Phytoplankton dynamics in the East China Sea in spring and summer as revealed by HPLC-derived pigment signatures

The community structure of phytoplankton was investigated from pigment composition in the East China Sea during an early spring bloom and a well-stratified summer season. Pigment concentration determined by high-performance liquid chromatography was interpreted using a matrix factorization program “CHEMTAX” (Mackey et al., 1996) to obtain the chlorophyll a biomass of phytoplankton groups at class level. There was a distinct horizontal heterogeneity in phytoplankton composition during spring. The spring bloom of diatoms was observed on the shelf, where weak stratification produced by overlying less saline surface water supported the bloom. In contrast, off the shelf where Kuroshio water prevailed, phytoplankton abundance was low and the community was composed mainly of prochlorophytes, chrysophytes, prymnesiophytes and chlorophytes. Patchy high concentrations of fucoxanthin were observed off the shelf, indicating transport of diatom-rich water from the shelf. In a clear contrast to the spring season, a “two-layer” distribution of phytoplankton prevailed both off and on the shelf in summer. There was a well-developed subsurface chlorophyll maximum at all the stations except near Changjiang estuary. In the upper mixed layer where nutrient salts were depleted, the phytoplankton community was characterized by high contribution of cyanobacteria followed by prochlorophytes, whereas in the chlorophyll maximum, prochlorophytes, chrysophytes and prymnesiophytes were most abundant. In both seasons, total amount of chlorophyll a was primarily determined by that of diatoms, and the chlorophyll a of procaryotes and eucaryotic flagellates did not exceed 0.4 and 1 μg l−1, respectively, suggesting herbivorous control. Active grazing of macrozooplankton also was indicated in the diatom patch by the presence of degradation products of chlorophyll a.