The fauna and rate of degradation of stranded kelp

The moisture content, residual dry mass and fauna associated with kelp stranded on a South African beach were monitored for 30 days following its deposition. Plants that were cast up separately dried much more rapidly, but declined in dry mass more slowly, than those deposited in banks. Both types of deposit lost at least half their dry mass within the first 7 days. The fauna colonizing the two types of wrack was similar, a total of 22 species of Coleoptera, three Diptera and two Amphipoda being recorded. Amphipoda and Diptera dominated the early successional stages, but only Coleoptera colonized older, direr wrack. Amphipoda were dramatically more abundant in wrack banks, as compared with single plants. Calculated consumption rates of the fauna were compared with actual declines in kelp mass and showed a good correlation in the case of banked material. Single plants, however, lacked the expected biomass of consumers, probably because Amphipoda, the most important forms, desert single plants by day and migrate below the sand, and are underestimated in a diurnal sampling programme. The role of stranded kelp in enriching the economy of sandy beaches and adjoining marine ecosystems is considered. © 1981 Academic Press Inc. (London) Ltd.