Chlorophyll budgets: Zooplankton grazing and phytoplankton growth in a temperate fjord and the Central Pacific Gyres1

Abstract

The processes controlling the daily production and fate of phytoplankton pigments were evaluated in a moderately productive fjord (Dabob Bay, Washington) and the oligotrophic open ocean (North and South Central Pacific Gyres). The processes included phytoplankton growth, macrozooplankton grazing, microzooplankton grazing, the downward vertical flux of pheopigments, photodegradation, dark degradation, cell sinking, cell senescence, and physical mixing. A model was constructed describing the dynamic budget of chlorophyll and pheopigments within the euphotic zone. The model provides estimates of phytoplankton growth, macrozooplankton grazing, and microzooplankton grazing based on field measurements of the vertical distribution of pigments, the vertical flux of pigments out of the euphotic zone, and the flux of solar radiation through the water column. The pigment‐specific rates (d−1) provided by the model are derived in situ without manipulation of either the phytoplankton or zooplankton, and the experiments are largely container‐free. Growth rates, averaged over the whole euphotic zone, varied seasonally in Dabob Bay (0.05–0.9 d−1) and ca. 67% of the total daily grazing rate was due to macrozooplankton herbivores. Phytoplankton growth rates, averaged over the whole euphotic zone, were ca. 0.2 d−1 in the oceanic gyres, regardless of season or location, and ca. 95% of daily grazing was due to microzooplankton herbivores. The model suggests that grazing and growth are in balance.