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

Determination of fine-scale vertical distribution of microbes and meiofauna in an intertidal sediment

A simple sampling device is described which produces thin (1 mm) sections of sediment cores. The sampler has been tested on fine sand of an intertidal sandflat and used to study the vertical distribution, over part of a tidal cycle in August, 1981, of migrating algae in the surface 20 mm of sand. Two species of Diplonies and one of Navicula showed marked changes in vertical distribution as the sandflat was flooded, but the distribution of bacteria in the sime samples did not show any change with tidal state. Spatial separation of different species of harpacticoid oppepods within the surface 20 mm of sand has also been demonstrated using this sampler, and the results suggest that different species may occupy particular fine-scale spatial niches within the sand column. The depth separation of nematode species was less well defined, except for two species with apparently the same feeding mode which were isolated from one another vertically

Secondary production of the benthos in an estuarine environment

Several projects relating directly or indirectly to energy flow through the benthic community on a mud-flat in the Lynher estuary, Cornwall, U.K. have been integrated by using two methods, firstly a steady-state energy-flow diagram and secondly a dynamic simulation model, in order to provide a better understanding of ecosystem function and as an aid to research planning. For each macrofauna species and meiofauna group an energy budget has been constructed in the form C = P + R + F + U (IBP terminology). This information, together with values for primary inputs of carbon, has enabled us to construct a quantitative diagram representing the flow of carbon between the faunal components of 1 m2 of mud over 1 year. The net annual production of macrofauna is 5.46 g C m-2, and of meiofauna 20•17 g C m-2. Of the meiofauna production, 3•34 g is utilized within the system, so that 16•83 g remains available to mobile carnivores. The macrofauna ingest 55•74 g of primary carbon annually, and the meiofauna 107•09 g. However, the meiofauna standing crop is only 0•49 times that of the macrofauna. Nematodes and copepods are energetically the most important meiofauna groups. Having achieved realistic simulations of secondary production of deposit-feeders, filter-feeders, Nephtys and meiofauna, it has been possible to investigate the effect on the system as a whole of a variety of hypothetical trophic relationships which are poorly understood, particularly the interactions between meiofauna and macrofauna. The growth of Nephtys on different diets is given as an example