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 C-14-labeled microalgae to undisturbed sediment cores with and without fauna. The Presence of benthic macrofauna stimulated release of (CO2)-C-14 only briefly during the initial phase (4 d); after 21 d the total amount of (CO2)-C-14, released was nearly equivalent with or without fauna. The first-order decay rate constant of the added organic matter based on the rate of (CO2)-C-14, production was 0.019 d-1 with fauna and 0.020 without. The release of [C-14]DOC from the sediment accounted for 4-5% of the total C-14 release. Only a small fraction of the C-14 added as algae was found as (CO2)-C-14, (max, 1.2%) or [C-14]DOC (max, 0.7%) in the pore water at any time during the experiment. In contrast, there was a significant increase of both sediment TCO2 (total inorganic C) production (70%) and O2 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.