EFFECTS OF PREDATION AND DISTURBANCE BY OPHIUROIDS ON SOFT-BOTTOM COMMUNITY STRUCTURE IN OSLOFJORD - RESULTS OF A MESOCOSM STUDY

Effects of 2 ophiuroids (Ophiura affinis and O. albida) on the structure of a soft-bottom community dominated by polychaetes were examined in a mesocosm. These 2 ophiuroids are common members of the soft-bottom community at 30 m depth in Oslofjord, Norway, where together they reach a density of 276 ind. m-2. The following 4 treatments were established and sampled after 6 and 12 wk: ophiuroid removal, ambient ophiuroid density, elevated ophiuroid density (3.6 times ambient), and an unmanipulated control. Polychaetes inhabiting the top 3 cm of the sediment were 30% and surface-dwelling, deposit-feeding, sedentary infauna 40% less abundant in the presence of ophiuroids than in their absence. Ophiuroids had no effect on abundances of taxa living deeper than 3 cm. No significant differences in infaunal abundances were detected among any of the treatments containing ophiurioids. The ophiuroid species manipulated in this study disturb the top 1 cm of the sediment while foraging. This behavior probably accounts for the specific effect of these ophiuroids on surface-dwelling taxa. Sediment traps collected significantly more surf ace sediment in the presence of ophiuroids than in their absence. A multivariate comparison of infaunal abundances among treatments indicated a slight effect of these ophiuroids on infaunal community structure. The ophiuroids also affected species diversity (Shannon-Wiener Index) which was higher in the absence of ophiuroids than in the elevated ophiuroid density treatment after 12 wk.