Species size distributions of benthic and pelagic Metazoa: evidence for interaction?

The benthic metazoan fauna at Station CS2 in the Celtic Sea has the familiar bimodal pattern of species size distribution, with a meiofaunal mode at 0.63 µg dry weight, a macrofauna mode at 2.03 mg and the trough between them at 60 µg. The pelagic community, on the other hand, has its centre of distribution corresponding with the position of the benthic trough. Differences in taxonomic composition cannot explain this pattern since within a single taxon, as exemplified by the Copepoda, there are marked size differences between the benthic and planktonic assemblage. Of the possible mechanisms of benthic/pelagic coupling which might influence species size distributions, interactions between macrobenthic larvae and holoplankton and the influence of the pelagic community on the nature of the benthic food supply are considered potentially important. Planktotrophic larvae of benthic macrofauna avoid competition with similarly sized meiobenthic organisms but settle when they reach the size of the holoplankton. Thus resource partitioning on a size basis between these larvae and the holoplankton may occur. Benthic communities enriched with non-phytoplankton organic material show size convergence of meiofauna and macrofauna, their species size distribution being similar to that of pelagic communities This may result from a relaxation of benthic/pelagic interaction.