title: Stable isotope-based trophic structure of pelagic fish and jellyfish across natural and anthropogenic landscape gradients in a fjord estuary.
abstract: Identifying causes of structural ecosystem shifts often requires understanding trophic structure, an important determinant of energy flow in ecological communities. In coastal pelagic ecosystems worldwide, increasing jellyfish (Cnidaria and Ctenophora) at the expense of small fish has been linked to anthropogenic alteration of basal trophic pathways. However, this hypothesis remains untested in part because baseline description of fish-jellyfish trophic dynamics, and the environmental features that influence them are lacking. Using stable isotopes of carbon (δ13C) and nitrogen (δ15N), we examined spatiotemporal patterns of fish and jellyfish trophic structure in greater Puget Sound, an urbanizing fjord estuary in the NW United States. We quantified niche positions of constituent species, niche widths and trophic overlap between fish and jellyfish assemblages, and several community-level trophic diversity metrics (resource diversity, trophic length, and niche widths) of fish and jellyfish combined. We then related assemblage- and community-level measures to landscape gradients of terrestrial-marine connectivity and anthropogenic influence in adjacent catchments. Relative niche positions among species varied considerably and displayed no clear pattern except that fish generally had higher δ15N and lower δ13C relative to jellyfish, which resulted in low assemblage-level trophic overlap. Fish assemblages had larger niche widths than jellyfish in most cases and, along with whole community trophic diversity, exhibited contrasting seasonal patterns across oceanographic basins, which was positively correlated to landscape variation in terrestrial connectivity. In contrast, jellyfish niche widths were unrelated to terrestrial connectivity, but weakly negatively correlated to urban land use in adjacent catchments. Our results indicate that fish-jellyfish trophic structure is highly heterogeneous and that disparate processes may underlie the trophic ecology of these taxa; consequently, they may respond divergently to environmental change. In addition, spatiotemporal variation in ecosystem connectivity, in this case through freshwater influence, may influence trophic structure across heterogeneous landscapes.
authors.affiliation: Department of Zoology University of British Columbia Vancouver BC Canada.
authors.forename: Sean M
authors.initials: SM
authors.lastname: Naman
authors.affiliation: NOAA Fisheries Northwest Fisheries Science Center Seattle WA USA.
authors.forename: Correigh M
authors.initials: CM
authors.lastname: Greene
authors.affiliation: NOAA Fisheries Mukilteo Research Station Mukilteo WA USA.
authors.forename: Casimir A
authors.initials: CA
authors.lastname: Rice
authors.affiliation: NOAA Fisheries Northwest Fisheries Science Center Seattle WA USA.
authors.forename: Joshua
authors.initials: J
authors.lastname: Chamberlin
authors.affiliation: NOAA Fisheries Northwest Fisheries Science Center Seattle WA USA; Present address: Washington Department of Fish and Wildlife Habitat Program Olympia WA USA.
authors.forename: Letitia
authors.initials: L
authors.lastname: Conway-Cranos
authors.affiliation: School of Aquatic and Fishery Sciences University of Washington Seattle WA USA.
authors.forename: Jeffery R
authors.initials: JR
authors.lastname: Cordell
authors.affiliation: NOAA Fisheries Northwest Fisheries Science Center Seattle WA USA.
authors.forename: Jason E
authors.initials: JE
authors.lastname: Hall
authors.affiliation: NOAA Fisheries Northwest Fisheries Science Center Seattle WA USA.
authors.forename: Linda D
authors.initials: LD
authors.lastname: Rhodes
keywords: Puget Sound; fish; food webs; jellyfish; stable isotopes; trophic structure