Importance of Picoplankton in Lake Superior

In Lake Superior, approximately 50% of total primary production is attributable to phytoplankton that pass through a 3-μm screen. The <3-μm size class is dominated by eukaryotic flagellates, nonmotile eukaryotic cells (1 μm), and chroococcoid cyanobacteria. Approximately 20% of total primary production is attributable to orange autofluorescent chroococcoid cyanobacteria (mean size = 0.7 μm) as determined by size fractionation and track autoradiography. These small prokaryotes exhibited abundances of 42 000 and 56 000 cells∙ml−1, maximum photosynthetic rates of 7 and 6 fg∙cell−1∙h−1, and growth rates of 1.5 and 0.8∙d−1 in the epilimnion and hypolimnion, respectively. A significant portion of this picoplankton (<1 μm) production may be consumed by heterotrophic protozoans in a "microbial loop."