Macrozooplankton Community Dynamics in Relation to Environmental Variables in Willapa Bay, Washington, USA

Willapa Bay is a large, economically and ecologically important estuary on the Washington coast, USA for which the zooplankton community has not previously been studied. Thus, in 2006 and 2007, six stations within Willapa Bay were sampled biweekly for macrozooplankton, chlorophyll, and various abiotic variables to elucidate the processes underlying community composition and dynamics. Nonmetric multidimensional scaling identified water temperature and upwelling values as major factors defining two distinct temporal communities. High densities and a community dominated by oceanic species (Calanus pacificus, Centropages abdominalis) marked the winter season, while summer (or the upwelling season) was dominated by estuarine species (Palaemonidae, Clevelandia ios). Smaller scale changes in the community were characterized by variation in chlorophyll a concentration and salinity and were marked by the presence of other taxa (Neotrypaea californiensis, Mysidae). These results point to the importance of physical processes, including the import of marine organisms and retention of estuarine organisms, in the structuring of the macrozooplankton community in Willapa Bay.