Identification of food sources of invertebrates from the seagrass Zostera marina community using carbon and sulfur stable isotope ratio and fatty acid analyses

Carbon and sulfur stable isotope ratios, as well as fatty acid composition of tissues, of dominant consumer species were determined and compared to th ose of potential food sources in an isolated community of Zostera marina in a shallow, sen-ii-enclosed inlet of the Sea of Japan, Of the 6 dominant sp ecies of invertebrates, 4 species were enriched in C-13, compared to all sa mpled carbon sources alternative to Z. marina, Among them, the grazing gast ropods Littorina squalida and Homalopoma sangarense exhibited the most enri ched delta C-13 values. On the dual delta C-13 versus delta S-34 plot, thes e mollusks occupy an intermediate position between Z, marina and epiphytes, suggesting nearly equal proportions of organic carbon from both nutritiona l sources. In lipids of H. sangarense there was a high content of the 18:1( n-7) acid characteristic of aerobic bacteria; however, another grazer (L. s qualida) showed the lowest content of bacterial fatty acids among all consu mers, Other highly C-13-enriched consumers were the surface-deposit-feeding mollusks, the gastropod Batillaria cumingii and the bivalve Macoma incongr ua; however, their delta S-34 values were markedly lower than those of any of the primary producers sampled, including Z marina. Although the high VC values of grazers and surface-deposit feeders are suggestive of a great con tribution of Z. marina organic carbon, no substantial concentrations of sea grass marker fatty acids were detected, Significant interspecific variation s of both the sulfur isotope ratios and the fatty acid composition of these consumers suggest that there are a variety of pathways by which seagrass o rganic matter reaches invertebrates at lower trophic levels of the communit y food web. Dominant filter feeders, the bivalves Ruditapes philippinarum a nd Pillucina pisidium, had carbon drastically different in isotopic composi tion from Z. marina organic matter, Body tissues of P. pisidium and especia lly its gills, which bear symbiotic bacteria, were dramatically depleted bo th in C-13 and S-34 compared to all sources of photosynthetically fixed car bon in the bay, Very low delta C-13 and delta S-34 values of R pisidium bod y together with a high content of 18: 1 (n-7) acid suggest a leading role o f sulfur-oxidizing symbiotic bacteria in the nutrition of this species. R. philippinarum was only slightly C-13-enriched, compared to POM, and was the only consumer which had the high concentration of fatty acids characterist ic of plankton, particularly 22:6(n-3). At the same time, it was much more S-34-depleted than would be expected, assuming negligible contribution of Z . marina detritus to its food. This mollusk showed a high content of branch ed fatty acids, especially the iso17:0 and anteiso17:0 acids characteristic of bacteria from sediment; this suggests that R. philippinarum assimilated notable amounts of bacteria from resuspended sediment. Furthermore, the co ntribution of S-34-depleted bacteria, which inhabit reduced sediment, to R. philippinarum nutrition was high enough to result in the observed depletio n of S-34 in mollusks. Further progress in food web studies of seagrass eco systems using a complex of multiple stable isotope and fatty acid analyses would appear possible on the basis of analysis of separate components of th e seagrass epiphytic community and micro- and meiobenthic organisms, inhabi ting surface sediments.