Abstract
We examined food selectivity on microalgae by aquatic insects in Hiji Steram, Ehime, whose main water source is a eutrophic reservoir, from July to August 2004. Abundance and composition of planktonic and epilithic algae, those of aquatic insects and their gut content were determined, followed by estimation of food selectivity using Chesson's alpha index. One of the dominant species in aquatic insects was the filter feeder Cheumatopsyche spp. (Hydropsychidae) during the study period. For phytoplankton, Coelastrum (Chlorophyceae) and Microcystis spp. (Cyanophyceae) were dominant throughout the study period and in August, respectively. Food selectivity of the insect on Coelastrum was negative, suggesting inedibility of Coelastrum for the insect. Positive food selectivity was detected for Microcystis in July, but, in August, the insect showed negative selectivity on Microcystis . In August, there were positive food selectivities on Pediastrum (Chlorophyceae) and Synedra (Bacillariophyceae), both of which were less dominant at the upper station. Thus, Microcystis is edible but not preferable prey for the insect. These results suggest that Cheumatopsyche spp. graze on Microcystis when there is no other edible prey available, and that the insect show selective grazing on some eukaryotic microalgae such as Pediastrum and Synedra .
