Seasonal and longitudinal variability of zooplankton assemblages along a river-dominated estuarine gradient

The Columbia River Estuary (CRE) on the Pacific coast of North America is a highly modified river-dominated estuary. Land use, human development and tributary input vary along the length of the estuary, which may impact the distribution of zooplankton; however the longitudinal and temporal variability of zooplankton have not been evaluated along the tidally-influenced length of the CRE, which extends 234 river kilometers (rkm) inland. To evaluate zooplankton dynamics along the estuarine gradient, zooplankton and environmental data were collected monthly from five sites distributed along 193 rkm (82% of the length) of the estuary over a 2-year period (2016-2018). We found that zooplankton abundance and assemblage structure exhibited strong seasonal dynamics and that abundance was higher in the year with lower river discharge. At the only saline site sampled near the river mouth, assemblages were comprised of greater proportions of estuarine and marine taxa relative to tidal freshwater sites. Zooplankton assemblages at the river mouth were best explained by salinity, discharge and temperature, while assemblages at tidal freshwater sites were best explained by temperature and season. Nonnative taxa, notably the calanoid copepod Pseudodiaptomus forbesi, comprised over 50% of zooplankton abundance at tidal freshwater sites during late summer - early autumn, but were largely absent at the river mouth. Assemblages were highly similar among tidal freshwater sites despite differences in tributary input, land use and development along the estuarine gradient. Our findings suggest that low residence times in river-dominated estuaries, such as the CRE, may contribute to homogenization of zooplankton assemblages, particularly during the high flow period and within the tidal freshwater reach.