The Influence of Anadromous Alewife on Maine Lakes and Streams: Using Nutrient Limitation Assays and Stable Isotopes to Track Marine-Derived Nutrients

Anadromous fish can act as nutrient subsidies to freshwater ecosystems when they return there to spawn. However, relatively few studies have quantified the role of alewife (Alosa pseudoharengus) as an ecologically important source of marine-derived nutrients (MDN) to lakes and streams. Primary producers in lakes and streams are often limited by nutrients, such as nitrogen and phosphorus. If alewife bring nutrients to lakes and streams, then the limitation of primary producers in those systems should be alleviated. Nutrient limitation assays and stable isotopes were used to examine the effects of alewife MDN on Maine lakes and their outlet streams. Nutrient limitation assays were run prior to, during, and after alewife runs and again after lake turnover. Alewife runs increased water nutrient concentrations in streams slightly, but not in lakes. There was also no coherent shift or alleviation of nutrient limitation in alewife lakes and streams compared to non-alewife systems. There was enrichment in δ13C and δ15N of lake zooplankton and resident fish in one alewife lake in comparison to a non-alewife lake. Additionally, white perch in Fields Pond, to which alewife access was restored by dam removal, were relatively more enriched in 15N after alewife re-introduction in 2011. There was less of an alewife effect on freshwater nutrient limitation than was expected, but the current densities of alewife runs were relatively low compared to historical counts and other areas of the northeastern U.S. Results from the stable isotope data suggest that some MDN were incorporated into lake food webs. This study has relevance in Maine given the current and proposed dam removals and diadromous fish restoration, which will restore access to historical freshwater habitat for native anadromous alewife