School of Engineering and Science - Jacobs University

CHAPTER III
The comparison of mesocosm chlorophyll a development with that in field revealed that
they differed only during the inflow of coastal waters, even though copepods as
potential predators were present potentially leading to a higher grazing impact on
phytoplankton. As we did not investigate phytoplankton grazing of microzooplankton
and copepods in the field we can only speculate about their relative importance.
However, the similarity of the chlorophyll a development suggests that the same
patterns of grazing as in the mesocosms were responsible for the development of
phytoplankton biomass in the field. Combined with the fact that copepods avoided
phytoplankton prey in the mesocosms this finding strengthens our result that spring
microzooplankton can be regarded as the key phytoplankton grazer during the
phytoplankton spring bloom and copepods apparently playing only a minor role at this
time of the year.
Optimal bloom exploitation through different feeding strategies of
microzooplankton
Different feeding strategies are recorded among heterotrophic dinoflagellates including
direct engulfment, pallium-feeding and peduncle- or tube-feeding (Jacobson &
Anderson, 1986, Gaines & Elbrächter, 1987). Ciliates are categorized as suspension,
raptorial, deposit and diffusion feeders (Müller & Weisse, 1994). Depending on the
feeding mode of the predators different prey is selected. Therefore, depending on the
zooplankton community present at specific times of the year, feeding habits are directly
mirrored by food selectivity patterns. Grazing selectivity itself also structures the
phytoplankton composition (Irigoien et al., 2005). During the course of our experiments
the microzooplankton community comprised a large variety of food preferences and
preferred size spectra according to grazer species, their own size and feeding mode.
Generally, dinoflagellates can feed on a wide range of prey (Jeong, 1999) and are likely
to be more quantitatively significant consumers of bloom-forming diatoms than
copepods (Sherr & Sherr, 2007). Species that dominated in our study (Gyrodinium spp.
and Protoperidinium spp.) are mainly associated with diatom blooms (Sherr & Sherr,
2007). Athecate Gyrodinium spp. (20-120 µm length) and thecate Protoperidinium spp.
(15-75 µm diameter) dominated the grazer assemblage. Dinoflagellates can feed and
grow on variable predator to prey size ratios between 5.2:1 and 0.15:1 (Naustvoll,
2000a, Naustvoll, 2000b). The upper limit of prey size reported by Naustvoll (2000a,
2000b) is probably not reached by naked phagotrophs such as Gyrodinium sp. as they
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