Role of microzooplankton during a Phaeocystis sp. bloom in the Oosterschelde (SW Netherlands)

The impact of microzooplankton on the plankton community was assessed during a bloom of Phaeocystis sp. in the Oosterschelde basin (SW Netherlands). Chlorophyll a concentration and Phaeocystis sp. abundance reached maxima values in the middle of the sampling period (~ 27 mg chl a m− 3 and 3.4 × 1010 cells m− 3, respectively). The bloom collapse coincided with the last sampling day, when accentuated decreases in chlorophyll a concentrations (~ 11 mg chl a m− 3) and Phaeocystis sp. cells numbers (~ 1.3 × 1010 cells m− 3) were recorded. Microzooplankton organisms were significant consumers of both phytoplankton and heterotrophic plankton. Although Phaeocystis sp. was the most consumed organism (336 ± 71 mg C m− 3 d− 1), microzooplankton impact on its standing stock was lower than on the stocks of other less abundant organisms. This impact was also lower during the peak of the bloom, when colonial forms of Phaeocystis sp. presumably predominated, than during the bloom collapse, when free-living cells were supposedly more abundant. The impact of microzooplankton on heterotrophic organisms was higher than on phytoplankton, and increased when the bloom collapsed. Picoheterotrophs experienced the highest impact on their standing stock (~ 75% d− 1) and production (~ 90% d− 1). These results demonstrate that during a Phaeocystis sp. bloom, the microbial food web was responsible for channelling a significant fraction of plankton biomass, either from direct consumption of Phaeocystis sp. cells or through consumption of heterotrophs, which would have been favoured by the high quantities of organic matter released during the bloom collapse.

► Microzooplankton consumed a significant fraction of Phaeocystis sp. biomass.
► Microzooplankton impact was higher during the bloom collapse, when cells were free-living.
► Heterotrophs were also consumed by microzooplankton simultaneously.
► We observed two main pathways for channelling Phaeocystis sp bloom biomass.