Man-Made Structures on Marine Sediments: Effects on Adjacent Benthic Communities

This study (1975-1977) examines the effect of man-made structures on natural sand bottom communities in shallow water in San Diego County, southern California, USA. While there were shallow scour effects to 15 m around some artificial reefs, the reefs had no measurable effect on sand ripple patterns, grain size, organic carbon or infauna beyond the scoured areas. Foraging by reef-associated fishes produced profound alterations in the epifauna populations of the sea pen Stylatula elongata. The sea pen densities were 4 to I0 m -2 before the reefs were established, but within 5 mo were eliminated from distances greater than 200 m around the reefs. On the other hand, densities of the tube-building polychaetes Diopatra spp. seemed to be enhanced in the immediate vicinity of the artificial reef. Oil platforms and bridge pilings seem to have much more profound effects on the nearby sand communities than do the relatively small artificial reefs. In addition to the elimination of sea pens, Diopatra spp. densities increased from < 1.0 m -2 in control areas to as many as 73 m -~ in the vicinity of oil platforms. Grain size and infauna were strongly affected by the oil platform. Vassar, 1975; Alfieri, 1975; Russell, 1975; Hanson and Bell, 1976; Sheehy, 1976; Allen et al., 1976) and on the fishes which they attract (Randall, 1963; Carlisle etal., 1964; Turner etal., 1969; Dewees and Gotshall, 1974; Higo, 1974; Russell, 1975; Prince, 1975; Prince and Gotshall, 1976; Allen etal., 1976; Sonnier etal., 1976; Talbot et al., 1978; Patten, 1981). With a few exceptions, little attention has been paid to the effect of these objects on the natural sedimentary communities in which most are placed. Introduced structures may alter soft-bottom habitats in several ways. They affect the wave field and alter current patterns causing scour and changes in sand ripple patterns and sediment grain size. They entrap drift algae and other organic material which, along with the activities and deaths of reef-associated organisms, can result in organic build-up in the sediment. Shells from barnacles and other fouling biota enter and modify sediments. Finally, predators attracted by the structure may forage on plants and animals that live in adjacent sediments. In this paper we examine the effect of man-made structures on the infaunal and epifaunal components of adjacent natural soft-bottom communities off southern California, USA.