MEPS 451:15-29 (2012)  -  DOI: https://doi.org/10.3354/meps09564

ABSTRACT: 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 d (14 to 28 April 2008) in 3 different mesocosms filled with natural water at Espegrend marine biological field station in Raunefjord, Norway. The carbon requirement was measured on mesozooplankton (the calanoid copepod Calanus finmarchicus) and 3 other size fractions of plankton—<200 µm (dominated by microzooplankton), <15 µm (dominated by nanoplankton including most of the phytoplankton) and particles passing GF/C filters (dominated by bacterioplankton)—by measuring oxygen consumption using an optode system with 2 SensorDish Readers. The respiratory carbon requirement showed no clear trend over time for any of the 4 groups. The mesozooplankton contributed the least to the total community carbon requirement, corresponding to <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 maximum of 50%). Feeding experiments on mesozooplankton (C. finmarchicus) 2 d before the peak in phytoplankton biomass showed that the copepods ingested from 2.4 to 4.3 times their respiratory carbon requirements, thus providing 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 present study confirms the important role of microorganisms in the biological carbon transformation through the food web during a phytoplankton spring bloom.

KEY WORDS: Plankton community · Respiratory carbon requirement · Energy transfer ·  Skeletonema marinoi · Mesozooplankton · Calanus