Long-term data assessment of chironomid taxa structure and function in theRiver Thames

Discharge of the River Thames is already regulated to prevent flooding and to satisfy demands for water. A flood alleviation channel is under construc tion, while a pumped-storage reservoir has been proposed to further regulat e discharge for water supply. The aim of this investigation was to assess t he suitability of available quantitative macroinvertebrate data for detecti ng significant change in biological structure and functioning, and substrat um composition, following the implementation of these schemes. Pupal exuvia e of Chironomidae (non-biting midges) were collected from eight surveys of the freshwater River Thames from 1977 until 1997. Multivariate and univaria te statistics were used to investigate biological variation between surveys , independently of variation between sites. Over 48% of biological variatio n was explained by the available physical and chemical data. Temporal varia tion was almost independent of spatial variation. River discharge was the m ost important environmental variable correlated with chironomid taxa abunda nces between surveys, particularly in the lower Thames. High concentrations of un-ionized ammonia occurred during 1977, the effects of this were also greatest in the lower Thames. Indicator taxa have been identified which can be used as a simple monitoring tool to track the effects of varying flow a nd nitrogen flux between surveys. Proportions of fine sediment-dwelling chi ronomids, and their functional feeding types, have not significantly change d at sampling points immediately above or below the proposed reservoir or t he imminent confluence with the flood alleviation channel over the 20-year survey period. These data provide the basis for detecting significant impac ts by such schemes if or when they are completed. This knowledge would bene fit any future plans to regulate or alleviate river flows. Copyright (C) 20 00 John Wiley & Sons, Ltd.