Size-related response of zooplankton to hydroclimatic variability and water-quality in an organically polluted estuary of the Basque coast (Bay of Biscay)

Seasonal, interannual and spatial variabilities were analysed for the > 100 μm and > 200 μm zooplankton assemblages in the estuary of Bilbao throughout 1997–2000, and related to hydroclimatic and water-quality factors. Seasonal and interannual patterns of abundance differed between assemblages, and only the abundance of the > 100 μm zooplankton was correlated with temperature. The large-sized zooplankton decreased more drastically than the small-sized zooplankton with decreasing salinity, and showed highest correlations with dissolved oxygen and water transparency. Seasonal changes were the major variability mode of zooplankton composition in both assemblages. Interannual changes associated to phytoplankton biomass and temperature were more evident in the small-sized zooplankton, whilst spatial differences related to oxygen depletion were more evident in the large-sized zooplankton. This indicates that small zooplankton was more sensitive to climate-related factors, while large zooplankton was more sensitive to water quality. Small copepods, cirriped larvae, appendicularians and Noctiluca were the main taxa responsible for the interannual variations in the > 100 μm assemblage. In the > 200 μm assemblage, large copepods were the main zooplankton responsible for the spatial variability related to oxygen depletion. Cnidarians and cladocerans responded mainly to decreases in salinity, and polychaete larvae were found to be unaffected by the decrease in dissolved oxygen and salinity. Results suggest that size-related differential responses of zooplankton should be considered when zooplankton monitoring is performed to assess the effect of climate forcing and pollution in coastal and estuarine environments.

► Seasonal, interannual and spatial variability was compared for two different-sized zooplankton assemblages.
► Seasonal changes were the major variability mode of zooplankton composition in both assemblages.
► Interannual changes were more evident in the small-sized zooplankton.
► Spatial differences were more evident in the large-sized zooplankton.
► Small zooplankton was more sensitive to climatic factors, while large zooplankton was more sensitive to water quality.