Long-term distribution patterns of mobile estuarine invertebrates (Ctenophora, Cnidaria, Crustacea: Decapoda) in relation to hydrological parameters

Between 1977 and 1992, semi-quantitative samples of macroinvertebrates were taken at fortnightly intervals from the Thames Estuary (UK) utilising the cooling water intake screens of West Thurrock power station. Samples were taken for 4 h over low water, the abundances of invertebrates recorded in 30 min subsamples and related to water volume filtered. Abundances of the major estuarine species have therefore been recorded every 2 wk for a 16 yr period, together with physicochemical parameters such as temperature, salinity and freshwater flow. Annual cycles of distribution were apparent for several species. Carcinus maenas exhibited a regular annual cycle, with a peak in autumn followed by a decrease in numbers over winter, relating to seasonal temperature patterns. Conversely, abundance of Crangon crangon was consistently lowest in summer, responding to seasonal changes in salinity, whilst Liocarcinus holsatus, Aurelia aurita and Pleurobrachia pileus were only present in summer samples, with P. pileus often in vast numbers (>100 000 per 500 million l). The estuarine prawn Palaemon longirostris showed no obvious sustained annual pattern, but evidence for a longer cycle of distribution was apparent. During 1989-1992 severe droughts in southeast England severely disrupted annual salinity patterns and coincided with a large increase in the Chinese mitten crab Eriocheir sinensis population. This included the first synchronised migration of adults in the UK. Settlement of young crabs during low-flow periods is suggested as an explanation for this population increase.