School of Engineering and Science - Jacobs University

CHAPTER III
DISCUSSION
The main finding of this study is that microzooplankton was the main grazer throughout
the whole period of the phytoplankton spring bloom 2009 in the North Sea at Helgoland
while copepods played only a minor role, especially as the densities we used were rather
high. Using a mesocosm set up and excluding mesozooplankton grazers allowed us to
follow plankton spring succession focussing on top-down control mechanisms by
microzooplankton solely. In fact, the close resemblance of the bloom in the mesocosms
to the natural situations (where mesograzers were present) further suggested that the
microzooplankton drives the spring dynamics of the phytoplankton community around
Helgoland. Furthermore, the combined approach of dilution grazing experiments and T.
longicornis bottle incubations allowed us to analyse microzooplankton and copepod
grazing and feeding preferences in the same plankton community.
Microzooplankton and T. longicornis impact on the phytoplankton bloom
While microzooplankton grazed on average 120% of the potential phytoplankton
production (P p ) in our experiments, average grazing impact of T. longicornis was 47%.
Microzooplankton showed an almost sevenfold higher specific ingestion rate (I c ) when
preying on phytoplankton in contrast to copepods. Whereas the removal of
phytoplankton by T. longicornis in our experiments was slightly higher than the 10-40%
given by Calbet (2001) for copepods on a global scale, the grazing impact of the
microzooplankton was around twofold higher than results reported by Landry & Calbet
(2004). They found an average grazing impact of 59-75% of P p by microzooplankton
across a spectrum of open-ocean and coastal systems, whereas the lower border (60%)
was found for estuarine systems with chlorophyll a values similar to those of our
experiment. During our study the high availability of food during the bloom situation
combined with a release from grazing pressure by metazoans enabled the development
of a high microzooplankton grazer biomass in the mesocosms. Our results therefore
should represent the maximum in microzooplankton grazing impact on phytoplankton
in coastal regions. On the other hand the copepod biomass we used in our grazing
experiments was at the upper level of field abundances at this time of the year (Greve et
al., 2004) and therefore represents the maximal expectable grazing impact of copepods.
Nevertheless, we found a much higher grazing impact by microzooplankton than by
copepods.
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