The overarching objective of this study was to identify important river and estuary habitats of young American shad by estimating the value of these habitats to fish production. Specifically, I characterized production dynamics and biogeochemical processes in the Mattaponi River, the most productive American shad nursery in Virginia’s portion of Chesapeake Bay; quantified the contribution of autochthonous and allochthonous organic matter (OM) to plankton, macroinvertebrates and young American shad in the Mattaponi River; identified the major trophic pathways that support American shad production within river and estuary habitats during ontogeny; and determined the main areas of habitat use within the Mattaponi River and York River estuary, from the larval stage to their ocean migration. Mattaponi River production dynamics were strongly influenced by river discharge; during periods of high discharge, primary production was suppressed and allochthonous OM comprised a large fraction of POM. These periods favored allochthony by zooplankton and macroinvertebrates; less than 40% of their production was supported by autochthonous production. Young American shad, which were consuming copepods and aquatic insects, were therefore also reliant on allochthonous OM, which was reflected in the stable isotope composition of their tissue. Further, spatial segregation of young American shad rearing in the freshwater nursery zone was identified through a novel application of an existing stable isotope turnover model, indicating that these river food webs are temporally and spatially distinct. During high discharge, elevated particulate OM concentration, plankton density, and juvenile American shad indices were observed, demonstrating that allochthonous OM provides an important subsidy to the metazoan food web and fish production in the Mattaponi River. Juveniles generally emigrated from the nursery zone in November and December, residing and feeding in the York River estuary and Chesapeake Bay before migrating to the ocean in February through April. Variable emigration strategies were observed; most juveniles likely emigrated at 2-5 g and spent weeks to months in the estuary, however a few emigrated at <2 g and directly swam into the lower estuary while others lingered in the freshwater habitat. American shad populations across the Atlantic coast have been in decline since the 1800s. Major causes for this decline include restricted access to historic spawning and nursery habitats and habitat loss. This study demonstrates that production and year-class strength of Mattaponi River American shad are influenced by allochthonous subsidies, including upland- and riparian-derived organic matter, and thus habitat quality is related to the health of the watershed. Further, Chesapeake Bay is an important overwintering habitat of young American shad. Protection of these habitats, and their connection to the watershed, is vital for conservation and restoration of Chesapeake Bay American shad.

Joel can be reached at Hoffman.Joel@epa.gov