Sublittoral sand dollar (Echinodiscus bisperforatus) communities in two bays on the South African south coast

Abstract

Macrofaunal community assemblages associated with Echinodiscus bisperforatus beds were investigated at three relatively sheltered, sandy subtidal areas in Plettenberg Bay and St. Francis Bay on the South African south coast. Macrofauna, meiofauna and sediment parameters together with sand dollar abundance and distribution were recorded along transects with stations at 2 m depth intervals from 4 m to 12 m. A wave-induced, depth-related turbulence gradient was evident with both mean particle size and sediment sorting decreasing with depth, whereas macrofauna diversity and biomass increased. Macrofaunal species assemblages corresponded to those of subtidal transition zones of more exposed beaches along the southern Cape coast. No single abiotic variable could be identified as the dominant influence on community structure. Distribution of sand dollars within these sites was found to be extremely patchy with densities ranging from 1–10 m ^-2 over a depth range of 4–10 m.