Pigment specific growth and grazing rates of phytoplankton in the central equatorial Pacific

Abstract

Dilution experiments were performed during two transects across the central equatorial Pacific (3°N–3°S, 140°W) to estimate growth and mortality rates of select phytoplankton groups distinguished by their characteristic pigments. The first transect was conducted in February–March 1992 during a moderate El Niño event; the second transect took place in August–September 1992, under normal upwelling conditions. Experiments with and without added nutrients (N, P, Fe, and Mn) indicated that growth rates were nutrient limited during El Niño conditions. Nevertheless, the enhanced growth rates with added nutrients during El Niño were less than rates without added nutrients during the normal upwelling period; therefore, nutrients alone did not account for all of the differences between cruises. Growth rates were different for the various algal groups. In August–September, diatoms grew at 1.7 d⁻¹, and prochlorophytes and prymnesiophytes at −0.5 d⁻¹. Grazing by microzooplankton balanced growth for algal groups exhibiting the lowest growth rates (i.e. prymnesiophytes and prochlorophytes). Although microzooplankton grazing on diatoms and pelagophytes was also significant, a substantial fraction of their growth escaped consumption, accounting for most of the net chlorophyll production of the phytoplankton community.