School of Engineering and Science - Jacobs University

CHAPTER IV
INTRODUCTION
One basic element of interest in ecology is the relationships of organisms and their
interactions (Begon et al., 2006). The most obvious interaction between two predators
preying on the same limiting resource is competition whereby both can be negatively
affected by each other and the more competitive predator theoretically outcompetes the
less competitive one (Gause’s law) (Gause, 1934). However, such competitive
exclusion is actually rarely observed in natural ecosystems, and many biological
communities appear to violate Gause's law, which led to Hutchinson’s term “paradox of
the plankton” (Hutchinson, 1961). Meant was the fact that in the seemingly
homogenous environment of the world’s oceans an incredibly high number of
phytoplankton species coexist, despite the fact that they all compete for the same
limiting resources (e.g. CO 2 , light and nutrients). One solution to this paradox is
externally imposed variability in the surrounding environment, such that the systems are
never in equilibrium. Another important factor may be found in the trophic interaction
between phytoplankton and zooplankton, with typical oscillatory (Scheffer et al., 2003)
or even chaotic population cycles (Beninca et al., 2008). Although considerable work
has been carried out on predation in single predator-prey relationships of planktonic
species, less is known about inter-specific interactions between planktonic predators.
Whereas mesozooplankton refers to larger metazoan grazers, microzooplankton is the
size fraction of heterotrophic planktonic organisms between 20 and 200 µm.
Microzooplankton has recently gained attention as the main grazer in the oceans,
capable of grazing up to 60-75% of the daily phytoplankton production (Landry &
Calbet, 2004). Interestingly, almost nothing is known about the inter-specific
interactions of predators within the microzooplankton. Although it includes a large
variety of taxonomic groups, the most important ones in terms of abundance are
heterotrophic dinoflagellates and ciliates (Capriulo et al., 1991).
Generally, the majority of marine planktonic ciliates feed on prey which is about one
tenth of their own size (Spittler, 1973, Heinbokel, 1978a, Jonsson, 1986) and mainly
nanoflagellates of different taxonomic groups. In contrast, heterotrophic dinoflagellates
seem to prefer food of their own size (Hansen, 1992) and they also can feed on prey that
is actually larger than the predator (Naustvoll, 2000a, Naustvoll, 2000b). Large
heterotrophic dinoflagellates thus tend to feed on larger planktonic organisms (e.g.
chain-forming diatoms), while small heterotrophic dinoflagellates feed mainly on
nanoflagellates. As a result of their preferred prey size, small heterotrophic
dinoflagellates potentially compete with larger planktonic ciliates for prey (Jakobsen &
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