EXPERIMENTAL-EVIDENCE OF EFFECTS OF THE HEART URCHIN BRISSOPSIS-LYRIFERA ON ASSOCIATED SUBTIDAL MEIOBENTHIC NEMATODE COMMUNITIES

An enclosure experiment was carried out in the benthic mesocosm facili ty of the Norwegian Institute for Water Research at Solbergstrand, Nor way, to determine the effects of the predatory/ disturbance activities of the heart urchin Brissopsis lyrifera on natural meiobenthic nemato de communities. Four 1 m(2) boxes, filled to a depth of 22 cm with fre sh sediment containing meiobenthic communities which had been collecte d from a 30 m deep site in the Oslofjord, were placed in the mesocosm. Large areas of the boxes were partitioned off with coarse mesh, 30 cm diameter cages which allowed meiofauna to move freely around within t he box but retained the Brissopsis within the caged areas. Brissopsis were added to the cages at two densities (equivalent to 28 and 71 indi viduals m(-2), respectively) with two replicates of each treatment per box (i.e. four cages per box). Meiofauna was sampled inside and outsi de the cages of each box after 20 weeks. Multivariate nematode communi ty structure varied between the uncaged sediment and the Brissopsis ca ges and between the cages with different Brissopsis densities. Univari ate measures of nematode community structure such as abundance were un affected by the Brissopsis treatments. The more abundant species gener ally decreased in tile presence of Brissopsis with the exception of Od ontophora sp. whose abundance increased with increasing Brissopsis den sity. The nematode abundance in different feeding groups varied betwee n treatments suggesting that Brissopsis altered the functional composi tion of the nematode community. High densities of Brissopsis are quite patchy in the Oslofjord and Brissopsis may act only locally as a keys tone species. However the presence of these localised high density pat ches of Brissopsis in the Oslofjord may increase the regional heteroge neity of nematode communities. (C) 1998 Elsevier Science B.V.