Distribution, abundance and size composition of heterotrophic dinoflagellates and ciliates in the Sargasso Sea near Bermuda

We examined the temporal variability in vertical distribution, abundance, size composition and biomass of heterotrophic dinoflagellates and ciliates at the JGOFS time-series station off Bermuda during August 1989 and March-April 1990. Abundances of heterotrophic dinoflagellates and ciliates ranged from 1.3 to 14.2 ml−1 in August and 2.0 to 28.6 ml−1 in number in March-April. Total biomass of these two groups ranged from 0.2 to 2.9 μg C 1−1; integrated euphotic zone biomass was twice as high in March-April than in August. Heterotrophic dinoflagellates were always more numerous than ciliates (0.9–28.6ml−1 compared to 0.1–5.0ml−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 dinoflagellate cells were < 20 pm in size, but larger cells generally dominated the biomass. The proportion of ciliates < 20 pm in size was highly variable between samples (range = 8–91 %, mean= 59%), but small cells were, on average, only 18% of the ciliate biomass. The temporal variability in taxonomic composition of these two groups was high, reflecting the mesoscale physical variability found in this transition region. This is the first report of the biomass and composition of the > 5 gmm heterotrophic protists in the oligotrophic subtropical Sargasso Sea.