School of Engineering and Science - Jacobs University

CHAPTER III
The effort to capture, handle, digest the prey and egest the excess carbon might have
been more energy demanding than the energy benefit the prey offered. Negative effects
due to poor food have been reported (Jensen & Hessen, 2007) and if predators have the
choice between good and bad food they naturally choose the good one. Other
microzooplankters, which feed on nutrient-limited phytoplankton represent the better
food when compared to the phytoplankton itself (Malzahn et al., 2010). Thus, an extra
effect introduced by “bad quality phytoplankton” may have been predation within
microzooplankton. Pronounced carnivory towards the end of phytoplankton blooms has
been described by Irigoien (2005) and in our experiment microzooplankton might also
have switched its feeding strategy. Towards the end of the bloom rotifers gained in
importance (up to 28% of biomass). About 10-40% of rotifer food can consist of
heterotrophic organisms of the microbial food web as rotifers are efficient predators on
protozoans (Arndt, 1993). It is therefore most likely that the combined effects of both,
predation within the microzooplankton especially by rotifers and the bad nutritional
quality of the food sources, resulted in an overall decline in microzooplankton
abundance.
The microzooplankton fate in a real bloom
Microzooplankton is able to compete with copepods for the same food sources and to
exploit food stocks more efficiently due to their fast metabolic abilities and growth
rates. They in turn are preferred food for higher trophic levels, e.g. mesozooplankton,
even if phytoplankton is available at high numbers but at low food quality (Hansen et
al., 1993). Microzooplankton contributes as a substantial part to copepods’ diets and it
is often positively selected (Nejstgaard et al., 1997, Fileman et al., 2007). Even in
predominately herbivorous species such as Acartia tonsa microzooplankton can make
up to 41% of the diet even when present in low abundances (Gifford & Dagg, 1988).
Grazing on microzooplankton by copepods can have severe trophic cascade effects. The
release of microzooplankton grazing pressure can promote nanoflagellates, an important
prey of ciliates, and thus affect bacterial abundance positively as bacteria are the main
food source of nanoflagellates (Zöllner et al., 2009). Even more pronounced effects
were reported on chlorophyll a concentration: Enrichment in copepod grazers reduced
microzooplankton biomass and led to overall higher chlorophyll a concentrations due to
the release of small sized flagellates from microzooplankton grazing (Sommer et al.,
2003, Sommer et al., 2005).
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