School of Engineering and Science - Jacobs University

DISCUSSION
(average 120%, Chapter III) even surpassed this range and showed the big potential and
importance of this grazer group in waters around Helgoland.
Copepods have long been considered the main herbivorous force in the plankton.
However, taking into account the grazing impact of copepods during the spring bloom
experiment (average 47%, Chapter IV) I showed that throughout the bloom phase the
microzooplankton was the more important phytoplankton grazer group, even though it
was always present in lower biomass concentrations (30-94 µg L -1 ) compared to
copepods (103 µg L -1 ). In addition, our findings (Chapter I) suggest that in the field
copepods biomass only plays a minor role when compared to microzooplankton. The
combination of those two facts stresses the importance of microzooplankton grazers in
the North Sea. Furthermore, my findings support results reported in other studies
(Calbet, 2001, Calbet & Landry, 2004, Putland & Iverson, 2007, Sherr & Sherr, 2007).
Irigoien (2005) proposed that microzooplankton grazing is of such importance that only
phytoplankton species which can escape control by microzooplankton are able to
bloom. Supporting this view, I showed in Chapter III that microzooplankton selective
grazing can also have a stabilizing function on the blooming phytoplankton assemblage,
leading to constant shares of the bloom-forming taxa. In my study the high grazing
impact of ciliates prevented small flagellates from blooming, whereas the selective
feeding of dinoflagellates led to a diatom bloom consisting of only three genera
(Rhizosolenia, Thalassiosira, Chaetoceros).
Microzooplankton is not only in direct competition with herbivorous mesozooplankters,
such as copepods (Hansen, 1992, Aberle et al., 2007, Sherr & Sherr, 2007), but it is also
an important food source for higher trophic levels (Kleppel, 1993). Thus,
microzooplankters play a fundamental role as trophic intermediaries. They link small
planktonic size fractions, unavailable to most metazoan consumers (Gifford, 1991), to
mesozooplankton (Klein Breteler et al., 1999). Grazing experiments with copepods
during the spring bloom experiments conducted in this thesis (Chapter III) showed that
microzooplankton was always an important food source for them. Several other studies
support this finding (Nejstgaard et al., 1997, Calbet & Saiz, 2005, Fileman et al., 2007,
Figueiredo et al., 2009, De Laender et al., 2010).
Nutritionally “poor” food can have negative effects on copepods (Schoo, 2010) and if
they have the choice they obviously actively choose food according to their nutritional
needs (Cowles et al., 1988, Kleppel, 1993).
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