Reprint Series No. 9

ASSEMBLAGE STRUCTURE, MICROHABITAT DISTRIBUTION, AND FOOD WEB LINKAGES OF EPIBENTHIC CRUSTACEANS IN PADILLA BAY NATIONAL ESTUARINE RESEARCH RESERVE, WASHINGTON.

Charles A. Simenstad, Jeffrey R. Cordell, Robert C. Wissmar, Kurt L. Fresh, Steven L. Schroder, Mark Carr, Gene Sandborn, and Mary Burg

1988

Bibliographic Citation
Simenstad, Charles A., Jeffrey R. Cordell, Robert C. Wissmar, Kurt L. Fresh, Steven L. Schroder, Mark Carr, Gene Sandbom, and Mary Burg. 1988. Assemblage structure, microhabitat distribution, and food web linkages of epibenthic crustaceans in Padilla Bay National Estuarine Research Reserve, Washington. Report to NOAA/OCRM/MEMD by University of Washington, Fisheries Research Institute (FRI-UW-8813). 60 pp. Seattle, Washington. Padilla Bay National Estuarine Research Reserve Reprint Series No. 9.

Abstract
In May 1986, the assemblage structure of epibenthic meiofauna and small macrofauna, their relative availability at different stages in the tidal cycle and in different microhabitats, and their trophic importance to fishes were examined in four habitats across a littoral flat gradient in Padilla Bay, Washington. Composition and standing stock of epibenthic crustaceans were described for three stages in the tidal cycle: (1) those in sediments during tidal exposure; (2) those carried by the leading edge of the inundating tide; and (3) those found in the benthic boundary layer during areal submergence. Microhabitat utilization of epibenthos living on the Bay's principal macrophyte, the eelgrass Zostera marina, was also examined among sections of eelgrass blades, which represented different degrees of epiphyte growth and blade morphology. Nematodes and harpacticoids numerically dominated surface sediments and the benthic boundary layer, harpacticoids predominated in the leading edge of the inundating tide and on Z.marina blades. Total epibenthos density and standing crop were an order of magnitude different in the three microhabitats, decreasing from 1 X 107 m-2 in the surface sediment to 1 X 106 m-2 during tidal inundation, and 1 X 105 to 1 X 104 m-2 in the benthic boundary layer. Harpacticoid densities on eelgrass blades with high epiphyte growth averaged twice as dense (113 100-cm-2) as on plants with low epiphyte growth (54 100-cm-2), excluding the basal segments adjacent to the substrate and rhizomes, where densities increased to almost 2000 100-cm-2 on the high epiphyte plant. Using numerical analysis techniques, we were able to discriminate eleven assemblages among six habitat clusters. Four assemblages were unique to specific habitats and/or microhabitats, while the others tended to be distributed ubiquitously across the littoral flat. Analyses of stomach contents of five fishes prominent in the estuary at this time - juvenile and adult surf smelt (Hypomesus pretiosus), juvenile Pacific herring (Clupea harengus pallasi), juvenile chum salmon (Oncorhynchus keta), adult Pacific sand lance (Ammodytes hexapterus), and adult threespine stickleback (Gasterosteus aculeatus) - indicated that epibenthic crustaceans, specifically harpacticoid copepods, dominated prey composition except in herring, which had fed on burrowing shrimp larvae. Furthermore, harpacticoid taxa in all the fishes' diets were predominantly (~50%-94% harpacticoid abundance) restricted to the three taxa - Harpacticus uniremis, Zaus sp. and Tisbe sp. The first two of these harpacticoid taxa were prominent members of one assemblage unique to Z. marina blades, particularly the plant with high epiphyte growth; while Tisbe sp. was most abundant in the eelgrass habitats, it was one of the taxa dispersed broadly throughout the flat. These results provide evidence that the predominant prey of both resident and non-resident estuarine fishes in Padilla Bay are harpacticoid copepods, which originate in eelgrass beds, and therefore extend the functional importance of eelgrass habitats beyond the habitat's resident fish community.