Physiological ecology of picoplankton in the North Sea

The distribution of cyanobacteria in the surface waters of the North Sea was measured during July 1987. Numbers of cyanobacteria ranged from 2.5x106 to 1.7x108 cells 1-1. In the majority of stations, cyanobacterial numbers were highest in the near-surface water and a subsurface maximum was found at only one station. The distribution of 14C among the end-products of photosynthesis was determined for picoplankton ( 1 μm throughout the North Sea. The majority of label was found in the protein fraction of both picoplankton and >1 μm phytoplankton; incorporation into lipids and polysaccharides plus nucleic acids was much lower. We interpret the large incorporation into protein to be a consequence of nutrient limitation of these natural assemblages. Photosynthetic parameters of the two size fractions were also determined. Assimilation number (PmB) and initial slope αβ were greater for the picoplankton fraction than for phytoplankton >1 μm but there was no evidence of significant photoinhibition of either fraction at irradiances up to 1 000 μE m-2 s-1.