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) util ising the cooling water intake screens of West Thurrock power station. Samples were taken for 4 h over low water, the abundances of inverteb rates recorded in 30 min subsamples and related to water volume filter ed. Abundances of the major estuarine species have therefore been reco rded every 2 wk for a 16 yr period, together with physicochemical para meters such as temperature, salinity and freshwater flow. Annual cycle s of distribution were apparent for several species. Carcinus maenas e xhibited a regular annual cycle, with a peak in autumn followed by a d ecrease in numbers over winter, relating to seasonal temperature patte rns. Conversely, abundance of Crangon crangon was consistently lowest in summer, responding to seasonal changes in salinity, whilst Liocarci nus holsatus, Aurelia aurita and Pleurobrachia pileus were only presen t in summer samples, with P. pileus often in vast numbers (>100 000 pe r 500 million l). The estuarine prawn Palaemon longirostris showed no obvious sustained annual pattern, but evidence for a longer cycle of d istribution was apparent. During 1989-1992 severe droughts in southeas t England severely disrupted annual salinity patterns and coincided wi th a large increase in the Chinese mitten crab Eriocheir sinensis popu lation. This included the first synchronised migration of adults in th e UK. Settlement of young crabs during low-flow periods is suggested a s an explanation for this population increase.