ABUNDANCE, VARIABILITY, AND POTENTIAL GRAZING IMPACT OF PLANKTONIC CILIATES IN THE OPEN SUB-ARCTIC PACIFIC-OCEAN

The abundance and variability of planktonic ciliates in the open subarctic Pacific were determined during four month-long cruises in 1987 and 1988. The ciliate community, numerically dominated by relatively small aloricate choreotrichs, was comparable in abundance to communities in a range of oceanic and neritic environments, including waters with much higher average chlorophyll concentrations. Integrated (0-80m) ciliate biomass was typically 100-200mgC m-2, although 3- to 4-fold higher levels were observed on two occasions in spring. Ciliate community biomass, in general, was dominated by large (>20mum width) individuals, although in August 1988 the biomass of smaller cells was as great or greater. The estimated grazing impact of the ciliate community averaged 20% of the primary production. On one instance in May 1988, however, a large biomass of ciliates led to an estimated grazing impact equivalent to 55% of phytoplankton production. While ciliates may be major phytoplankton grazers during sporadic ciliate ''blooms'', dino- and other heterotrophic flagellates, which make up the bulk of microheterotroph biomass, must normally be of equal or greater importance as herbivores in this ocean region.