The influence of drought-induced low freshwater flow on an upper-estuarine macroinvertebrate community

Severe drought in the Thames catchment during 1989 and 1990 resulted in increased water abstraction from the river Thames, reducing the amount of freshwater flowing into the Thames estuary over Teddington weir. Increased salinity levels were recorded in the upper estuary, related to the decrease in flow, which in turn affected the community of freshwater invertebrates inhabiting a site below Teddington weir. A large decrease in BMWP score was recorded following increased abstraction in both 1989 and 1990. However, the disappearance of taxa was not directly related to their perceived sensitivity to organic pollution, with Asellidae, Caenidae and Unionidae being amongst the first families to be affected. Multidimensional scaling ordination indicated that the community demonstrated a cyclical pattern of disturbance and recovery related to variations in salinity and flow-dependent influx of organisms via invertebrate drift. The study indicates that reducing freshwater flow below a critical level can have detrimental effects on the diversity of macroinvertebrate communities in certain sections of tidal rivers.