The fate of intertidal microphytobenthos carbon:: An in situ 13C-labeling study

At two intertidal sites (one sandy and one silty, in the Scheldt estuary, The Netherlands), the fate of microphytobenthos was studied through an in situ C-13 pulse-chase experiment. Label was added at the beginning of low tide, and uptake of C-13 by algae was linear during the whole period of tidal exposure (about 27 mg m(-2) h(-1) in the top millimeter at both sites). The C-13 fixed by microphytobenthos was rapidly displaced toward deeper sediment layers (down to 6 cm), in particular at the dynamic, sandy site. The residence times of microphytobenthos with respect to external losses (resuspension and respiration) were about 2.4 and 5.6 d at the sandy and silly stations, respectively. The transfer of carbon from microphytobenthos to benthic consumers was estimated from the appearance of C-13 in bacterial biomarkers, handpicked nematodes, and macrofauna. The incorporation of C-13 into bacterial biomass was quantified by carbon isotope analysis of polar lipid derived fatty acids specific for bacteria. The bacterial polar lipid-derived fatty acids (i14:0, i15:0, a15:0, i16:0, and 18:1 omega 7c) showed rapid, significant transfer from benthic algae to bacteria with maximum labeling after 1 d. Nematodes became enriched after 1 h, and C-13 assimilation increased until day 3. Microphytobenthos carbon entered all heterotrophic components in proportion to heterotrophic biomass distribution (bacteria > macrofauna > meiofauna). Our results indicate a central role for microphytobenthos in moderating carbon flow in coastal sediments.