Effects of biological disturbance by<Emphasis Type="Italic"> Monoporeia affinis</Emphasis> (Amphipoda) on small-scale migration of marine nematodes in low-energy soft sediments

Free-living marine, benthic nematodes quickly colonise sediments where physical forces are strong enough to suspend them into the water column. In the absence of such forces colonisation is much slower and is more likely to be affected by biological factors. The aim of the study was to investigate if nematodes disperse more readily in the presence of biological disturbance where physical disturbance is rare or non-existent. Amphipods are able to greatly rework sediments, and thereby induce disturbance to the infauna. A laboratory experiment with the amphipod Monoporeia affinis and nematodes from a low-energy, 30-m-deep location was conducted in mesocosms where the nematodes were given the choice to colonise azoic sediment at three amphipod densities, zero, low and high. Each area of azoic sediment in the mesocosms was divided into three equilateral sections from the nematode source, i.e. 10, 23 and 36 cm. At termination, after 7 weeks, there were no significant differences in nematode abundance and assemblage structure between treatments despite considerable biological disturbance created by the amphipods. The number of nematodes was 16%, 15% and 11% of the total numbers in the source at the three sections 10, 23 and 36 cm, respectively. There were distinct differences in the nematode community composition between distances, with the small surface-dwelling taxon Leptolaimus spp. being a rapid and the numerically dominant coloniser of the azoic sediments. Migration of nematodes over short distances is likely to be slow in the absence of strong physical forces. To our knowledge, this is the first paper ever that investigates the influence of macrofauna on nematode short-range migration.Communicated by L. Hagerman, Helsingør