The effects of ditching operations on the benthic fauna of a stream in an agricultural area of Quebec
A quantitative and qualitative study of the benthos of Ruisseau des Anges (Montcalm County, Quebec), in an agricultural area, in 1986 to 1988 was undertaken to evaluate the effects of ditching operations in 1987. Sampling included upstream zones where the ditching occurred, downstream stations, and control areas. All streams were enriched, particularly in nitrogen, and their benthic fauna was dominated by Tubificidae (Oligochaeta). Even the control area had a reduced fauna dominated by Chironomidae (Diptera), Sphaeriidae (Mollusca), Elmidae (Coleoptera) and Sialidae (Megaloptera), all resistant to pollution. The ditching operations eliminated the accumulated mud and destroyed the vegetation winch choked the streams. This allowed a colonization by Chironomidae. However, by the fall of 1988 much silting had occurred and the Tubificidae reestablished, though not in the high densities of 1986. Any improvement of the benthic fauna was marginal and shortlived, as silting and organic pollution restored the degradated conditions in the absence of erosion control. The use of several biotic indices to assess stream conditions showed that these are often unsatisfactory in lowland streams.