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The Influence of Periphyton Abundance on Baetis bicaudatus Distribution and Colonization in a Small Stream

Colonization, distribution, and immigration of Baetis bicaudatus on rock surfaces was observed in a small Idaho (USA) stream. Baetis densities on rocks were assessed by direct observation through clear plastic viewing boxes. Baetis distributions among regions on individual rocks was related to periphyton abundance. Densities of Baetis on periphyton patches reflected the relative abundance of periphyton. Among rocks, distributions also were most strongly influenced by periphyton abundance. Water velocity and substrate size did not greatly influence distribution patterns. Chlorophyll a was the best indicator of periphyton abundance. Immigration rates, determined through time-lapse photography, were as high as 5.8 individuals/100 cm2/hr. Over 50% of nymphs arrived from the drift. At such immigration rates, the proportion of immigrants that emigrate per unit time would have to be high to maintain densities observed on rocks after 48 hr. The results of this study show that Baetis bicaudatus abundance on rock surfaces is an interplay between immigration rate and nymphal response to food abundance.