School of Engineering and Science - Jacobs University

CHAPTER III
Strombidium capitatum, which is known to be directly dependent on flagellate prey
(Stoecker & Silver, 1990) and forms cysts as soon as unfavourable conditions occur
(Kim et al., 2008). Simultaneously with the disappearance of its potential prey
(dinoflagellates < 15µm) S. capitatum started to form cysts and abundance of the ciliate
population collapsed within one week. After the decrease of strombidiids and strobilids,
Cyclotrichium sp. a ciliate which also feeds on diatoms (Xu et al., 2005) started to
dominate and towards the end of the bloom, bacterivorous ciliates (Acineta sp., Euplotes
sp.) gained importance.
In contrast, dinoflagellates showed unclear succession patterns compared to ciliates.
While two groups disappeared due to predation by ciliates (thecate and athecate
dinoflagellates < 15 µm), the remaining dinoflagellate community remained relatively
stable until the end of the experiment. This was most likely related to the fact that
diatoms, the preferred prey of dinoflagellates (Sherr & Sherr, 2007), were always
present. The opposing patterns found for dinoflagellates and ciliates might also be
related to a contrasting ecological strategy of these groups. Dinoflagellates are
considered to have lower growth rates than ciliates (Hansen, 1992) and therefore their
ability to react rapidly to enhanced food availability is limited. On the other hand,
dinoflagellates can prey on almost every organic particle present in the oceans (Jeong,
1999, Tillmann, 2004). They also have a higher starving potential (Hansen, 1992,
Menden-Deuer et al., 2005), and thus can survive periods of food shortage (Sherr &
Sherr, 2007). In contrast, ciliates can respond rapidly to enhanced food availability
showing growth rates higher than those of dinoflagellates (Strom & Morello, 1998) but
their potential to survive starvation periods is low (Jackson & Berger, 1985, Hansen,
1992) and they are more restricted to certain prey items (Tillmann, 2004). Thus, the
succession of microzooplankton observed in the present study is mainly triggered by the
availability of food and contrasting survival strategies. Ciliates can be considered as
specialists and dinoflagellates more as generalists.
Factors determining the microzooplankton bloom in the mesocosms
Interestingly, microzooplankton biomass declined to values close to start values at the
end, even though a considerable amount of phytoplankton food was still available.
Besides predator-prey relationships that negatively influence the predator when its prey
is absent, this pattern could be due to changes in the food quality of the preferred food.
With the duration of the bloom phytoplankton got increasingly nutrient-depleted
(Schoo, 2010) and therefore did not meet the nutritional needs of the microzooplankton.
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