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 structu re of a soft-bottom community dominated by polychaetes were examined i n a mesocosm. These 2 ophiuroids are common members of the soft-bottom community at 30 m depth in Oslofjord, Norway, where together they rea ch a density of 276 ind. m-2. The following 4 treatments were establis hed and sampled after 6 and 12 wk: ophiuroid removal, ambient ophiuroi d density, elevated ophiuroid density (3.6 times ambient), and an unma nipulated 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. Op hiuroids 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 ma nipulated in this study disturb the top 1 cm of the sediment while for aging. This behavior probably accounts for the specific effect of thes e ophiuroids on surface-dwelling taxa. Sediment traps collected signif icantly more surf ace sediment in the presence of ophiuroids than in t heir absence. A multivariate comparison of infaunal abundances among t reatments indicated a slight effect of these ophiuroids on infaunal co mmunity structure. The ophiuroids also affected species diversity (Sha nnon-Wiener Index) which was higher in the absence of ophiuroids than in the elevated ophiuroid density treatment after 12 wk.