The influence of crevice size on the protection of epilithic algae from grazers

1. This study investigated how the size of crevices might affect their effectiveness as refuges for diatom-dominated algal assemblages from the grazing minnow Campostoma anomalum. 2. Crevice size was manipulated by making fired-clay substrates, using moulds to produce eight substrates with pits from 1.17 to 22.0 mm diameter. Non-pitted clay-stones and limestone were also tested. Cages were used to control the access of Campostoma to arrays of the 10 different algal-colonised substrates. The grazing treatments were: open and grazed, caged and ungrazed, and a grazed cage control. The experiment was replicated in eight large outdoor tanks. After 3 weeks, substrates were brushed and chlorophyll a concentrations of the removed algae and the algae remaining in pits were measured. 3. The experiment was field-validated by exposing arrays of substrates to grazing Campostoma in five pools of a limestone stream. 4. The clay-stone and limestone substrates accrued similar algal biomass and assemblages. 5. Smaller crevices provided more protection against grazing than larger crevices. Specifically, pits with openings smaller than 2 mm protected the enclosed algal assemblages in both the tank and field experiments. Larger pits provided less protection and pits over 7 mm in diameter were heavily grazed and may even be preferentially grazed by Campostoma. 6. None of the tested pit sizes were protective against larval chironomid grazers in the tank experiment, demonstrating that differences in the grazer size influence the effectiveness of crevice refuges.