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

Abstract

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−1; pigment‐specific microzooplankton grazing rates ranged from 0.0 to 0.6 d−1. 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 µm) phytoplankton species; large diatom species seemed to be unaffected by microzooplankton grazing. Higher grazing rates were measured on pigments with the lowest standing stocks.