School of Engineering and Science - Jacobs University
School of Engineering and Science - Jacobs University
School of Engineering and Science - Jacobs University
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DISCUSSION<br />
1986, Tillmann, 2004) consisting mainly <strong>of</strong> flagellates (Kivi & Setälä, 1995). Apart<br />
from obvious exceptions, ciliates can thus be classified as rapid reaction food specialists<br />
<strong>and</strong> din<strong>of</strong>lagellates more as generalists with longer response times but greater<br />
persistence. These ecological strategies were confirmed by succession patterns visible<br />
during my spring bloom experiments (Chapter III). The observed seasonality <strong>of</strong> both<br />
groups in the monitoring data (Chapter I) also supported this to a certain extent. Ciliates<br />
played a key role during spring (Riegman et al., 1993) as they responded more quickly<br />
to increasing phytoplankton concentrations (flagellates) <strong>and</strong> formed an earlier peak than<br />
din<strong>of</strong>lagellates. These in turn displayed longer lasting biomass maxima especially<br />
during the summer months.<br />
Let us apply these findings to a simplified situation in the field shortly before a bloom<br />
<strong>of</strong> different phytoplankton species, e.g., small flagellates <strong>and</strong> different diatoms.<br />
Furthermore let us neglect top down control <strong>of</strong> din<strong>of</strong>lagellates <strong>and</strong> ciliates <strong>and</strong> ask the<br />
question: Concerning the different features <strong>of</strong> din<strong>of</strong>lagellates <strong>and</strong> ciliates, what would<br />
be the effect <strong>of</strong> both grazer groups on bloom formation? The resulting scenario could be<br />
as follows:<br />
The phytoplankton constituting the preferred food <strong>of</strong> the ciliates, e.g. flagellates, would<br />
be eaten before being able to form a bloom due to the effective grazing <strong>of</strong> their fastgrowing<br />
ciliate predators. Ciliates therefore would prevent their preferred prey from<br />
blooming. The other phytoplankters would not be controlled by ciliates. They would be<br />
able to grow faster as they would be grazed by their more slowly growing din<strong>of</strong>lagellate<br />
predators. These phytoplankton species, e.g. different diatoms, would be the bloomforming<br />
species. Although this depiction is an oversimplification <strong>of</strong> the real<br />
mechanisms that drive phytoplankton blooms a similar scenario has been observed<br />
during the spring bloom experiments reported in this study (Chapter III). Such sizedifferential<br />
grazing control promoting diatom spring blooms has also been reported<br />
from other studies (Riegman et al., 1993, Brussaard et al., 1995) in the North Sea.<br />
Interactions between microzooplankton predators<br />
The relationships <strong>and</strong> interactions between species are <strong>of</strong> fundamental interest in<br />
ecology (Begon et al., 2006). Although the interactions <strong>of</strong> microzooplankton with other<br />
members <strong>of</strong> the marine food web, especially phytoplankton <strong>and</strong> mesozooplankton<br />
(Calbet, 2008), have been investigated in some detail, studies on the interactions<br />
between members <strong>of</strong> the microzooplankton are rare (Stoecker & Evans, 1985, Jakobsen<br />
& Hansen, 1997). Due to the wide range <strong>of</strong> members <strong>of</strong> different taxonomic groups <strong>and</strong><br />
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