Past ecosystem dynamics in Connecticut's coastal freshwaters: Linking paleoecology and nutrient loading models

Background/Question/Methods

Anadromous alewives (Alosa pseudoharengus) were once abundant throughout Eastern North America and served as important drivers of ecosystem dynamics in the region’s coastal freshwaters. In the 400 years since Euro-American colonization in North America, anadromous alewife runs have declined by an order of magnitude or more throughout their range. In the past anadromous alewife likely served as major drivers of lake nutrient dynamics by importing substantial quantities of marine-derived nutrients to the region’s freshwaters. At the same time that alewife populations in the region have declined, nutrient inputs from the landscape have increased as human land use has intensified. We asked how lake nutrient dynamics in the region have changed over the past 400 years. We examined nutrient loading in four lakes in coastal Connecticut by modeling past fluxes of allochthonous P and N from anadromous alewife runs and the surrounding landscape. We complemented these models with a paleoecological analysis of nitrogen stable isotopes (δ¹⁵N) and nutrients (total C, organic C, N, C:N, P) from lake sediment cores in each of our four study watersheds. 

Results/Conclusions

Our nutrient loading models suggested that prior to Euro-American contact, anadromous alewife runs provided up to 90-99% of allochthonous P inputs and up to 60-95% of allochthonous N inputs to Connecticut’s coastal freshwaters. Today’s alewife runs account for only 23% of P and 5% of N inputs, while inputs from the landscape and atmosphere account for 77% and 95% of P and N inputs respectively. From pre-Euro-American contact to present nutrient inputs from the landscape increased 20-40% in watersheds with minimal land use and up to 500% in watersheds with extensive land use. Our paleoecological study suggested that nutrient cycles changed profoundly in the past 400 years. Total and organic C were highly correlated and decreased in association with land use change. C:N ratios also decreased with land use change, suggesting that primary production became more autochthonous. δ¹⁵N and P increased with human land use, suggesting that nutrient inputs from human activity increased. N generally decreased with human land use. Overall, our study suggested that anadromous alewife shifted from being a major source of freshwater nutrient inputs to serving as a minor source compared to inputs from human activity.