Ultraphytoplankton abundances and chlorophyll a concentrations in ice-covered waters of northern seas

The abundances and chlorophyll aconcentrations (Chl a) of ultraphytoplankton (<5 m) were determined at four ice-covered sites in northern seas, i.e. southeastern Hudson Bay, Saroma-ko Lagoon, Resolute Passage and the Northeast Water Polynya. Numbers of total ultraphytoplankton were low, ranging from 3.6 x 10⁷ to 9.7 x 10⁹ cells m^-3, which confirms the overall paucity of ultra-phytoplankton in cold waters. Concentrations of <5 m Chl a varied between 0.002 and 10.8 mg m^-3, which accounted for 0.2-99.7% of total Chla. Chlorophyll a concentrations of ultraphytoplankton can thus reach high values and make up a substantial fraction of total Chl a. Ultraphytoplankton were ubiquitous, but they showed high among- and within-site variability in abundance, biomass and contribution to total Chla concentrations. The ultraphytoplankton comprised primarily eukaryotes and prokaryotic phycoerythrin-rich cyanobacteria, but also some cryptomonads and phycocyanin-rich cyanobacteria. Concentrations of ultraplanktonic eukaryotes reached 7.8 x 10⁹ cells m^-3, but were generally <5 x 10⁹ cells m^-3, whereas the maximum concentration of prokaryotes was 6.2 x 10⁹ cells m^-3. The concentrations of eukaryotes and prokaryotes were related, overall, to water mass characteristics, i.e. temperature, salinity, percent irradiance, and concentrations of nitrate and ammonium. Depending on sites, the abundances of eukaryotes were positively liked to salinity, percent irradiance, nitrate and ammonium, whereas the abundances of prokaryotes were positively correlated with ammonium and nitrate. Phycocyanin-rich cyanobacteria were generally confined to brackish waters (Hudson Bay). The highest cell numbers of ultraphytoplankton were found at temperatures of <0.5C and salinities of >30 p.s.u.