Multivariate analysis of stable-isotope ratios to infer movements and utilization of estuarine organic matter by juvenile weakfish (Cynoscion regalis)

Canonical discriminant analysis of the natural carbon, nitrogen, and sulfur stable isotopes in tissues of young weakfish (Cynoscion regalis) captured in Delaware Bay was used to infer movements of juveniles and to estimate the utilization of estuarine organic matter in this marine transient species. Two gradients emerged in the data. The first was associated with contributions from tidal salt marsh macrophytes, Spartina alterniflora in the polyhaline lower Bay and Phragmites australis in the brackish upper Bay. The second was associated with the expected change in phytoplankton carbon-isotope values along the salinity gradient in addition to the availability of macrophyte-derived organic matter within the Bay. The separation of the gradients reflected differences in the relative contribution of phytoplankton to weakfish secondary production in open waters versus marsh habitats. As they grew, discrepancies between the anticipated isotopic signatures of juvenile weakfish collected in a specific habitat and their actual signatures were interpreted as down-bay movements based on known life-history patterns. The size-specific differences in the isotopic signatures of weakfish suggested that stable-isotope data can be used to gauge the relative magnitude of marsh-derived organic matter exported from Delaware Bay via movements of juveniles out of the estuary.