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INTRODUCTION Pomeroy (1974) stated that “although the ocean food web has been investigated for more than a century, several recent discoveries indicate that the classical textbook description of a chain from diatoms through copepods and krill to fishes and whales may in fact be only a small part of the flow of energy”. Indeed, expanded food web models were found to be a better fit to reality and explained energy fluxes in the food web more accurately (Pace et al., 1984). We now know that pelagic trophic relationships constitute highly complex web-like interactions between members of various groups. In addition, some consumers which were formerly classified as “top predators” are now known to be ingested by their “prey”, at least in some life-stages, e.g. during their larval or juvenile stages. This is the case, when, as an example, copepods feed on fish eggs or fish larvae (Turner et al., 1985, Yen, 1987). The closer food web interactions are studied, the more relationships in terms of cross-linkages and loopings appear, even where this has not been previously expected. Such studies are thus fundamental to dealing with our increasing demands upon the ocean (Pomeroy, 1974). Although our knowledge of pelagic food web structures has improved during the second half of the last century, the role of many organisms in this pelagic food web, as well as interactions between them, are still poorly understood or as yet undiscovered. Autotrophic phytoplankton forms the basis of the pelagic food web and the manner in which this resource is used by herbivores is decisive for the transport of energy to higher trophic levels such as fish (De Laender et al., 2010). Only recently the crucial role of microzooplankton organisms as the probably most important primary consumers in the ocean has been addressed (Landry & Calbet, 2004). In addition, microzooplankton is increasingly viewed as an irreplaceable food source for higher trophic levels (Stoecker, 1990, Montagnes et al., 2010). This group thus interacts with a wide range of trophic levels in the marine food web. However, despite years of research a lot of questions relating to microzooplankton are still unanswered. The microzooplankton – long neglected phytoplankton grazers Traditionally, planktonic crustaceans (copepods) were considered to be the main herbivores and meanwhile another group of phytoplankton grazers in the oceans has been overlooked for a long time: The microzooplankton. The term microzooplankton refers to the size fraction of heterotrophic planktonic organisms between 20 and 200 µm (Sieburth et al., 1978). It consists of taxonomically diverse groups of protozoa (e.g. ciliates, dinoflagellates and other heterotrophic 2
INTRODUCTION flagellates) and metazoa (e.g. rotifers, nauplii and other planktonic larvae). However, the numerically most important components within this group are heterotrophic dinoflagellates and ciliates (Capriulo et al., 1991). Microzooplankton only started receiving more attention when Azam et al. (1983) coined the term microbial loop. Where dissolved organic matter (DOM) released by phytoplankton is utilised by heterotrophic bacteria, heterotrophic nanoflagellates consume these bacteria and are in turn prey for microzooplankton organisms. Via this microbial loop energy in the form of DOM released by phytoplankton is returned to the main food chain (Azam et al. 1983). Subsequent investigations showed that microzooplankton not only plays a significant role in transferring energy to higher trophic levels (Sherr et al., 1986) but that it can also consume up to 60–75% of the daily phytoplankton production (Landry & Calbet, 2004). Early continental shelf models including microzooplankton assumed that they only feed on phytoplankton fractions smaller than 60 µm (Pace et al., 1984). However, we now know that they have a broad food spectrum (Smetacek, 1981, Jeong, 1999) placing them in direct competition with copepods for bigger phytoplankton (Hansen, 1992, Aberle et al., 2007). Recent studies even show that dinoflagellates can be the most important grazers during diatom blooms (Sherr & Sherr, 2007). Irigoien et al. (2005) went one step further hypothesizing that phytoplankton blooms can only occur when microalgae are released from microzooplankton grazing pressure. This relationship was also experimentally shown by Sommer et al. (2005). Only recently a growing number of studies have started to investigate the role of microzooplankton as phytoplankton grazers (Calbet & Landry, 2004, Fonda Umani et al., 2005, Irigoien et al., 2005, Putland & Iverson, 2007, Sherr & Sherr, 2007). Although their pivotal role as phytoplankton grazers especially during phytoplankton blooms has now been recognised, less is known about the functional diversity of microzooplankton. Crucial for an understanding of the ecological role of microzooplankton is more research on its abundance, species composition, seasonal distribution and succession patterns as well as the biotic and abiotic factors influencing all of these aspects. Another blank area in our knowledge about microzooplankton concerns investigations on its capacity for food selectivity. Scarcely anything is known about the plasticity in microzooplankton food preferences and how this can influence bloom assemblages, both of the phytoplankton prey and the microzooplankton predators. Although fundamental to ecological considerations, to date, interactions within the microzooplankton community, e.g., competitive patterns or inter-specific predation 3
Page 1 and 2: The role of heterotrophic dinoflage
Page 3: “What we know is a drop, what we
Page 7: INTRODUCTION INTRODUCTION Understan
Page 11 and 12: INTRODUCTION The remaining species
Page 13 and 14: INTRODUCTION Figure 4: Diplopsalis
Page 15 and 16: INTRODUCTION compared to dinoflagel
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Page 20 and 21: RESEARCH AIMS Apart from the pure g
Page 22 and 23: OUTLINE OF THE THESIS the two diffe
Page 25 and 26: CHAPTER I CHAPTER I Dinoflagellates
Page 27 and 28: CHAPTER I Dinoflagellates and cilia
Page 29 and 30: CHAPTER I INTRODUCTION Marine resea
Page 31 and 32: CHAPTER I counted out at 200-fold m
Page 33 and 34: CHAPTER I Dinoflagellates closely f
Page 35 and 36: CHAPTER I Figure 3: Biomass [µgC L
Page 37 and 38: CHAPTER I prevents confusion with o
Page 39 and 40: CHAPTER I Figure 6: Comparison of c
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Page 46 and 47: CHAPTER II ABSTRACT Grazing experim
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Page 50 and 51: CHAPTER II Experiment 2 Experiment
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Page 54 and 55: CHAPTER II Experiment 2 Microzoopla
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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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CHAPTER III Table 3: Temora longico
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CHAPTER III Table 4: Temora longico
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CHAPTER III The comparison of mesoc
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CHAPTER III Flagellates contributed
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CHAPTER III The effort to capture,
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CHAPTER III in words and deeds. Man
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CHAPTER III Table 2 Microzooplankto
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CHAPTER III Table 3b Temora longico
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CHAPTER III Table 4b Temora longico
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CHAPTER IV 102
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CHAPTER IV ABSTRACT The intra-speci
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CHAPTER IV Hansen, 1997). In turn,
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CHAPTER IV out in F/2 medium over 7
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CHAPTER IV the same patterns in G.
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CHAPTER IV Figure 1: General develo
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CHAPTER IV Favella ehrenbergii (Fig
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CHAPTER IV (24, 48, 72 hours of inc
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CHAPTER IV Growth response of G. do
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CHAPTER IV due to pre-conditioning
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CHAPTER IV This was also confirmed
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DISCUSSION described by competition
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DISCUSSION to know who they are. Th
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DISCUSSION Microzooplankton grazers
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DISCUSSION Figure 1: A rather simpl
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DISCUSSION 1986, Tillmann, 2004) co
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DISCUSSION To my knowledge similar
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DISCUSSION transported around the w
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SUMMARY by its larger competitor. T
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REFERENCES Buskey, E.J. & Stoecker,
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REFERENCES Gentsch, E., Kreibich, T
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REFERENCES Jeong, H.J., Yoo, Y.D.,
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REFERENCES Montagnes, D.J.S., 2003.
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REFERENCES Rose, J.M. & Caron, D.A.
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REFERENCES Teixeira, I.G. & Figueir
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