Pigment-specific rates of phytoplankton growth and microzooplankton grazing in the open subarctic Pacific Ocean

A long-standing hypothesis states that low year-round phytoplankton biomass in the open subarctic Pacific Ocean is maintained by herbivorous grazing. To evaluate the balance between phytoplankton growth and microzooplankton grazing, we carried out seawater dilution experiments at two subarctic Pacific locations in June and September 1987. Pigment-specific phytoplankton growth and grazing rates were obtained from dilution experiments by HPLC separation of phytoplankton pigments. This approach allowed us to look at the relationship between growth and grazing rates for different phytoplankton taxa. A wide range of rates was observed during any given experiment. Pigment-specific growth rates ranged from 0.0 to 0.8 d-l; pigment-specific microzooplankton grazing rates ranged from 0.0 to 0.6 d-l. The highest growth rates appeared attributable to large diatoms, as indicated by both pigment and cell-count data. Grazing rates were most closely matched with growth rates for small (< 10 pm) phytoplankton species; large diatom species seemed to be unaffected by microzooplankton grazing. Higher grazing rates were measured on pigments with the lowest standing stocks. The subarctic Pacific Ocean apparently never experiences a phytoplankton bloom. Canadian weathership data collected between 1966 and 1976 show that chlorophyll concentrations only rarely reach 0.5 rg liter-’ and never exceed 1.8 Kg liter-’ (Anderson et al. 1977); these observations have been corroborated during subsequent cruises to the subarctic before and during project SUPER (subarctic pacific ecosystem research). These low phytoplankton standing stocks persist in spite of high (minimum 67 PM) euphotic zone concentrations of NO,and seasonal increases in primary productivity. A long-standing hypothesis proposed to account for year-round low phytoplankton Acknowledgments We thank the members of the SUPER project for support and encouragement, and the captain and crew of the RV Thomas G. Thompson for assistance at sea. Ralf Goericke standardized the HPLC and assisted with sample analysis. Dave Kirchman performed the DFAA analyses. Bruce Frost assisted with statistics, editing, and clear thinking in general. Comments of Evelyn Lessard, Dora Henry, and two anonymous reviewers improved earlier versions of this manuscript. This work was supported by NSF grants OCE 8613943 to N. A. Welschmeyer and OCE 86-13621 to B. W. Frost. stocks in the subarctic Pacific states that control of phytoplankton production is due to grazing by herbivorous zooplankton (McAllister et al. 1960). Because measurements of grazing by several Neocalunus species (the dominant suspension-feeding macrozooplankter in the subarctic Pacific) show that they are able to graze only 10% of the daily phytoplankton production in this region, herbivorous microzooplankton are considered likely candidates for consumers of a large fraction of the phytoplankton production (Frost 1987; Miller et al. 1988). It is further hypothesized that the permanent ha&line at about 120 m in the North Pacific prevents winter mixing beyond this depth. This should allow a low level of primary production year-round and, hence, survival of microzooplankton in winter in numbers that allow them to respond immediately to spring phytoplankton production increases (Evans and Parslow 1985; Frost 1987). To test the hypothesis of grazer control of phytoplankton biomass, project SUPER personnel visited the subarctic Pacific in June and September 1987. As part of this program, we measured the growth and graz-