The importance of benthic macrofauna in decomposition of microalgae in a coastal marine sediment

The effectiveness of macrofauna in decomposing organic matter was studied in microcosms by adding 14C-labeled microalgae to undisturbed sediment cores with and without fauna. The presence of benthic macrofauna stimulated release of 14C02 only briefly during the initial phase (4 d); after 21 d the total amount of 14C0, released was nearly equivalent with or without fauna. The firstorder decay rate constant of the added organic matter based on the rate of 14C0, production was 0.019 d-’ with fauna and 0.020 without. The release of [14C]DOC from the sediment accounted for 4-5% of the total 14C release. Only a small fraction of the 14C added as algae was found as 14C0, (max, 1.2%) or [14C]DOC (max, 0.7%) in the pore water at any time during the experiment. In contrast, there was a significant increase of both sediment TCO, (total inorganic C) production (7OOh) and 0, uptake (17%) rates in the presence of animals. The discrepancy between the effect of macrofauna on fluxes of labeled vs. unlabeled compounds indicates that the surface deposit feeders primarily affected decomposition of old and relatively refractory sediment organic matter. Much of the organic matter produced in estuarine and coastal marine areas is decomposed by aerobic and anaerobic microbial processes in the sediment (Smith 1974). The rate of decomposition is generally re