Abundance and biomass of pico-, nano-, and microplankton on a transect across Nordvestbanken, north Norwegian shelf, in 1994

The seasonal variation of suspended microplankton on a transect across Nordvestbanken in 1994 revealed that pico- and nanoplankton flagellates and monads (< 2 mu m and 2-20 mu m, respectively) entirely determined total phytoplankton numbers and biomass. From March to May and from August to October nanoflagellates and monads comprised on average 90 % and up to 98 % of total phytoplankton biovolume. Only during the maximum diatom and dinoflagellate abundance in June and July, and during the appearance of Halosphaera viridis at selected stations on the mid shelf, did flagellates comprise less than 60-90 % of total biovolume. In general, the abundances of picoplankton, coccolithophorids and Phaeocystis pouchetii were low, never comprising more than a few percent of total biovolume. The estimated total biomass of pico-, nano- and microplankton (from biovolume), phytoplankton (from chlorophyll and epifluorescence estimates) and larger protozooplankton (from biovolume) in the upper layers were on average 3-16 g C m(2), 1-5 g C m(-2) and 15-400 mg C m(-2), respectively. The biomass of ciliates was low throughout the investigation and only one substantial peak in June/July was recorded over the entire shelf. Despite significant consumption of nitrate and silicate, large cells such as diatoms and dinoflagellates were not abundant on the shelf. They must have been removed from the water column, and it is speculated that this was accomplished mainly by mesozooplankton grazing. The dynamics of flagellates, diatoms, and protozooplankton are interpreted as the result of variable grazing pressure by mesozooplankton on protozoa and diatoms, and by protozoa on flagellates.