Anadromous alewives (Alosa pseudoharengus) contribute marine-derived nutrients to coastal stream food webs

Abstract

Diadromous fish are an important link between marine and freshwater food webs. Pacific salmon ( Oncorhynchus spp.) strongly impact nutrient dynamics in inland waters and anadromous alewife ( Alosa pseudoharengus ) may play a similar ecological role along the Atlantic coast. The annual spawning migration of anadromous alewife contributes, on average, 1050 g of nitrogen and 120 g of phosphorus to Bride Brook, Connecticut, USA, through excretion and mortality each year. Natural abundance stable isotope analyses indicate that this influx of marine-derived nitrogen is rapidly incorporated into the stream food web. An enriched δ15N signal, indicative of a marine origin, is present at all stream trophic levels with the greatest level of enrichment coincident with the timing of the anadromous alewife spawning migration. There was no significant effect of this nutrient influx on water chemistry, leaf decomposition, or periphyton accrual. Dam removal and fish ladder construction will allow anadromous alewife to regain access to historical freshwater spawning habitats, potentially impacting food web dynamics and nutrient cycling in coastal freshwater systems.