THE EFFECT OF THE 2012 ALEWIFE MIGRATION ON NUTRIENT DYNAMICS IN NEQUASSET LAKE, WOOLWICH MAINE

Anadromous fish, such as alewives (Alosa pseudoharengus) provide an important link between coastal watersheds and the Atlantic Ocean along the Gulf of Maine. Alewives contribute marine-derived nutrients (MDN) in the form of nitrogen to freshwater lakes via excretion and mortality as they migrate upstream during spawning season. Previous attempts to detect MDN in the sedimentary record have provided equivocal results. Freshwater biota or the size of current alewife migrations may have a significant effect on the sedimentary MDN signal. The focus of this project is to determine the degree to which MDN were imported into Nequasset Lake, Woolwich Maine. These data represent the initial findings of an expanded, multi-institutional, multi-year study currently underway.

The Nequasset Lake watershed covers an area of ~50 square kilometers and provides drinking water to the city of Bath and three other communities in Maine. Every spring, alewives return to Nequasset Lake to spawn, accessing the lake through a fish ladder adjacent to the water control dam. In April and May 2012, alewife counts were performed at the top of the fish ladder by volunteers of Trout Unlimited and Kennebec Estuary Land Trust. Water samples were collected from the top of the fish ladder, and from the 4 major stream inlets, and analyzed for nutrient concentrations (TDN, NO₃^-, NH₄⁺) to construct a nitrogen budget. Additional samples were collected for δ¹⁵N_NO3-analysis from April to August to trace marine-derived nitrogen from the alewives in the lake.

During peak migration, TDN concentrations at the top of the fish ladder were correlated to fish counts, indicating that the fish were importing a significant amount of nitrogen into the lake. Furthermore, the δ¹⁵N_NO3- of the lake shows an enriched signal during the spawning period, perhaps reflecting the presence of MDN in the middle of the lake. Increases in MDN have the potential to affect lake productivity as it is immediately available for uptake by primary producers. Thus, MDN observed in Nequasset Lake may have a profound effect on the lake’s ecosystem. If labile MDN is used by biota right away little evidence may be stored in the sedimentary record. Analyzing lake sediments for δ¹⁵N_NO3- may shed new light on the historic and archaic magnitude of alewife runs and MDN additions.