ANNUAL VARIATION IN THE ABUNDANCE AND SIZE OF HETEROTROPHIC NANOFLAGELLATES ON THE SW COAST OF FINLAND, THE BALTIC SEA

Annual variation and vertical distribution in the abundance and cell volume of heterotrophic nanoflagellates (HNF) was studied on the SW coast of Finland, the Baltic Sea. HNF cell numbers and mean cell volume varied annually from 90 to 10(4) cells ml-1, and from 3 to 32 mum3, respectively, with maxima in early summer. The proportion of choanoflagellates in the HNF community was 0-23%. Statistical analysis revealed the vertical differences in HNF abundance to be insignificant, but vertical differences in the size structure of HNF communities were found, especially during thermal stratification. The majority (>80%) of HNF were small (maximum dimension 2-4 mum); the proportion of large (> 7 mum) cells were only 2-4% of the HNF abundance. An empirical equation for the relationship between HNF cell length and volume is presented, and the measurement of flagellate volume by epifluorescence microscopy is discussed.