ABUNDANCE, BIOMASS AND DISTRIBUTION OF HETEROTROPHIC DINOFLAGELLATES DURING THE NORTH-ATLANTIC SPRING BLOOM

The abundance, biomass, size and vertical distribution of heterotrophic (aplastidic) dinoflagellates were measured over several weeks in drogued water masses of the northeast Atlantic Ocean. Sampling was conducted in 1989 and 1990 in the vicinity of 47-49-degrees-N and 18-19-degrees-W as part of the JGOFS and BOFS studies of spring blooms in oceanic waters. Two size classes of dinoflagellates were distinguished: <20 mum cells, measured and enumerated using a sophisticated true-color image analysis system, and >20 mum cells, quantified via Utermohl microscopy. Dinoflagellates were an abundant component of the microzooplankton assemblages in the upper 200 m. In 1989, abundance ranged from 2 to 67 cells ml-1, equivalent to 0.1-3.5 mugC l-1. Heterotrophic dinoflagellates were more abundant in 1990, with 13-414 cells ml-1, equivalent to 0.9-18.3 mugC l-1. Nanodinoflagellates (<20 mum cells) dominated the assemblages, representing 99% of total numbers and 70-88% of total carbon biomass of heterotrophic dinoflagellates. The average carbon content of <20 mum cells was 32 pgC (1990) to 51 pgC (1989), compared with 1.5 ngC (1990) to 1.7 ngC (1989) for >20 mum cells. Estimates of their grazing impact, derived from literature data on growth rates and conversion efficiencies, ranged from 8 (1989) to 25% (1990) of total primary production, and up to 52% of <5 mum primary production. It is hypothesized that their greatest impact may occur beneath the mixed layer.