PHYTOPLANKTON VARIABILITY IN THE CENTRAL AND EASTERN TROPICAL PACIFIC

An extensive set of measurements of phytoplankton production, biomass and composition, and microzooplankton grazing from the coast of Peru t o 170 degrees W during 1992, together with similar data collected over the previous decade, has allowed recalculation of the primary product ion supported by equatorial upwelling and improved description of the variability in phytoplankton properties. Equatorial region surface chl orophyll and phytoplankton biomass were low, averaging 0.2 mu g 1(-1) and 20 mu g Cl-1, respectively, and showed low variance. Phytoplankton in the open ocean of the tropical Pacific were dominated by small (< 5 mu m) solitary organisms, primarily prochlorophytes, Synechococcus, eukaryotic picoplankton, haptophytes and dinoflagellates, while coasta l populations were dominated by larger organisms or colonies (primaril y diatoms). At a few open ocean locations high numbers of diatoms were found. The chlorophyll maximum observed in the equatorial Pacific was a function of increased chlorophyll per cell rather than an increase in cell numbers. Surface phytoplankton carbon to chlorophyll was highl y variable and a function of available irradiance and upwelling streng th. On the order of 40% of the particulate nitrogen retained by GF/F f ilters was estimated to be phytoplankton nitrogen. Phytoplankton growt h rate estimates using daily carbon uptake and phytoplankton carbon es timated from microscopic enumeration ranged from 0.55 to 0.70 day-1. E stimates of growth rates from dilution experiments gave estimates of t he order of 1 day(-1) and microzooplankton grazing rates that were sig nificantly lower, 0.4 day(-1). The mean mass specific grazing rate for microzooplankton was estimated to range from 1.6 to 1.8 day(-1). The mean productivity for the equatorial Pacific from 90 degrees to 180 de grees W, 5 degrees N-5 degrees S, was estimated to be 900 mg C m(-2) d ay(-1) for the period from 1990 to the present, twice that estimated p reviously. The maximum f-ratio (new to total production) was estimated to be 0.36. Assuming that between 25 and 50% of the upwelled nitrate is never taken up by phytoplankton between 5 degrees N and 5 degrees S . new production would be 162-244 mg C m(-2) day(-1) and f would range from 0.18 to 0.27. Copyright (C) 1996 Elsevier Science Ltd.