Limnol. Oceanogr., 44(6), 1999, 1522–1529

This study explored the mechanisms by which crevices act as refugia for small algae during abrasive disturbances. Four substrates with different crevice features were subjected to three levels of abrasion in a stream-based experiment. Substrates were sand-grain caddisfly cases and three glass rod substrates that mimicked caddisfly cases in size and shape, as well as forming a gradient of larger crevices (i.e., smooth with no crevices, sanded with shallow pits, and scratched with deeper grooves). Substrates were subjected to no, low, or high abrasion levels during 18 d of incubation by pulling substrates through a sand–gravel mixture in floating enclosures that housed the experiment. Diatom density, relative biovolume, location in crevices, detritus accumulation, and filamentous algal density were compared among substrate types and abrasion levels. Across all abrasion levels, diatom and filamentous algal densities were higher on caddisfly cases than on all three glass rod substrates. Although abrasion greatly reduced overall diatom density, diatoms within crevices were largely protected. Crevice size influenced diatom size and composition. The small crevices of sanded rods contained primarily small diatoms of a single species (Achnanthidium minutissimum), whereas the larger crevices of scratched rods and caddisfly cases contained a greater range of diatom size and, consequently, greater species diversity. Detritus accumulated in all crevices, and accumulation increased with level of abrasion. Diatom biovolume decreased and location in crevices increased with the level of abrasion on the less textured smooth and sanded glass rods. Abrasion level had little effect on algal assemblages on caddisfly cases. This study demonstrates that crevices can be important refugia from abrasive disturbances, and that the size of crevices relative to organismal size can influence the protective value of crevices. Algae in stony streams and in rocky intertidal zones experience disturbance in association with water movement and grazing. Disturbances resulting from water movement include the effects of floods in streams (Fisher and Grimm 1988; Resh et al. 1988; Biggs and Close 1989), abrasion by waterborne objects, such as logs in intertidal zones (Dayton 1971; Sousa 1984), and siltation from mobilized bedload (Seapy and Littler 1982; Power 1990). Grazers both consume and dislodge algae (Scrimgeour et al. 1991). These disturbances can and often reduce local algal populations in marine and freshwater habitats, yet both ecosystems are characterized by spatially dense, taxonomically diverse algal communities. Small size may be advantageous in resisting the mechanical forces of waves (Koehl 1984), and perhaps consequently, organisms in marine intertidal zones are smaller than organisms in sheltered marine and terrestrial habitats (Denny et al. 1985). Likewise, stream algae are generally small, and the combination of small size and prostrate morphology increases resistance to loss from disturbances such as spateinduced floods (Blenkinsopp and Lock 1994; Peterson 1996) and grazing (Jacoby 1987; Steinman et al. 1987; Lamberti et al. 1989). Small size has the additional benefit of enabling organisms to fit within crevices. For brevity, the term crevices is used to denote recessed areas of surfaces, such as pits, scratches, cracks, and small spaces among sessile or-