Stable isotopes' as trophic tracers: combining field sampling and manipulative labelling of food resources for macrobenthos

We combined 3 different approaches to determine the relative importance of microphytobenthos production as food for intertidal macrobenthic animals: ( 1) the natural abundance of stable-isotope ratios of carbon and nitrogen, ( 2) an in situ deliberate tracer addition of C-13-bicarbonate, which was tra nsferred through the benthic food chain after its incorporation by benthic algae, and (3) a dual labelling experiment in a flume, where pelagic and be nthic algae were labelled with N-15 and C-13, respectively. The results of the 3 approaches confirmed the high importance of microphytobenthos as a fo od source for (surface) deposit feeders. Despite the clearly demonstrated r esuspension of benthic algae at high current velocities, suspension feeders appeared to depend almost exclusively on pelagic algae (and possibly detri tal carbon) as a food source. Based on the results of the experiments, we d etermined an approximate degree of dependence on microphytobenthos for diff erent species of intertidal macrobenthos. The macrobenthic biomass at 5 stu dy locations, when weighted by these coefficients, correlated very well wit h measured productivity of the microphytobenthos.