Organization and patterns of interactions in a subtidal sand community on an exposed coast

Abstract

Temporal and spatial patterns of distribution of the macroscopic epifauna of an exposed nearshore sand habitat in southern California were monitored from early 1970 to early 1975. This community is dominated by relatively large, long-lived suspension feeders that experience low successful recruitment and low predation rates. Of the 3 distinct zones, the shallow shoreward zone (water depths 2.6 to 6.4 m) is characterized by a harsh physical environment of unstable, shifting sands and strong wave surge, few species and individuals (mainly the clam Tivela stultorum and the sea pansy Renilla kolliker~], and a relatively low biomass. In the middle zone (water depths 6.4 to 9.1 m) there is moderate surge but relatively little sand fluctuation, and a high density (up to 1200 m-2) of sand dollars Dendraster excentricus. The sand dollars form a bed that has distinct seaward and shoreward edges; densities increase toward the seaward margin. The shoreward edge is relatively stable and is located where major sand fluctuations increase significantly toward the shore. The outer edge of the bed expands seaward in the fall and contracts shoreward in the spring. This middle zone shows the greatest numbers of total individuals and total biomass of the 3 zones. The seaward zone (water depths of 9.1 to 13.1 m) is physically the most stable. The surge velocity is generally low and sand movement is negligible. The zone is dominated by R. kollikeri, the sea pen Stylatula elongata, the tube worm Diopatra ornata and the sand star Astropecten verrilli, but shows the greatest species diversity (mainly as motile predators), few individuals, and a relatively low biomass. The shoreward distribution of many of these various motile forms appears to be physically limited during the summer by wave surge and biologically limited during the winter by competition with migrating sand dollars. Fishes and crustaceans, which occur over all zones, are primarily evident at night. However, unlike animals from hard substrate marine communities, they have no ecological equivalents within the community by day. Long-term fluctuations and periodic successful recruitment in some of the species have been detected.