Abstract
Water samples from the Celtic Sea were concentrated and examined by scanning and transmission electron microscopy. Cyanobacteria were abundant and had maximum cell dimensions ranging from 0.5-1.0 .mu.m. In addition to these prokaryotic cells, numerous eukaryotic algae were present which had maximum cell dimensions only slightly larger (0.85-2 .mu.m) than the cyanobacteria; these eukaryotic cells were ''Chlorella-like'' green algae, possessing neither scales nor flagella. Many of these eukaryotic algae and all of the cyanobacteria would pass through a 1 .mu.m pore-size sieve and were the organisms responsible for the high rates of primary production by picoplankton in the Celtic Sea.