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 13 C puls‐chase experiment. Label was added at the beginning of low tide, and uptake of 13 C 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 13 C 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 silty stations, respectively. The transfer of carbon from microphytobenthos to benthic consumers was estimated from the appearance of 13 C in bacterial biomarkers, handpicked nematodes, and macrofauna. The incorporation of 13 C 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 18:1ω7c) showed rapid, significant transfer from benthic algae to bacteria with maximum labeling after 1 d. Nematodes became enriched after 1 h, and 13 C 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.