Some effects of organic pollution on the distribution and seasonal incidence of Chironomidae in riffles in the River Cole

Abstract

SUMMARY. The effects of a sewage effluent discharge into the River Cole, Birmingham, were determined by regular collections of water and biological samples from six comparable stations, one above and five below the sewage effluent. The effluent reduced the dissolved oxygen concentration and increased the ammonia, orthophosphate and BOD concentrations in the water. The effects were much more severe during the summer.The Chironomidae were very important constituents of the benthic macroinvertebrate community at the stations downstream of the effluent. Larval densities of over 30,000 m−2 were recorded. Eleven species were common and of these Chironomus riparius was the most tolerant. There was considerable variation in the seasonal incidence of the different species. Brillia longifurca was most abundant in the spring while Cricotopus bicinctus was most abundant in the summer. Brillia longifurca, Paratrichocladius rufiventris, Prodiamesa olivacea, Micropsectra atrofasciata and Eukiefferiella claripennis had three peaks in numbers of larvae during the year.The increase in the polluted state of the river in the summer resulted in changes in the chironomid populations. Some were suppressed, e.g. E. claripennis, while C. riparius extended its range and achieved maximum abundance further downstream during the summer.