DISTRIBUTION, ABUNDANCE AND SIZE COMPOSITION OF HETEROTROPHIC DINOFLAGELLATES AND CILIATES IN THE SARGASSO SEA NEAR BERMUDA

We examined the temporal variability in vertical distribution, abundan ce, size composition and biomass of heterotrophic dinoflagellates and ciliates at the JGOFS time-series station off Bermuda during August 19 89 and March-April 1990. Abundances of heterotrophic dinoflagellates a nd ciliates ranged from 1.3 to 14.2 ml(-1) in August and 2.0 to 28.6 m l(-1) in number in March-April. Total biomass of these two groups rang ed from 0.2 to 2.9 mu g Cl-1 integrated euphotic zone biomass was twic e as high in March-April than in August. Heterotrophic dinoflagellates were always more numerous than ciliates (0.9-28.6 ml(-1) compared to 0.1-5.0 ml(-1)). On average, heterotrophic dinoflagellates constituted 50% of the total biomass of these two groups (46-59% in August and 29 -64% in March-April) in the upper 150 m. Greater than 92% of the dinof lagellate cells were <20 mu m in size, but larger cells generally domi nated the biomass. The proportion of ciliates <20 mu m in size was hig hly variable between samples (range=8-91%, mean=59%), but small cells were, on average, only 18% of the ciliate biomass. The temporal variab ility in taxonomic composition of these two groups was high, reflectin g the mesoscale physical variability found in this transition region. This is the first report of the biomass and composition of the >5 mu m heterotrophic protists in the oligotrophic subtropical Sargasso Sea. Copyright (C) 1996 Elsevier Science Ltd.