with the sediment record, we were able to confirm a significant increase in primary production in these lakes inferred from the sediment record during stage 2. The limnological monitoring records also provided context for interpretation of a negative δ¹³C_org excursion of 0.5‰ to nearly 2‰ in the three largest study lakes during stage 2. These lakes became increasingly more algal-rich and eutrophic based on water-quality data and a concurrent increase in %C and organic carbon accumulation rates and decrease in atomic C/N ratios. The unanticipated decline in the δ¹³C_org during maximum eutrophication may reflect a shift in the relative proportions of atmospheric CO₂ and recycled CO₂ originating from decomposition or oxidation of organic matter that was supplied to the dissolved inorganic carbon pool and subsequently used by phytoplankton during photosynthesis. The post-1990 increase in δ¹³C_org may be the result of re-engineering of the ecosystem by zebra and/or quagga mussels.