Methods for biological monitoring: Biological interactions in communities of subtidal sediments

Predation has been demonstrated to be of fundamental importance in structuring benthic communities in the intertidal zone. The aim of the present investigation was to elucidate some of the effects of predation on structuring communities in unvegetated subtidal sediments. Field manipulative cage experiments were performed on sediment in the inner part of the Oslofjord (Norway), where the species composition was typical for a moderately organically enriched sediment. Sediment from this area was transferred to an area not suspected a priori to be seriously affected by organic pollution, and the effect of predation on the fauna was evaluated. Predation effects were not observed in the Oslofjord experiments partially because of extensive obstruction of recruitment to the sediment by settlement ofPolydora antennata on the cage; however, further experiments are in progress in this area. No such settling was observed in the unpolluted area. Here 63 taxonomic groups were identified, 57 in the control and 50 in the cages; 43 taxonomic groups were found in both the cage and the control. The total number of individuals was significantly higher in the cage (4779) than in the control (2849). The fauna recruited to the sediment in the cage responded to decreased predation by macropredators by a significant reduction in diversity. Of the 10 most abundant groups 3 (Tellinacea, Syllidae andPholoë minuta) were significantly more abundant in the cage than in the control; 3 others (Prionospio malmgreni, Microphthalmus abberans and Paraonidea) were significantly more abundant in the control. It is concluded that in an unpolluted area predation is an important factor in controlling numbers of at least some of the most abundant species. However, the effect of predation does not seem to be of the same importance in the subtidal as has previously been recognized for unvegetated intertidal mudflats.