THE INFLUENCE OF DROUGHT-INDUCED LOW FRESH-WATER FLOW ON AN UPPER-ESTUARINE MACROINVERTEBRATE COMMUNITY

Severe drought in the Thames catchment during 1989 and 1990 resulted i n increased water abstraction from the river Thames, reducing the amou nt of freshwater flawing 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 fres hwater 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 direct ly related to their perceived sensitivity to organic pollution, with A sellidae, Caenidae and Unionidae being amongst the first families to b e affected. Multidimensional scaling ordination indicated that the com munity demonstrated acyclical pattern of disturbance and recovery rela ted to variations in salinity and flow-dependent influx of organisms v ia invertebrate drift. The study indicates that reducing freshwater fl ow below a critical level can have detrimental effects on the diversit y of macroinvertebrate communities in certain sections of tidal rivers .