EXPERIMENTAL-EVIDENCE FOR THE ROLE OF BRISSOPSIS-LYRIFERA (FORBES, 1841) AS A CRITICAL SPECIES IN THE MAINTENANCE OF BENTHIC DIVERSITY AND THE MODIFICATION OF SEDIMENT CHEMISTRY

The effects on infaunal diversity and sediment chemistry of bioturbati on/feeding activity by different densities of the heart urchin Brissop sis lyrifera are quantified in an experiment conducted in the benthic mesocosm facility of the Norwegian Institute for Water Research at Sol bergstrand, Norway. Using sediment from Bjornhordenbukta, a small, she ltered bay in Oslofjord, areas were subjected to 20 weeks of continuou s disturbance from urchins at densities equivalent to 28 and 71 indivi duals m(-2), whilst other areas remained undisturbed. Low density trea tments, reflecting the natural field densities observed during collect ion of the sediment, produced higher infaunal beta diversity than the heavily disturbed or control treatments and this could be attributed t o a decrease in competitive exclusion. This is consistent with the pre dictions of the intermediate disturbance hypothesis (Connell, 1978). B ioturbation also caused a significant change in the chemistry of the s urface sediment increasing oxygenation, decreasing the rates of denitr ification and increasing the precipitation of phosphate. It is conclud ed that the disturbance activity of Brissopsis lyuifera may play a vit al role in the maintenance of regional diversity and in the mediation of geochemical processes. (C) 1998 Elsevier Science B.V. All rights re served.