Microbial Regeneration of Nutrients from the Decomposition of Macrophyte Debris on the Shore

Abstract

A mass balance for the decomposition of kelp in a sand beach microcosm shows that microbial incorporation of nitrogen from kelp debris is achieved within 7 d with an efficiency of as much as 94.2 % of the particulate and dissolved carbon. Although comparatively high concentrations of carbon and ammonia appear in the leachates from kelp, these amount only to 0.2 % of the carbon and 1.5 % of the nitrogen originally present in the kelp debris. Estimates of the input of macrophyte debris to the shore from a kelp bed at Kommetjie, Cape Peninsula, South Africa, suggest that 206 g C m-' d-' are cast onto the strandline and that the organic nitrogen input is 12 g m-' d-l. The carbon returned to the sea is thus 0.4 g m-' d-' while the nitrogen ist 0.2 g m-' d-' Although the ammonia released during decomposition of kelp debris on the strandline may have some strictly localised significance in the immediate subtidal zone, it is estimated that such leachates could meet only 0.09 % of the nitrogen requirements of primary production in the nearby kelp bed, or approxin~ately 0.4 % of those of surf zone phytoplankton. Primary production in the adjacent subtidal comn~unities is thus dependent mainly on in sjtu remineralisation of nutrients and on local upwelling, rather than on the regeneration of nutrients from material cast onto the strandline.