The Role of Tidal Salt Marsh as an Energy Source for Marine Transient and Resident Finfishes: A Stable Isotope Approach

Abstract

Stable isotope ratios of carbon, nitrogen, and sulfur (δ13C, δ15N, and δ34S) in bay anchovy Anchoa mitchilli and white perch Morone americana from Delaware Bay were a function of capture location and, for restored tidal salt marshes, possibly a function of the relative position of the marsh restoration trajectory. White perch collected in polyhaline restoration and reference (Moore's Beach) sites had stable isotope signatures that reflected contributions from both benthic microalgae and Spartina alterniflora. Isotope values from white perch captured at the reference site were slightly enriched compared with those captured at the restoration sites. At upper estuary oligo–mesohaline locations, Phragmites australis contributed to the isotopic composition of both species. Although P. australis was not dominant at the reference marsh (Mad Horse Creek), it also seemed to influence the flow of nutrients into these species. White perch were not collected in open waters of Delaware Bay, but bay anchovy were abundant at offshore sampling stations. A striking result of this study was that stable isotopes that originated in macrophytes and benthic microalgae of salt marshes occurred ubiquitously among bay anchovy collected several kilometers from shore. Interestingly, fish from Mad Horse Creek, a reference marsh where Phragmites was abundant only along creek banks, were intermediate in their isotopic composition. Although benthic microalgal signatures varied both within and among sites, the isotopic composition of bay anchovy and white perch was clearly influenced by these primary producers across all marsh types. Particulate organic matter (POM) appeared to play a lesser role in bay anchovy and white perch nutrition, although depletion of POM sulfur due to the presence of inorganic sulfur may have led to underestimates of the contribution of phytoplankton. Further studies are required to elucidate the relative role of phytoplankton in the trophodynamics of these species.