Effects on invertebrate populations of drought-induced changes in estuarine water quality

Between the years 1989 and 1992, severe drought conditions occurred in the Thames estuary catchment, significantly reducing freshwater flows into the estuary and affecting estuarine water quality. Long-term data assembled for water quality between 1977 and 1992 were combined with invertebrate abundance data obtained from the intake screens of West Thurrock power station over the same period. This allowed an examination of the influence of drought-induced changes in water-quality parameters on the populations of the most abundant mobile estuarine invertebrates. For 8 invertebrate species, seasonal and monthly means for pre-drought (1977 to 1988) and drought (1989 to 1992) abundances were calculated and tested for significant differences. Time-series data were used to estimate regression models explaining observed variations in populations of Carcinus maenas, Crangon crangon and Gammarus spp. during both periods. Stability tests were used to establish significant differences in pre-drought and drought models. Significant mean seasonal and monthly differences were found for C. maenas and Gammarus spp. in both winter and summer, with significant summer differences being apparent for C. crangon and Palaemon longirostris. Regression analyses confirmed these results for the modelled species and highlighted the significance of drought-related changes in temperature for all 3 crustaceans, with dissolved oxygen being an additional significant variable for C. crangon. The reported significant changes in abundance of the studied species during drought conditions have important implications for the structure and dynamics of estuarine food webs, as C. maenas, Gammarus spp., and particularly C. crangon, are either important predators on invertebrates and juvenile fish or major food items for estuarine fish species.