School of Engineering and Science - Jacobs University

SUMMARY
SUMMARY
This thesis investigates the role of microzooplankton in the food web of the North Sea.
For a basic understanding of the importance and seasonal distributions of dinoflagellates
and ciliates in the North Sea a monitoring of both groups was established on a weekly
basis and continued for 2.5 years. The results show that dinoflagellates and ciliates are
key organisms in terms of planktonic biomass in the North Sea around Helgoland and
constitute a highly diverse and abundant community with a clear seasonal trend.
Dinoflagellate biomass surpassed that of ciliates, especially during summer months,
whereas ciliates played an important role at the onset of the phytoplankton spring
bloom.
The evaluation of methodological sources of error in standard grazing experiments
resulted in a new set-up technique, which was tested and shown to represent an
improvement of the state-of-the-art method, conserving fragile microzooplankton
species and grazer biodiversity.
Applying the new technique, experiments on the grazing impact and food selectivity of
microzooplankton and copepods were carried out during key phases of a typical North
Sea spring bloom. The grazing experiments showed the overall important role of ciliates
and dinoflagellates as phytoplankton grazers when compared to copepods. Their grazing
impact terminated the bloom after roughly three weeks. Microzooplankton food
selectivity shaped the bloom and at the same time led to a stabilized assemblage of the
bloom-forming diatoms. Copepods, in turn, selectively fed on microzooplankton and
could therefore be regarded as the main top down controlling factor of this group. As
the positive selection for microzooplankton food gained importance during the course of
the bloom, when nutrients became limiting for the phytoplankton, microzooplankton
can be regarded as an important dietary component for copepods, particularly in cases
of nutritional shortfall in phytoplankton.
Microzooplankton does not only interact with other functional groups of the food web
but also with other members of the microzooplankton. Such interactions were
investigated within a three species model system consisting of two different Helgoland
microzooplankton predators and their common prey. The system included the
possibility of “intraguild” predation as the smaller dinoflagellate predator could be a
prey for the larger ciliate predator. In spite of theoretical assumptions that predicted the
extinction of the smaller predator a commensalistic relationship between both predators
was shown. The smaller predator was promoted by prey items that were pre-conditioned
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