Progressive decoupling between phytoplankton growth and microzooplankton grazing during an iron-induced phytoplankton bloom in the Southern Ocean (EIFEX)
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Dilution experiments were performed to quantify growth and mortality rates of phytoplankton groups (as defined by pigment markers) for 5 wk in an iron-induced phytoplankton bloom during the European Iron Fertilization Experiment (EIFEX) conducted in the Southern Ocean. Rates could be reliably measured for the 2 main groups, diatoms and prymnesiophytes. Mean phytoplankton intrinsic growth rates were around 0.23 d−1, without a significant temporal trend. Mortality rates, however, decreased with time (from ~0.3 to ~0.06 d−1), leading to an increase in decoupling between phytoplankton growth and microzooplankton grazing. The decrease in grazing was correlated with the decrease in concentrations of small microprotozooplankton (<60 μm). As a consequence, net growth in the dilution experiments increased from around 0 d−1 up to 0.13 d−1 in the last days of the experiment, 35 d after the initial iron fertilization. This pattern did not reflect the dynamics of net phytoplankton accumulation ...