Long-term fluctuations in algal cover and populations of hermit crabs and gastropods at Enewetak Atoll

Populations of hermit crabs and gastropods living in the middle and upper intertidal zones of the ocean-facing reef flat on Enewetak Island were censused four times over a 10-year period. Similarity analyses suggest that species population sizes in a topographically complex and algae-covered section of the reef flat fluctuated much more than those in a section with significantly less topographic relief and macroscopic algal cover. Sampling in a patchy environment contributed some of the variation in population sizes in the algae-covered area. However, much of the topographic complexity is provided by clumps of algal turf which may be susceptible to cropping and dislodgement by strong waves. Although algal cover in 1973 and 1980 was similar, I did not measure algal turf thickness. Reductions in thickness could limit the algal turfs effectiveness as an invertebrate refuge against physical and biological mortality factors, resulting in high mortalities of invertebrates during and after periods of heavy surf. The smaller population fluctuations in the less topographically complex area may reflect the stability of its physical refuges.