Does transported seagrass provide an important trophic link in unvegetated, nearshore areas?

The contribution of detritus from seagrass and other primary producers to faunal production in unvegetated nearshore areas was examined primarily using stable isotopes. Fish, macroinvertebrates, meiofauna and primary producers (seagrasses, macroalgae, seston and benthic microalgae) were sampled from sites in south-western Australia. All samples were analysed for delta C-13 and delta N-15 values and fish aut contents were determined. delta C-13 values for seagrasses in the region were high compared to other inacrophytes, ranging from 49.9 to -8.2 parts per thousand compared to -19.8 to - 12.67 parts per thousand for macroalgae. The delta N-15 values ranged between 4.0 and 7.77 parts per thousand for the red, brown and green algae, and between 3.2 and 5.9 parts per thousand for seagrasses. Seston and benthic microalgae samples had a mean delta C-13 value of -12.8 and -14.0 parts per thousand, respectively, and their delta N-15 values were comparable to the macroalgae. All invertebrate fauna had mean delta C-13 values considerably lower than seagrasses. However, individual samples harpacticoid copepods and polychaetes had a value as high as -11.7 parts per thousand. delta N-15 values for consumers were higher than those of the primary producers, except for copepods and amphipods. The delta C-13 values for fish had a relatively small range, between -16.6 and -13.1 parts per thousand, and the delta N-15 values of fish were elevated compared to the invertebrates and primary producers, ranging mostly between 10.0 and 12.6 parts per thousand. Mixing model analysis based on delta C-13 values indicated that seagrass ranked low as a likely carbon source for all invertebrates other than harpacticoid copepods at a single site and some samples of polychaetes. The delta C-13 values for fish were similar to those of a combination of harpacticoid and calanoid copepods, amphipods and polychaetes. The consumption of harpacticoid copepods by some fish species indicates that Amphibolis and Posidonia species in south-western Australia can contribute to the food web of unvegetated nearshore areas as detritus, but brown algae is likely to make a greater contribution. At least for the time of year that was sampled, the flow of detrital seagrass material into the foodweb may be mediated by specific detrivores, in this case harpactacoid copepods, rather than by all detritivores. (c) 2005 Elsevier Ltd. All rights reserved.