Methods for ecological monitoring: Biological interactions in a rocky subtidal community

Whilst both abiotic and biotic factors affect communities, biological interactions are widely believed to be the most important factors structuring subtidal communities. Among the potential biological interactions a few “key ” species may regulate species fluctuations. A rocky subtidal community in the Oslofjord (Norway) has been investigated using stereophotography, field experiments and manipulations (settlement plates, scraping, cages). The purpose was to develop a method for biological monitoring of chronic pollution effects. Around 60 species were identified in the community. The most permanent occupants wereLithothamnion sp. (25–30% cover) andPomatoceros triqueter (ca. 20% cover). The most important fluctuation was induced by the rapid growth of the hydroidLaomedea longissima to a 100% cover in June and its rapid disappearance. Free space covered more than 30% except in the period with large hydroid occurrence. The most active settlement period was in summer and autumn. The most important predators observed wereCoryphella sp.,Asterias rubens andPasmmechinus miliaris.Coryphella grazed upon the hydroids andAsterias andPsammechinus foraged on settled organisms which resulted in an increase of free space. Predation and recruitment in that order are probably the two most important factors structuring the studied community. The “key ” species were therefore the three predators. A monitoring programme should concentrate on these predators and their influence on the success of recruitment and the abundance of hydroids and free space, since these species control the natural fluctuations in the studied community. The stereophotographic method combined with simple settlement-plate experiments seems suitable for such a monitoring programme.