Biodegradation and Carbon Flow Based on Kelp (Ecklonia maxima) Debris in a Sandy Beach Microcosm

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Abstract
Primary decomposition of intertidally stranded kelp E. maxima in a mixed substrate microcosm is effected by bacteria rather than by fungi, which are generally responsible for the initial decomposition phases of debris in other habitats. Initial colonization by cocci along the junctions of epidermal cell walls leads to lysis and release of cell contents. Lysed cells are then colonized by bacterial rods. High concentrations of leachates, reaching 5640 mg C l-1 subsequently appear beneath the decomposing kelp. Over 90% of the C in these leachates is utilized by bacteria during drainage through a strip of sand 1 m long, 50 cm wide and 12 cm deep, although on an open beach other organisms, e.g. nematodes, may be capable of direct absorption of such organic matter. Calculation of C flow via grazing invertebrates and through bacteria shows that 23-27% of the C in kelp is converted to bacterial C. This relationship occurs despite wide variations in direct consumption by grazers, since much of the consumed material is returned to the microcosm as faeces. Estimates of the bacterial C in equilibrium with the kelp cast up on the strandline suggest that 1648 g of kelp C is deposited per meter of strandline each 8 days and that this is capable of supporting a bacterial production of 444.9 g C m-1 of beach per 8 day cycle. The residual 73-77% of kelp C which is not incorporated into bacteria is mineralized by the sand beach microbes within 8 days. The microbial community occupies a central role in the rapid regeneration of inorganic materials necessary to support the characteristically high primary production of the kelp bed.