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. This change is anticipated to reestablish previous spawning runs of 2+ million alewives by reconnecting the freshwater and marine ecosystems.  In order to characterize current food web connections and nutrient limitation prior to alewife return, a food web assessment was initiated in 2013.  Water samples have been taken from ten sites within the watershed, including lakes, impoundments, and the main stem of the river.  Fish, aquatic insects and zooplankton were also sampled from each site for stable isotope analysis.  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.  In 2014, samples will also be taken from a reference watershed with an established alewife run.  Nutrient diffusers will be used to determine nutrient limitations at each site.  This data will provide the groundwork for monitoring marine-derived nutrient cycling as alewives return to the St Croix River.