Persistence of phytoplankton responses to different Si:N ratios under mesozooplankton grazing pressure: a mesocosm study with NE Atlantic plankton

We fertilised 12 mesocosms with NE Atlantic phytoplankton with different Si:N ratios (0:1 to 1:1). After 1 wk, we added mesozooplankton, mainly calanoid copepods at natural densities to 10 of the mesocosms; the remaining 2 mesocosms served as controls. A trend of increasing diatom dominance with increasing Si:N ratios and species-specific correlations of diatoms to Si:N ratios were not changed by the addition of mesozooplankton. Large unicellular and chain-forming diatoms, thin-walled dinoflagellates (Gymnodiniales) and ciliates were reduced by copepod grazing while armoured dinoflagellates remained unaffected. Nanoplanktonic flagellates and diatoms profited from the addition of copepods, probably through release from ciliate grazing.