Depending on the availability of food, the biomass (weight of living material per unit area) of the sea-bed varies from place to place. Biomass may be expressed as 'rough weight' of fresh material, with or without shells, or more accurately as a 'dry weight' of organic material obtained after the removal of shells and drying of the remaining tissue to constant weight, allowance being made for any inorganic material present in the residue, i.e. ash-free dry weight. Dry weights of between 1 g and 35 g m~2 have been found beneath shallow water off the Danish coast. Holme (1953, 1961) found a mean dry weight of about 11 gm~2 for the biomass of the macrobenthos at 20 stations in the English Channel off Plymouth, the percentages of different animal groups being as shown in Table 6.2. In areas of exceptional productivity, biomass dry weights as high as 100-200 gm~ 2 occur.
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