The size distribution of organisms in the Celtic Sea: from bacteria to Metazoa

The main features of the size distribution of pelagic and benthic organisms are described, with particular reference to comprehensive studies at a single station, CS2, in the Celtic Sea. These are: 1. A more or less even distribution of biomass in all size classes of pelagic autotrophs. 2. Five size groups of pelagic heterotrophs separated from each other by roughly 10³ differences in individual weight, with three well-defined gaps in the size spectrum between the four smallest size modes. 3. Benthic organisms with three size modes, the microbial peak between the two smallest pelagic modes, the meiofaunal peak between the size of pelagic ciliates and herbivorous macrozooplankton, and the macrobenthic peak at about the same size as the carnivorous macrozooplankton. Differences in the positions of the microbial peaks are thought to be associated with the different nutritional environments of free-living and surface-attached bacteria. Other features of the pelagic heterotroph spectrum are explicable in terms of the known limits to size ratios between prey and predator for suspension feeders. These limits do not apply to the benthos, the size distribution of which is largely determined by physical constraints of the sedimentary environment and the optimisation of size-related life history characteristics. Thus, constraints on body size are entirely different in the two systems, and we see little evidence for coupling between the pelagos and benthos which might result in complementary patterns of size distribution, except perhaps for interactions between the pelagic larvae of macrobenthos and the permanent macrozooplankton at the upper end of the size spectrum.