Vertical distribution and partitioning of organic carbon in mixed, frontal and stratified waters of the English Channel

The vertical distribution of plankton is described for 3 stations representative of stratified, frontal and vertically mixed regions of the western English Channel in summer. All components of organic carbon, representing dissolved and particulate organic carbon, phytoplankton, bacteria, protozoans, micro- and meso-zooplankton, were estimated independently. Major differences were found in the abundance and species composition of the phytoplankton, and in the relative proportions of different groups of heterotrophs. In the frontal region the phytoplankton (26.5 g C m⁻²) was composed of an essentially monospecific, surface population of the dinoflagellate Gyrodinium aureolum; by contrast, under well-stratified conditions small naked flagellates (0.42 g C m⁻²) forming a sub-surface chlorophyll maximum were dominant, and the tidally mixed waters were characterised by diatoms (7.91 g C m⁻²). At each station the estimated standing stock of heterotrophs was between 2.3 and 3.2 g C m⁻², 10 to 30% of which consisted of bacteria. Hence the phytoplankton was the dominant compartment in the frontal and mixed regions, whereas the Zooplankton biomass considerably exceeded that of the phytoplankton in the well-stratified water. The ecological implications of these carbon distribution patterns are discussed.