The effect of bacterivorous nematodes on detritus incorporation by macrofaunal detritivores: A study using stable isotope and fatty acid analyses

Several studies have investigated the effect of nematodes on microbial degradation of macrophyte detritus, but little is known about the potential effect of bacterivorous nematodes on productivity of macrofaunal detritivores. We investigated the influence of the bacterivorous nematode Rhabditis (Pellioditis) mediterranea on the incorporation rate of isotopically-labelled macroalgal detritus by the amphipod Paramoera chevreuxi in a laboratory microcosm. The fatty acid composition of amphipods, nematodes, and macroalgal detritus was characterized to help determine the pathway of detritus incorporation by amphipods. The potential importance of R. mediterranea as a source of highly unsaturated fatty acids (HUFAs) to higher trophic levels was also investigated.We found no clear evidence for an effect of nematodes on the incorporation rate of fresh macroalgal detritus by amphipods, although there was some indication that the type of detritus (i.e. the green Chaetomorpha sp. vs the red Polysiphonia sp.) is important in determining the nature and extent of the relationship between nematodes and macrofaunal detritivores. Fatty acid data indicated that nematodes did not contribute significantly to the diet of amphipods when detritus was present, and there was no evidence that nematodes affected the pathway of detritus incorporation by amphipods. Amphipods incorporated Chaetomorpha sp. detritus about 10 times faster than Polysiphonia sp. detritus despite the higher C/N ratio and lower HUFA content of the former. This suggests that other factors, such as the presence of grazer deterrent compounds, are important in determining the palatability of macroalgal detritus. Amphipods fed exclusively on nematodes retained high HUFA levels but suffered high mortality. The burrowing behaviour of nematodes is suggested as the most likely factor limiting their availability to predators.Results suggest a limited interaction between amphipods and bacterivorous nematodes in detrital food webs. Further experiments are needed to test the wider applicability of these findings to different nematode and macrofaunal taxa, and for different types of detritus.