Copepod Assemblage Dynamics in a Snowmelt-Dominated Estuary

Abstract

What limits secondary production in the estuaries of snowmelt-dominated basins? Due to substantial differences in their annual hydrographs, zooplankton dynamics in these estuaries are expected to differ from better-studied pluvial systems. We hypothesized that water residence time limited zooplankton production in the undammed, snowmelt-dominated estuary of the Fraser River, Canada. We sampled zooplankton and associated environmental variables bi-weekly to monthly in slough and channel areas of the estuary over a 33-month period. Annual mean copepod abundance at channel stations was low (2005 ± 238 ind. m−3) in comparison to other studied estuaries, with minima occurring during spring, concurrent with the freshet. Higher abundances (11,905 ± 3606 ind. m−3) observed at slough stations were attributed to estuarine copepods. Differences in abundance and species composition between slough and channel areas, and the coincidence of seasonal minima with the freshet, suggest that the low water residence time of this system limits zooplankton density and production. Heavy channelization of the Fraser River Estuary has further reduced water residence time in the estuary, likely affecting prey availability to fish and invertebrate predators. Given that peak river discharge in snowmelt-dominated basins occurs during the relatively short spring-summer growing season, the impacts of channelization on zooplankton production in the estuaries of snowmelt-dominated basins may be greater than in river-dominated estuaries subjected to different flow regimes.