Kinetics of Nutrient Uptake and Release by Phytoplankton in East China Sea: Model and Mesocosm Experiments

The aim of this study was to investigate the influence of nutrient additions on phytoplankton growth and provide a method to estimate kinetic parameters of nutrient uptake and release by marine phytoplankton. Experiments with phosphate (PO4)-enriched mesocosms were performed in the East China Sea in autumn 1997 and spring 1998. The maximum concentration of chlorophyll a (Chl-a) in the control enclosures was 19.2 μg l−1 and 4.42 μg l−1, while Chl-a increased to 74.9 μg l−1 and 17.2 μg l−1 in the PO4-enriched enclosures in autumn 1997 and spring 1998, respectively. Values of specific rates for uptake and release of PO4 and NO3 by the phytoplankton community, kup and krel, were estimated with data from the mesocosm experiments by means of a kinetic model. The model was developed on the basis of ERSEM (European Regional Seas Ecosystem Model) II, by considering the nutrient uptake attributable to phytoplankton assimilation and the release mainly by way of excretion processes. Values of kup and krel were 2.03±0.03 d−1 and 0.76±0.34 d−1 for uptake and release of PO4 by the autumn phytoplankton community, mainly the diatom Skeletonema costatum, in 1997, whereas values were 1.24±0.21 d−1 and 0.53±0.04 d−1 for spring phytoplankton, mainly the dinoflagellate Prorocentrum dentatum, in 1998. However, no significant differences in kup and krel were observed for uptake and release of NO3 by autumn and spring phytoplankton communities; the mean values were 0.76±0.09 d−1 for kup and 0.30±0.16 d−1 for krel. These results demonstrate that PO4 is one of the most important factors for the phytoplankton growth in the vicinity of the Changjiang Estuary, with uptake rates in autumn larger than in summer.