An isotopic analysis of ecosystem connectivity: Are anadromous Alosa herring marine nutrient vectors?

This thesis examines the role of herring of the genus Alosa as vectors of marine-derived nutrients to freshwater invertebrates. Alosa feed in the Atlantic Ocean and migrate to coastal streams of the eastern US to spawn each spring. In order to investigate whether the temporary presence of these fish is a nutrient source for the stream ecosystem, stable isotope analysis was employed. Since oceanic organisms will be relatively enriched in nitrogen-15 and carbon-13 in comparison to the stream components, those isotopes can be used as a chemical tracer. Significant enrichment in 15N and 13C was found in stream invertebrates in correlation with the Alosa spawning migration. Application of a mixing model to the data yielded the estimate that approximately 20% of the diet of the invertebrate Gammarus fasciatus in late April had come from Alosa-derived nutrients.