Seasonal, annual, and spatial variation in the development of hard bottom communities

Abstract

The development of hard bottom communities has been studied on test panels in Helgoland Harbour (German Bight) since 1977. Settlement and growth of epibenthic species was examined monthly. Natural variation in different seasons, years, and at three stations (the latter, only in 1981 and 1982) was investigated. At Station A (Binnenhafen), barnacles(Balanus crenatus) and polychaetes(Polydora ciliata) were always among the first settlers in spring. They were followed by other barnacles(Elminius modestus, Balanus improvisus) and by colonial ascidians(Botryllus schlosseri). The latter species often dominated from August to October, and tended to overgrow the barnacle populations.E. modestus showed strong annual variation, probably due to extremely low winter temperatures: after the cold winter of 1978/79, its populations were less dense than in previous years. In 1981 they recovered, and settlement increased again, but the cold winter 1981/82 damaged the population again. At Station B (Nordosthafen), mussels(Mytilus edulis) soon covered barnacles and empty space. By October they had monopolized the fouling community. At Station C (Sudhafen), barnacle settlement in spring was followed by an overgrowth of hydrozoans (Laomedea spec.). In summer, ascidians (Ciona intestinalis andAscidiella aspersa) settled and began to dominate. Barnacles were weaker in the competition for space as opposed to later colonizers at all three stations.