Macrofauna recolonization of subtidal sediments. Experimental studies on defaunated sediment contaminated with cruce oil in two Norwegian fjords with unequal eutrophication status. I. Community responses.

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Abstract
Experiments were performed by placing oiled and unoiled defaunated sediment in boxes (0.2 m2) on the sea floor in 2 Norwegian fjords, the eutrophicated Oslofjord (4 boxes, April to July 1980) and the non-eutrophicated Raunefjord (12 boxes, February 1981 to March 1982). In the Oslofjord no negative effects of the added oil (3920 ppm wet weight sediment) on macrofauna community structure were seen after 3 mo. Thus restoration of the community took less than 3 mo during a spring/summer situation in a soft bottom area with a species composition dominated by opportunistic species (Polydora spp.). In the Raunefjord the effect of the added oil (4520 ppm) was shown in reduced species diversity after 4 mo. and altered k-dominance curves, distribution of individuals among species, Hurlbert''s rarefaction curves and multidimensional scaling plots. The added oil reduced the mean equilibrium number of species per box from 80 to 55. The time needed to reach 90% of this equilibrium was shorter in the oiled boxes (259 d) than in the control boxes (466 d). Effects were most severe after 4 mo on the filter-feeders and surface deposit feeders, after 9 and 13 mo on subsurface deposit feeders. Immigration rates were similar (0.27 species d-1 box-1) in both treatments during June to November; however, from November to March the rate was higher in control boxes (0.24) than in oiled boxes (0.17). Mean extinction rates in boxes (species/species d-1 box-1) were, however, larger in the oiled boxes than in the control boxes during both periods (Oil: 0.0048, 0.0033; Control: 0.0027, 0.0024). For both fjords animals retained on a 250 .mu.m sieve generally showed higher densities in the oiled sediment. It is concluded that restoration of the benthic macrofauna after oil contamination of sediment takes longer in a noneutrophicated area than in a eutrophicated area. The reduced density of macrofauna in the oiled boxes in the Raunefjord was caused by toxic response to oil directly, or by secondary effects leading to increased mortality, rather than by reduced settlement. Macrofauna recolonization is easily affected by oil-contamination in a non-eutrophicated area; however separation of oil-specific responses in benthic communities from responses to other disturbances can hardly be demonstrated without relevant controls.