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DISCUSSION 1986, Tillmann, 2004) consisting mainly of flagellates (Kivi & Setälä, 1995). Apart from obvious exceptions, ciliates can thus be classified as rapid reaction food specialists and dinoflagellates more as generalists with longer response times but greater persistence. These ecological strategies were confirmed by succession patterns visible during my spring bloom experiments (Chapter III). The observed seasonality of both groups in the monitoring data (Chapter I) also supported this to a certain extent. Ciliates played a key role during spring (Riegman et al., 1993) as they responded more quickly to increasing phytoplankton concentrations (flagellates) and formed an earlier peak than dinoflagellates. These in turn displayed longer lasting biomass maxima especially during the summer months. Let us apply these findings to a simplified situation in the field shortly before a bloom of different phytoplankton species, e.g., small flagellates and different diatoms. Furthermore let us neglect top down control of dinoflagellates and ciliates and ask the question: Concerning the different features of dinoflagellates and ciliates, what would be the effect of both grazer groups on bloom formation? The resulting scenario could be as follows: The phytoplankton constituting the preferred food of the ciliates, e.g. flagellates, would be eaten before being able to form a bloom due to the effective grazing of their fastgrowing ciliate predators. Ciliates therefore would prevent their preferred prey from blooming. The other phytoplankters would not be controlled by ciliates. They would be able to grow faster as they would be grazed by their more slowly growing dinoflagellate predators. These phytoplankton species, e.g. different diatoms, would be the bloomforming species. Although this depiction is an oversimplification of the real mechanisms that drive phytoplankton blooms a similar scenario has been observed during the spring bloom experiments reported in this study (Chapter III). Such sizedifferential grazing control promoting diatom spring blooms has also been reported from other studies (Riegman et al., 1993, Brussaard et al., 1995) in the North Sea. Interactions between microzooplankton predators The relationships and interactions between species are of fundamental interest in ecology (Begon et al., 2006). Although the interactions of microzooplankton with other members of the marine food web, especially phytoplankton and mesozooplankton (Calbet, 2008), have been investigated in some detail, studies on the interactions between members of the microzooplankton are rare (Stoecker & Evans, 1985, Jakobsen & Hansen, 1997). Due to the wide range of members of different taxonomic groups and 134
DISCUSSION a high diversity of nutritional strategies in the microzooplankton (Sherr & Sherr, 2002), interactions between microzooplankters can be as variable as the players themselves comprising different nuances of competition as well as predation or neutralism. Investigations on possible interactions are made difficult by problems with culturing microzooplankton, particularly ciliates (Gifford, 1985). During one part of this thesis I focused on the interactions between small heterotrophic dinoflagellates and large ciliates and succeeded in taking them into culture. The experimental organisms, according to the monitoring data “key” model species at Helgoland Roads, were isolated from North Sea samples and cultures were established. As a result of their preferred prey size, small heterotrophic dinoflagellates potentially compete with bigger planktonic ciliates for prey (Jakobsen & Hansen, 1997). The system I investigated included the possibility of intraguild predation realized when the ciliate preys on the smaller dinoflagellate. Other studies with the same interactive conditions of the predators have dealt with two competing ciliates (Stoecker & Evans, 1985) or with a ciliate and a dinoflagellate species (Jakobsen & Hansen, 1997). Unlike such studies, results presented in Chapter IV of this study showed that interactions between intraguild prey and predator are more complex and not just a combination of competition and predation between the predators. Diehl & Feissel (2000) developed a theoretical framework for three-level food chains including omnivory. One fundamental condition in this is that the intraguild prey must be the superior resource competitor, otherwise it will be outcompeted by the intraguild predator. My experiments with both predators as single grazers suggested that the intraguild predator was the superior competitor. Against all expectations and theoretical suggestions (Diehl & Feissel, 2000, Diehl & Feissel, 2001) no negative effect on the intraguild prey was detected in the three-species treatments conducted during my thesis. The small heterotrophic dinoflagellate grew even faster in the presence of its competing intraguild predator. Looking closer into the interactive patterns I found a kind of commensalistic relationship preceding other possibilities of interaction. The small dinoflagellate was promoted by pre-conditioned prey items produced by the ciliate and showed the same growth rates as its competitor. Due to a faster growing predator biomass this led to a more efficient use of the resource in the system. Theoretical assumptions suggest that the promotion of the intraguild prey is of such magnitude that it can potentially favor a stable coexistence of both predators which is of further relevance as both predators co-occur in the same environment. 135
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The role of heterotrophic dinoflage
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“What we know is a drop, what we
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INTRODUCTION INTRODUCTION Understan
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INTRODUCTION flagellates) and metaz
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INTRODUCTION The remaining species
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INTRODUCTION Figure 4: Diplopsalis
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INTRODUCTION compared to dinoflagel
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INTRODUCTION “seawater dilution t
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RESEARCH AIMS Apart from the pure g
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OUTLINE OF THE THESIS the two diffe
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CHAPTER I CHAPTER I Dinoflagellates
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CHAPTER I Dinoflagellates and cilia
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CHAPTER I INTRODUCTION Marine resea
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CHAPTER I counted out at 200-fold m
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CHAPTER I Dinoflagellates closely f
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CHAPTER I Figure 3: Biomass [µgC L
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CHAPTER I prevents confusion with o
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CHAPTER I Figure 6: Comparison of c
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CHAPTER I should therefore occur on
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CHAPTER II 38
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CHAPTER II ABSTRACT Grazing experim
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CHAPTER II application, the alterna
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CHAPTER II Experiment 2 Experiment
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CHAPTER II Ciliates The three most
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CHAPTER II Experiment 2 Microzoopla
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CHAPTER II Asterisk * 1 symbolizes
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CHAPTER II DISCUSSION Transfer tech
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CHAPTER II bottles when siphoning w
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CHAPTER II ACKNOWLEDGEMENTS This st
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CHAPTER III 58
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CHAPTER III ABSTRACT Mesocosm exper
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CHAPTER III the prey (Cowles et al.
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CHAPTER III The mesocosms were stir
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CHAPTER III Net growth of plankton
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CHAPTER III prior to the experiment
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CHAPTER III incubation bottles with
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CHAPTER III Data analysis To monito
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CHAPTER III General development of
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CHAPTER III Ciliate community After
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CHAPTER III Other microzooplankton
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CHAPTER III Microzooplankton Experi
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CHAPTER III Figure 6: (6a) General
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Page 98 and 99: CHAPTER III The effort to capture,
Page 100 and 101: CHAPTER III in words and deeds. Man
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Page 110 and 111: CHAPTER IV ABSTRACT The intra-speci
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Page 116 and 117: CHAPTER IV the same patterns in G.
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Page 128 and 129: CHAPTER IV This was also confirmed
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Page 132 and 133: DISCUSSION described by competition
Page 134 and 135: DISCUSSION to know who they are. Th
Page 136 and 137: DISCUSSION Microzooplankton grazers
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Page 142 and 143: DISCUSSION To my knowledge similar
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Page 148 and 149: SUMMARY by its larger competitor. T
Page 150 and 151: REFERENCES Buskey, E.J. & Stoecker,
Page 152 and 153: REFERENCES Gentsch, E., Kreibich, T
Page 154 and 155: REFERENCES Jeong, H.J., Yoo, Y.D.,
Page 156 and 157: REFERENCES Montagnes, D.J.S., 2003.
Page 158 and 159: REFERENCES Rose, J.M. & Caron, D.A.
Page 160 and 161: REFERENCES Teixeira, I.G. & Figueir
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