Marine-Derived Nutrient Cycling in the St. Croix River, Maine

Three fish-ways at main-stem dams on the St. Croix River had been unilaterally closed to diadromous fish passage since the late 1980s by the State of Maine.  Collaboration between State, national and First Nations participants led to the reopening of these fish-ways in 2013. In order to characterize current food web connections and nutrient limitation prior to alewife return, a food web assessment was initiated in 2013.  In 2014, a reference site with an established alewife run was also sampled to characterize the temporal influx of marine nutrients. Fish, aquatic insects and zooplankton were sampled from each site on both rivers for stable isotope analysis.  Nutrient diffusers were used to determine nutrient limitation at each site.  2013 data indicate alewives were enriched in both δ¹³C and δ¹⁵N compared to freshwater fish and invertebrates.  Trophic level differences were distinct between fish and invertebrates.  This data will provide a baseline for monitoring marine-derived nutrient cycling as alewives return to the St. Croix River.