Recruitment and development of epibioses on artificial and cleared substrata at two sites in Gullmarsfjorden on the Swedish West Coast

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Abstract
Recruitment and community development on artificial and natural scraped-off substrata were compared between two sites at a depth of 15m in Gullmarsfjorden on the Swedish west coast. The sites are separated by a distance of about 9 km. At each site, three asbestos panels were suspended freely from an aluminium rod in the water column away from the vertical rock wall. Three similar panels were hung against the rock wall. Additionally, three 0.25 m2 areas ofrock wall were cleared ofliving organisms and three similar areas were left undisturbed as controls. The experiment was performed twice. Panels, scraped-off areas and controls were monitored by stereophotography. Recruitment and community development among treatments were quantitatively and qualitatively different between sites. Within sites, recruitment and community development differed between artificial substrata and cleared natural substrata. At the exp.sed site, however, panels positioned on the rock wall were colonized by the same annual ascidians as the scraped-off areas. Recruitment to both artificial and natural substrata was by settling larvae as lateral growth and migration from adjacent organisms did not occur. Established assemblages in the control areas were relatively persistent during the experiments and did not receive any substantial recruitment of the annual ascidians Ascidiella scabra and Ciona intestinalis, which otherwise influenced the community structure on the panels and cleared natural substratum. Cionarecruited more heavily on panels away from the rock wall and Ascidiellaon panels against the rock wall and cleared natural substratum. This pattern is explained by differential settlement rather than the effects of predation and dislocation. Recruitment of ascidians was found to be synchronous at the two stations.