RECONSTRUCTING PAST PRIMARY PRODUCTIVITY OF FOUR NEW YORK FINGER LAKES USING MULTIPLE PROXIES

Eutrophication is one of the most pressing water quality issues in freshwater systems worldwide. Sediment cores provide a valuable means of reconstructing changes in lake trophic state because when dated using ¹³⁷Cs and ²¹⁰Pb, they permit inferring the factors involved in eutrophication at a high temporal resolution. Box cores were collectedand analyzed from four New York Finger Lakes (Conesus, Honeoye, Canandaigua, Cayuga) to reconstruct shifts in the trophic status over the last century using several paleo-productivity proxies. These lakes were chosen because they currently reflect a wide range of trophic states, mixis types, morphological conditions, and land use in their respective watersheds. The principal objective of this study is to evaluate the use of the stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic composition of bulk organic matter to infer paleoproductivity across the trophic gradient of these freshwater lakes by analyzing modern sediments and assessing stratigraphic changes in the cores.

The atomic C/N ratios and Suess-corrected δ¹³C indicate that sedimented organic matter is algal in origin. In all lakes, the wt.% organic carbon and nitrogen gradually increased since the early 1900s. The δ¹⁵N also increased upcore in the three lakes in agriculturally-dominated watersheds. The δ¹⁵N in Honeoye was lower than the other lakes and remained constant because it is a shallow, polymictic lake in a forest-dominated watershed. The δ¹³C gradually declined in all lakes during the 1940-1990s before increasing again upcore. This trend is muted in Honeoye because of the influence of Hurricane Agnes (1972). Increased nutrient loading and primary productivity likely explains the trend in δ¹³C. Although δ¹³C values typically increase with increasing biological productivity as more ¹²C is removed during photosynthesis, δ¹³C values may become more negative if primary producers are dominated by chemoautotrophic or methanotrophic microbes, typical of eutrophic-hypereutrophic lakes. When plotted against each other, the δ¹³C and δ¹⁵N of sediment from each lake show good separation in isotope space. The δ¹³C and δ¹⁵N was lowest in the most productive lakes and highest in the least productive lakes.