Partition of planktonic respiratory carbon requirements during a phytoplankton spring bloom

• We studied the effect of variable phytoplankton biomass and dominance of the diatom Skeletonema marinoi on the planktonic community respiratory carbon requirement over a period of 14 days (14-28 April, 2008) in 3 different mesocosms filled with natural water at Espegrend marine biological field station by Raunefjord, Norway. The carbon requirement was measured on mesozooplankton (the calanoid copepod Calanus finmarchicus) and three different size fraction; <200 µm (dominated by microzooplankton), <15 µm (dominated by nanoplankton including most of the phytoplankton) and particles passing GF/C (dominated by bacterioplankton) by measuring oxygen consumption with an optode system with two SensorDish Readers. The respiratory carbon requirement showed no clear trend over time for any of the four groups. The mesozooplankton contributed the least to the total community carbon requirement, corresponding to less than 6% of primary production. In contrast, microzooplankton and nanoplankton consistently dominated the community carbon requirement, corresponding to > 50% of the primary production, while bacterioplankton showed an intermediate and variable contribution (ca <20% with a max of 50%). Feeding experiments on mesozooplankton (C. finmarchicus) two days before the peak in phytoplankton biomass, showed that the copepods ingested 2.4-4.3 times their respiratory carbon requirements, thus giving a high potential for growth. Respiratory carbon requirements of mesozooplankton were not significantly related to dominance or quantity of food available, whereas the respiratory carbon requirements of other groups were all related to the production of 22:6(n-3) fatty acid. The study confirms the important role of microorganisms in the biological carbon transformation through the food web during a phytoplankton spring bloom.

Nyckelord

Plankton community

respiratory carbon requirement

Skeletonema marinoi

mesozooplankton

Calanus

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