School of Engineering and Science - Jacobs University

CHAPTER III
µgC prey µgC predator -1 d -1 ) was the lowest detected in our experiments leading to a
total daily ingestion I of 58.02 µgC L -1 d -1 (Table 2). Microzooplankton selected for
flagellates (except flagellates 5 µm), Rhizosolenia spp. and Chaetoceros spp. (Table 2).
In the late post-bloom phase (Experiment 4), grazer biomass (33 µgC L -1 ) was as low as
before the bloom and again saturated feeding was detected in 5 phytoplankton species
(Appendix Table 1). Beside the now dominating dinoflagellates, rotifers became as
important as ciliates. The community grazed 48% of P i and 80% of P p at a rate of 0.39
d -1 (g) and filtered 0.39 L d -1 (F) (Table 1+2). I c (0.97 µgC prey µgC predator -1 d -1 )
increased to a value similar to Experiment 2 resulting in a total daily ingestion I of
32.13 µgC L -1 d -1 . Again instantaneous growth µ 0 (0.52 d -1 ) of the phytoplankton
exceeded the growth in fertilized bottles (0.41 d -1 ) (Table 1). Selectivity was similar to
Experiment 1 whereas also the category ‘other diatoms’ was selected.
Microzooplankton predator-prey relationships
A direct coupling between ciliate and flagellate biomass was observed. This resulted in
a strong suppression of flagellate biomass from 26 to 10 µgC coincident with the ciliate
peak (Figure 6a). This was most pronounced for thecate and athecate dinoflagellates <
15 µm which disappeared totally during the Strombidium capitatum bloom (Figure 6b).
Simultaneously with the disappearance of both dinoflagellate groups S. capitatum
started to form cysts and its population collapsed totally within one week. With the
break-down of strombidiid and strobilid biomass at the end of March a relaxation from
grazing pressure enabled the flagellates to regenerate again.
The majority of dinoflagellate species we found in the mesocosms are reported to prefer
diatom prey. As diatoms did hardly change in composition, dinoflagellate succession in
the mesocosms was not as pronounced as for the ciliates. While two groups disappeared
during the first half of our experiment due to predation by ciliates as described above,
the remaining dinoflagellate community composition remained relatively constant until
the end of the experiment.
We also found a strong predator-prey relationship between a thecate amoeba and
Chaetoceros spp.. The apparently parasitic amoeba was found to be attached only to
cells of this genus. The abundance of the amoeba followed the development of
Chaetoceros spp. and showed a similar decline towards the end of the experiment.
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