A survey of the bottom fauna of streams in the Scottish Highlands part III seasonal changes in the fauna of three streams

Abstract

1. The paper deals with seasonal changes in the composition and amount, by number and weight, of benthic organisms in riffles of three streams of different character (Shelligan Burn, River Almond, Allt dos Mhuicarain). Of the three streams the Shelligan Burn has the richest, and Allt dos Mhuicarain the poorest, chemical composition and surrounding vegetation. The River Almond has similar surroundings to Allt dos Mhuicarain but contains a much greater concentration of dissolved ions. 2. The technique used to sample the bottom fauna is critically examined and compared with that using the Surber sampler. 3. The mean standing stocks of bottom fauna, as dry weight per m2, were 2.88 g for the Shelligan Burn, 0.93 g for the River Almond and 0.70 g for Allt dos Mhuicarain. The greatest weight of bottom fauna in the Shelligan Burn occurred in May, and in the River Almond in April and May, when a high proportion of the organisms were of a large size prior to emergence. After their emergence the numbers of animals fell by 1/4 but the weight of animals present fell by 1/2. The largest numbers of animals in the Shelligan Burn occurred in July and February. 4. The animals collected from the three streams were separated into 67 taxonomic categories, 37 of which were species. Fifty-three categories were represented in the Shelligan Burn, 45 in the River Almond and 49 in Allt dos Mhuicarain. Thirty-four categories, of which 17 were species, were common to all three streams. The fauna of the River Almond was similar in composition but quantitatively less than that of the Shelligan Burn. Allt dos Mhuicarain usually had less dense populations of the species present in the Shelligan Burn and River Almond. Some differences between the faunas of the streams can be attributed to differences in the chemical composition of the streams and special features of Allt dos Mhuicarain. 5. Details of the seasonal changes in the numbers ofLeuctra inermis, Amphinemura sulcicollis, Isoperla grammatica, Ephemerella ignita andRhithrogena semicolorata caught in the three streams, and the seasonal changes in their biomass and size-frequency distribution in the Shelligan Burn are given. The sampling programme extended over three winters of differing severity, but no differences were found in the sizes attained in mid-February each year, by species of bottom fauna usually considered to grow during the winter months. 6. Riffle stretches make up the greatest part of the three streams examined, but there are pools of slower flowing water and, in the Shelligan Burn, they constitute about 10% of the surface area. Riffles and pools of the Shelligan Burn were sampled in April and September. The riffles contained a higher numerical density of bottom fauna than the pools in the spring and in the autumn. There was no significant difference in the biomass densities of the bottom fauna in the two habitats.Leuctra spp. andBaetis spp. were several times more common in the riffles than in the pools.Ecdyonurus spp. were found in greater concentration in the pools than in the riffles in April, but were more concentrated in the riffles in September. The heavier, burrowing species such asDicranota spp., other Tipulidae andEiseniella spp. were more densely distributed in the pools than in the riffles.Chloroperla torrentium occurred in equal densities in the riffles and pools.