Effects of anadromous alewife on seasonal patterns of nutrient limitation in Maine lakes and streams

Background/Question/Methods

Anadromous alewife (Alosa pseudoharengus) link marine and freshwater environments, and may subsidize streams and lakes with nutrients via excretion, gametes and carcasses. Currently, the potential role of alewife as a nutrient vector to freshwater systems is poorly known. In the northeastern USA, alewife runs have declined dramatically over the past 150 years mostly due to damming. The goal of this study was to measure the ecological effects of alewife nutrient subsidies to lakes and streams using nutrient limitation assays and to compare limitation patterns of linked lake-stream systems. We evaluated alewife nutrient subsidies by examining the nutrient limitation status of 7 lakes and outlet streams with and 7 without alewife access. Sampling was done before, during and after alewife runs and after fall lake turnover. Nutrient-diffusing substrates and microcosm bags were used in streams and lakes respectively, to test for nitrogen, phosphorus or co-limitation of primary producers. Lake and stream water dissolved inorganic nitrogen  (DIN) and total phosphorus (TP) was measured seasonally. Seasonal patterns of carbon and nitrogen stable isotopes in zooplankton and fish were examined at a subset of the lakes to determine if marine nutrients transported by alewife were incorporated into the lake food web.

Results/Conclusions

During in-migration, DIN in alewife streams was marginally higher by 10.49 μg/L (P=0.06) than in reference streams, but was not different between the two for other sampling periods. Stream TP, and lake DIN and TP showed no increase during alewife presence. Nutrient limitation remained steady across all periods in alewife streams, while limitation of reference streams increased slightly during the run period followed by a significant decrease (P<0.01).  This indicates a possible alewife subsidy that deterred increasing limitation during runs as seen in reference streams. Some alewife streams exhibited a shift from co-limitation to nitrogen or no limitation following alewife presence, but this also occurred in one reference stream. Few lakes and their associated outlet streams (4 of 14) had similar nutrient limitation patterns. Lake zooplankton were enriched in ¹⁵N in one of two alewife lakes during spawning compared to reference lakes. While alewife may have subsidized some of the systems, we found no distinct, coherent signature of alewife as nutrient sources across systems. This may be due to variation in alewife run size or role of other limiting factors among systems. Further research should be conducted as dam removals on northeastern U.S. rivers allow for returns of more alewife.