Microzooplankton herbivory during the diatom–Phaeocystis spring succession in the eastern English Channel

The dilution technique was used to investigate microzooplankton grazing and phytoplankton growth in the eastern English Channel during the diatom–Phaeocystis spring succession from January 2009 to June 2009. Four periods were defined based on phytoplankton composition: Periods 1, 2 and 4 composed of distinct diatom communities of small (5–20 μm length) to larger cells (20–120 μm) in colonies; Period 3 characterized by the Phaeocystis globosa bloom. Dilution experiments were conducted before, during and after the P. globosa bloom. Microzooplankton carbon consumption (from 18.1 to 360.9 μg C L− 1 d− 1) often equalled or exceeded phytoplankton production (from 1.7 to 129.0 μg C L− 1 d− 1) in particular at the end of the P. globosa bloom when microzooplankton grazed on previously formed phytoplankton biomass. Results of size-fractionated dilution experiments, conducted with distinct grazer communities, suggested different roles for ciliates and dinoflagellates. Ciliates appeared to be very efficient grazers of small diatoms (5–10 μm) and P. globosa free cells, whereas dinoflagellates grazed on both larger diatoms (> 10 μm; P1) and small P. globosa colonies. Ciliates and dinoflagellates did not seem to compete for food resources, as they were oriented towards different phytoplankton size classes.

Research highlights
► Herbivory during the spring phytoplankton bloom in the eastern English Channel.
► 12 dilution experiments carried out with total and size-fractionated Chla.
► Experiments were run with different predator/prey communities.
► Ciliates are likely consumers of free cells of Phaeocystis globosa.
► Dinoflagellates are likely consumers of Phaeocytis colonies and diatoms.