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Marine Ecology Progress Series

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MEPS 278:67-75 (2004)  -  doi:10.3354/meps278067

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

Ulrich Sommer1,*, Thomas Hansen1, Herwig Stibor2, Olav Vadstein3

1Leibniz-Insitut für Meereswissenschaften, Düsternbrooker Weg 20, 24105 Kiel, Germany
2Zoologisches Institut, Karl-Ludwigs-Universität, Karlstrasse 23-25, 80333 München, Germany
3The Biological Station, Norwegian University of Science and Technology, 7491 Trondheim, Norway

ABSTRACT: 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.

KEY WORDS: Phytoplankton · Zooplankton · Nutrient ratios · Mesocosms

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