Effects of predation and disturbance by ophiuroids on soft-bottom community structure in Oslofjord: results of a mesocosm study

Abstract

Effects of Ophiura affinis and O. albida on the structure of a soft-bottom community dominated by polychaetes were examined in a mesocosm. These ophiuroids are common members of the soft-bottom community at 30m depth in Oslofjord, Norway, where together they reach a density of 276 ind.m-2. Polychaetes inhabiting the top 3cm 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 deepr than 3cm. No significant differences in infaunal abundances were detected among any treatments containing ophiuroids. The ophiuroid species manipulated in this study disturb the top 1cm of the sediment while foraging. This behaviour probably accounts for the specific effect of these ophiuroids on surface-dwelling taxa. The ophiuroids also affected species diversity which was higher in the absence of ophiuroids than in the elevated ophiuroid density treatment after 12 wk. -from Author