Predation and sediment disturbance effects of the intertidal polychaete Nereis virens (Sars) on associated meiofaunal assemblages

A microcosm experiment was carried out to determine the effects of the acti vity of the burrowing polychaete Nereis virens (Sars) on the associated mei ofauna. The sediment basin (76 x 41 cm) was filled with 10 cm of sandy sedi ment previously sieved with a l-mm mesh to remove any undesired macrofauna and macrodetritus. Fifteen 13-cm long polyvinyl-chloride (PVC) tubes (I.D. = 10 cm) were pushed into the sediment to partition treatments. Nereis were added to the tubes at two densities, low (N = 1) and high (N = 3). Five tu bes were: used as controls (no Nereis), while two sets of five tubes were u sed for the low (L) and high (H) density treatments, respectively. After 14 days, meiofauna was sampled by coring. Cores were cut into three slices: s urface (0-1 cm), subsurface (1-5 cm), and deep (5-10). High densities of Ne reis (H) significantly affected nematodes, harpacticoid copepods, and naupl ii abundance. However, lower abundances were found only in the top cm of th e sediment. Moreover, a significant number of dead nematodes found in this sediment layer of treatment H allowed a distinction between sediment distur bance effects and predation effects. Sediment disturbance caused by Nereis may be related to an intensive "ploughing" of surface sediment during food- searching activity. Diversity indices were affected only in the top cm of t he sediment with generally lower values in treatment H. Differences in the relative survival of the different feeding groups were found in treatment H , where microvores and deposit feeders respectively showed greater and lowe r survival. Multivariate analysis (multidimensional scaling) revealed signi ficant differences in nematode species composition among treatments in all sediment layers. It is concluded that N. virens significantly affects meiof auna mostly by disturbance of the top cm of the sediment where its predatio n represents an influent force as well. The structure of nematode assemblag es in subsurface and deeper sediment layers is also affected, most likely b y changes in redox conditions caused by the bioirrigating effects of Nereis burrows. (C) 2000 Elsevier Science B.V. All rights reserved.