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 facility of the Norwegian Institute for Water Research at Solbergstrand, Norway, to determine the effects of the predatory/ disturbance activities of the heart urchin Brissopsis lyrifera on natural meiobenthic nematode communities. Four 1 m(2) boxes, filled to a depth of 22 cm with fresh sediment containing meiobenthic communities which had been collected 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 the box but retained the Brissopsis within the caged areas. Brissopsis were added to the cages at two densities (equivalent to 28 and 71 individuals m(-2), respectively) with two replicates of each treatment per box (i.e. four cages per box). Meiofauna was sampled inside and outside the cages of each box after 20 weeks. Multivariate nematode community structure varied between the uncaged sediment and the Brissopsis cages and between the cages with different Brissopsis densities. Univariate measures of nematode community structure such as abundance were unaffected by the Brissopsis treatments. The more abundant species generally decreased in the presence of Brissopsis with the exception of Odontophora sp. whose abundance increased with increasing Brissopsis density. The nematode abundance in different feeding groups varied between treatments suggesting that Brissopsis altered the functional composition of the nematode community. High densities of Brissopsis are quite patchy in the Oslofjord and Brissopsis may act only locally as a keystone species. However the presence of these localised high density patches of Brissopsis in the Oslofjord may increase the regional heterogeneity of nematode communities. (C) 1998 Elsevier Science B.V.