Time-dependency of microzooplankton grazing and phytoplankton growth in the subarctic Pacific

Abstract

Dilution experiments were conducted on SUPER Program cruises in June 1987 and May and August 1988 to assess the role of microzooplankton in controlling phytoplankton stocks in the subarctic Pacific. Net growth rates of chlorophyll a varied in individual experiments from −0.4 to +0.7d −1. Experiments incubated for 48h gave higher net estimates than 24h incubations (0.01 to 0.22 d −1 for different cruises), exaggerating the imbalance between growth and grazing. Specific growth rates (μ) and grazing mortality (m) for 24h incubations were approximately balanced for the June and May cruises, and net growth estimates from the dilution experiments predicted changes in chlorophyll concentrations for May that closely matched those observed in the field, A major decline in phytoplankton abundance in the middle of May coincided with a high abundance of ciliates. Cell counts indicated that Synechococcus and small autotrophic nonflagellates were always kept in check by microzooplankton grazing, even when chlorophyll indicated uncontrolled phytoplankton growth in August 1988 experiments. Diatoms showed high growth potential in most incubations and dominated among the cells that bloomed in August. Our results support the hypotheses that micrograzers are major consumers of phytoplankton in the subarctic Pacific and that their grazing can control some elements of the phytoplankton community. However, growth limitation, presumably from iron deficiency, remains essential to the explanation of phytoplankton control in mid to late summer.