An Investigation of the Distribution of the Mayfly Fauna of a Lancashire Stream

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Abstract
The numbers of nymphs of Ecdyonurus torrentis, Heptagenia lateralis, Baetis rhodani, and Rithrogena semicolorata in a 530 sq. cm. area at each of 140 stations in Broadhead Brook and 100 stations in Wayoh Stream, Lancashire, were counted every month from September 1949 to August 1951. At each station the current and depth were measured. The temp., pH, calcium and oxygen concns. were measured at various points. Seasonal variations in numbers of E. torrentis followed the course of the life-cycle and were also affected by flood. Freezing of the stream caused a decrease in numbers. The numbers of R. semicolorata followed the life-cycle closely. H. lateralis was found to be very much affected by floods. B. rhodani was little affected by floods, and together with R. semicolorata repopulated Broadhead Brook after the fauna had disappeared during an excessive growth of algae. E. torrentis and H. lateralis moved to shallow regions at the emergence periods and during floods. R. semicolorata moved to deeper water at the time of emergence. B. rhodani is randomly distributed most of the year.

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