L Journal of Experimental Marine Biology and Ecology, 228 (1998) 241–255 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 a ,b , a * Stephen Widdicombe , Melanie C. Austen a Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL12PY, UK b Department of Biological Science, University of Plymouth, Drakes Circus, Plymouth, UK Received 9 May 1997; received in revised form 29 January 1998; accepted 7 February 1998 Abstract The effects on infaunal diversity and sediment chemistry of bioturbation / feeding activity by different densities of the heart urchin Brissopsis lyrifera are quantified in an experiment conducted in the benthic mesocosm facility of the Norwegian Institute for Water Research at Solbergstrand, Norway. Using sediment from Bjørnhordenbukta, a small, sheltered bay in Oslofjørd, areas were subjected to 20 weeks of continuous disturbance from urchins at densities equivalent to 28 and 71 22 individuals m , whilst other areas remained undisturbed. Low density treatments, reflecting the natural field densities observed during collection of the sediment, produced higher infaunal b diversity than the heavily disturbed or control treatments and this could be attributed to a decrease in competitive exclusion. This is consistent with the predictions of the intermediate disturbance hypothesis (Connell, 1978). Bioturbation also caused a significant change in the chemistry of the surface sediment increasing oxygenation, decreasing the rates of denitrification and increasing the precipitation of phosphate. It is concluded that the disturbance activity of Brissopsis lyrifera may play a vital role in the maintenance of regional diversity and in the mediation of geochemical processes.  1998 Elsevier Science B.V. All rights reserved. Keywords: Diversity; Disturbance; Bioturbation; Macrofauna; Nutrient chemistry 1. Introduction When addressing factors associated with the maintenance of diversity, disturbance is * Corresponding author. Corresponding address: Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 2PY, UK. Tel: 1 44 1752 633 100; fax: 1 44 1752 633 101; e-mail: S.WIDDICOMBE@PML.AC.UK 0022-0981 / 98 / $ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S0022-0981(98)00032-X