Phytoplankton Losses and Gains During Coastal Upwelling

Abstract

We investigated phytoplankton growth and microzooplankton grazing in coastal water off Oregon during the upwelling period. Phytoplankton growth and microzooplankton grazing rates were determined by applying the dilution method for water collected from the mixed layer. Distinct patterns in relative nutrient and phytoplankton biomass levels (measured as chlorophyll) allow characterization of prebloom, bloom, and postbloom conditions. Both prebloom and bloom periods yielded high phytoplankton growth rates (0.6-1.5 d−1, respectively) and grazing rates (around 0.7 d−1), but prebloom periods were characterized by a smaller phytoplankton biomass than bloom periods. The nitrate-to-chlorophyll ratio of prebloom and bloom periods were much greater than 1 μmol/μg Chl. Postbloom situations also were characterized by high phytoplankton biomass, but had a nitrate-to-chlorophyll ratio smaller than 1 μmol/μg Chl. Phytoplankton growth rates were low (around 0.1–0.2 d−1) or negative, and grazing rates were low or not measurable.