For the mean annual biomass of the benthic fauna in shallow water around the British Isles we will take a value of 10 g dry wt-m-2 (see Section 6.4.2). Part of this is benthic predators, some estimates indicating about 25 per cent, leaving a standing stock of 7.5 g dry wt of benthic herbivores feeding on detritus and bacteria. With a calorific value of 5 kcal-g-1 dry wt we have an energy content of mean standing stock of benthic herbivores of 37.5 kcal-m-2.
The conversion efficiency of these animals is likely to be as high as in herbivorous zooplanktonts, so we will take a value of 0.2 for GCE. Compared with zooplanktonts many of the benthic animals are much bigger and live longer - years rather than weeks - so the rate of turnover is low relative to the weight of standing stock. On the assumption that the stock produces its own weight of new tissue in a year, an assumption supported by experimental evidence (Buchanan and Warwick, 1974; Hibbert, 1976; Hughes, 1970; Kay and Brafield 1973; Warwick and Price, 1975; Warwick et al., 1978), the energy intake necessary to produce this is 37.5/0.2 = 187.5 kcal-m-2-yr-1. Because we have also assumed that 25 per cent of this comes from direct assimilation of digested detritus and the remainder from ingestion of bacteria, we have detrital and bacterial contributions of 47kcal-m-2-yr-1 and 140 kcal-m-2-yr-1 respectively.
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