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Development of mesozooplankton biomass, feeding activity and reproductive success of the copepod Calanus helgolandicus were compared in blooms of natural plankton in 7 mesocosms on the west coast of Norway between 28 August and 28 September 1996. Nutrient content and turbulence in enclosures of 27 m 3 natural sea water were manipulated in situ in order to generate blooms dominated by diatoms and flagellate-microzooplankton respectively. The diatom-dominated mesocosms reached the highest algal biomasses. Calanus helgolandicus generally preferred non-diatom food, such as ciliates and metazoans, and showed similar ingestion rates in both systems. The female body-carbon-specific nauplii production was highest (10.6% d-1) during a flagellate-microzooplankton bloom, before it dropped to ca 1.7% d-1 during a bloom of Dictyocha speculum. In the diatom-dominated water it dropped to as …