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

Discharge of the River Thames is already regulated to prevent flooding and to satisfy demands for water. A flood alleviation channel is under construction, while a pumped-storage reservoir has been proposed to further regulate discharge for water supply. The aim of this investigation was to assess the suitability of available quantitative macroinvertebrate data for detecting significant change in biological structure and functioning, and substratum composition, following the implementation of these schemes. Pupal exuviae of Chironomidae (non-biting midges) were collected from eight surveys of the freshwater River Thames from 1977 until 1997. Multivariate and univariate statistics were used to investigate biological variation between surveys, independently of variation between sites. Over 48% of biological variation was explained by the available physical and chemical data. Temporal variation was almost independent of spatial variation. River discharge was the most important environmental variable correlated with chironomid taxa abundances 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 and nitrogen flux between surveys. Proportions of fine sediment-dwelling chironomids, and their functional feeding types, have not significantly changed at sampling points immediately above or below the proposed reservoir or the imminent confluence with the flood alleviation channel over the 20-year survey period. These data provide the basis for detecting significant impacts by such schemes if or when they are completed. This knowledge would benefit any future plans to regulate or alleviate river flows. Copyright © 2000 John Wiley & Sons, Ltd.