Biological Oceanography

(^) Since the dominant primary producers of vast oligotrophic ocean regions, and
during long seasons in temperate zones, are Synechococcus and Prochlorococcus and
an array of miniscule autotrophic eukaryotic phytoplankton, Hflag are responsible for
a very large part of the initial transfer of primary production into pelagic food webs.
That first grow-out experiment was very clean. Later work (e.g. Calbet et al. 2001)
shows that the outcome can have many patterns. However, in most cases with near-
zero apparent growth in unfractionated water, filtrates of very fine filters generally
show elevated growth rates. Microflagellates (Hflag) are generally present and
feeding. Evaluation by Calbet et al. (2001) of Hflag with epifluorescence microscopy
using proflavin (distinguishes Hflag from autotrophs) and nuclear stains showed
shared numerical dominance of cells <2 and 2–3 μm, and an inverse dominance of
biomass by relatively rare (a few percent by numbers) forms >10 μm (Fig. 9.6), both
in the mixed layer and at the deep chlorophyll maximum.
Fig. 9.6 (a) Proportions of numerical abundance and (b) proportions of biomass of
size fractions of heterotrophic flagellates in the North Pacific subtropical gyre. Bars
from top to bottom are size fractions: <2, 2–3, 3–4, 4–5, 5–10, and >10 μm.
(^) (After Calbet et al. 2001.)
(^) Fonda Umani and Beran (2003) and Calbet et al. (2008) have combined a dilution
protocol with size fractionation to evaluate grazing on pico- and nano-autotrophs by
the <10 μm community. Studies were quarterly in the Adriatic and monthly off
Barcelona, respectively. This combined approach is certain to be applied more widely
to evaluate the feeding rates of specific size classes of microzooplankton. For the
moment the results need replication. One of the problems of both dilution and cascade
experiments has been the long intervals between trials. Sometimes experiments have
been monthly – as off Barcelona, or quarterly – as in the Adriatic, or just once, such
that the likely grazer–autotroph and micropredator–grazer oscillations are not