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CHAPTER IV due to pre-conditioning of the prey for G. dominans via immobilisation of the prey items by the tintinnid. Enhanced “intraguild” predation? From a theoretical perspective “intraguild” predation would be profitable for the toppredator in a double sense (Polis et al., 1989). First, the predatory competitor would benefit directly by ingesting the other predator as food and second, eating a competitor would indirectly release the predator from competition pressure. In our case G. dominans was also a potential prey organism for its larger competitor F. ehrenbergii. Food selectivity of F. ehrenbergii was not directly measurable in our two-predator treatment because we could not discriminate between the shares of each predator in the observed grazing rates. However, given the highest growth rates (first experiment) of G. dominans measured throughout our experiments when F. ehrenbergii was present it seems reasonable that F. ehrenbergii did not feed exclusively on G. dominans. Although not directly measured we can at least assess the likelyhood of predation of F. ehrenbergii on G. dominans. The growth rate of G. dominans was promoted by a factor of 2.6 when fed artificially immobilised prey. Applying this factor to its growth in treatments of the first experiment containing only G. dominans as a single predator of S. trochoidea provides an estimate on how G. dominans growth would have resulted, taking into account the positive effect of immobilized prey in the absence of the predation by F. ehrenbergii. Using this estimate we could predict the observed growth rates of G. dominans in the two predator treatment after 24 hours quite well (i.e. real growth mean: 0.58 d -1 , estimate mean: 0.55 d -1 ). The estimate indicated predation on G. dominans after 48 and 72 hours as predicted growth was much higher than the observed one. However, as food density effects were not included, this has to be seen as a fairly rough result. Nevertheless, predation has been shown in the treatments with G. dominans as the only prey organism for the tintinnid. Whereas it first ingested G. dominans in numbers comparable to the other prey organism S. trochoidea, F. ehrenbergii ingestion and also growth declined to a minimum even when prey was still available. One possible explanation could be that predation pressure induced predator avoidance mechanisms in G. dominans. Increased escape velocity as reported for other dinoflagellates (Jakobsen et al., 2006) seems unlikely because we did not detect any change in swimming speed or behaviour in the presence of the tintinnid. Toxicity is also not reported for G. dominans and other ciliate species are able to feed on this dinoflagellate without negative effects for the predator (Jeong et al., 2004). In the past 120
CHAPTER IV feeding rates of F. ehrenbergii have been shown to be inhibited by a number of dissolved free amino acids (Strom et al., 2007b) which could theoretically also be released by G. dominans. However, in our work growth rates of the tintinnid in the presence of the smaller predator were the same as when preying on S. trochoidea alone and thus such a chemical influence could be rejected. A different picture was observed in the last experiment with a new F. ehrenbergii culture. Here we detected a mortality rate of around -0.22 d -1 in the presence of F. ehrenbergii indicating predation on the smaller G. dominans. However, a pronounced selective predation on G. dominans that would also promote the autotrophic prey S. trochoidea due to the partial release of grazing pressure (Stoecker & Evans, 1985) was not observed in our experiments. Competitive predator relationship with a commensalistic element Our findings are in contrast to results of another study where no difference in growth rates was found for a dinoflagellate or its potential ciliate predator competing for the same prey when compared to the single predator treatments (Jakobsen & Hansen, 1997) and another study where intraguild predation between the predators favoured the prey (Stoecker & Evans, 1985). Even if both predators competed for the same prey organism in our experiments G. dominans was directly supported by the presence, especially by the feeding, of the other predator leading to a higher efficiency in resource exploitation. This observed paradox could only be solved when looking at the feeding behaviour of F. ehrenbergii. The dinoflagellate directly benefited from immobilised but not ingested prey cells of the tintinnid. Benefits from “pre-conditioned prey” have been reported for dinoflagellates before, e.g. when feeding on faecal pellets of copepods (Poulsen & Iversen, 2008). Although G. dominans can feed on different planktonic prey in the laboratory (Nakamura et al., 1995a) it is often highly abundant during red tides of mobile dinoflagellate prey (Nakamura et al., 1995b, Kim & Jeong, 2004). Interestingly, G. dominans selected strongly for immobilised dinoflagellates in our experiments even if mobile prey was available in the same concentration. This is most probably related to the feeding habit of G. dominans. Gyrodinium species display a smooth pre-capture swimming behaviour around the prey before it is captured and ingested (Hansen, 1992). Taking this habit and the swimming speed of the prey organism S. trochoidea into account it is clear that immobile prey cells are easier captured by G. dominans even if there were higher encounter rates with swimming prey (Gerritsen & Strickler, 1977). 121
Page 1 and 2:
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
Page 76 and 77: CHAPTER III incubation bottles with
Page 78 and 79: CHAPTER III Data analysis To monito
Page 80 and 81: CHAPTER III General development of
Page 82 and 83: CHAPTER III Ciliate community After
Page 84 and 85: CHAPTER III Other microzooplankton
Page 86 and 87: CHAPTER III Microzooplankton Experi
Page 88 and 89: CHAPTER III Figure 6: (6a) General
Page 90 and 91: CHAPTER III Table 3: Temora longico
Page 92 and 93: CHAPTER III Table 4: Temora longico
Page 94 and 95: CHAPTER III The comparison of mesoc
Page 96 and 97: CHAPTER III Flagellates contributed
Page 98 and 99: CHAPTER III The effort to capture,
Page 100 and 101: CHAPTER III in words and deeds. Man
Page 102 and 103: CHAPTER III Table 2 Microzooplankto
Page 104 and 105: CHAPTER III Table 3b Temora longico
Page 106 and 107: CHAPTER III Table 4b Temora longico
Page 108 and 109: CHAPTER IV 102
Page 110 and 111: CHAPTER IV ABSTRACT The intra-speci
Page 112 and 113: CHAPTER IV Hansen, 1997). In turn,
Page 114 and 115: CHAPTER IV out in F/2 medium over 7
Page 116 and 117: CHAPTER IV the same patterns in G.
Page 118 and 119: CHAPTER IV Figure 1: General develo
Page 120 and 121: CHAPTER IV Favella ehrenbergii (Fig
Page 122 and 123: CHAPTER IV (24, 48, 72 hours of inc
Page 124 and 125: CHAPTER IV Growth response of G. do
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
Page 138 and 139: DISCUSSION Figure 1: A rather simpl
Page 140 and 141: DISCUSSION 1986, Tillmann, 2004) co
Page 142 and 143: DISCUSSION To my knowledge similar
Page 144 and 145: DISCUSSION transported around the w
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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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