MICROZOOPLANKTON GRAZING, PIGMENTS, AND COMPOSITION OF PLANKTON COMMUNITIES DURING LATE SPRING IN 2 NORWEGIAN FJORDS

The coupling between primary and secondary production, and its impact upon vertical flux of organic matter, was studied in a fjord and a poll in late spring. We report here the abundance and vertical distribution of phototrophic and heterotrophic plankton; their growth rates and grazing by microzooplankton; and the degradation products of chlorophyll a due to grazing. Both fjord and poll contained abundant nano- and microplankton communities. Typical peak concentrations were 2-4 x 10(3) cells ml-1 of photosynthetic nanoplankton, 1-3 x 10(3) cells . ml-1 of heterotrophic (aplastidic) nanoplankton, 4-8 x 10(4) cells . ml-1 of cyanobacteria, and 4-9 ciliates . ml-1. Maximum abundances occurred within the euphotic zone. The dominant phytoplankton were chlorophyll c-containing taxa, principally prymnesiophytes and chrysophytes. Grazing by < 200 mum microzooplankton generally removed 50-100 % and 20-100 % of daily production by < 10 mum and < 2 mum cells, respectively, including autotrophic and heterotrophic taxa. Planktonic ciliates were numerically important microzooplankton. Phaeophorbides were the dominant form of phaeopigments. The three main phaeophorbide forms decreased in concentration during experimental incubations, while phaeophytins and chlorophyllide a were differentially produced by microzooplankton grazing. When corrected for losses not related to grazing, grazing rates estimated from pigments were similar to those derived from cell enumerations. Based upon measured primary production and sedimentation rates, little newly formed particulate organic matter was apparently available for export to depth. The key attribute to explain the close coupling between production and consumption was the small cell size of phototrophs and the accompanying importance of microzooplankton.