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CHAPTER III the prey (Cowles et al., 1988) and according to their own life stages (Mauchline, 1998). Microzooplankton species are in contrast often difficult to culture and the rare laboratory investigations on feeding behaviour have focussed on easily cultivable species (Flynn et al., 1996, Hamels et al., 2004). Given the diverse feeding modes within the microzooplankton community, also food preference and selectivity are likely to be highly diverse in this group of grazers. As there is still a lot of debate on the feeding habits and the selectivity of microzooplankton, investigations on microzooplankton grazing under conditions as close to nature as possible while considering inter-specific interactions in the plankton as well as the cumulative effect of the differences in selectivity of the present grazers are imperative. Our investigations focused on North Sea plankton communities at Helgoland Roads. Although this station has been sampled for plankton since 1962, the microzooplankton has hardly been investigated so far and, in contrast to other plankton components, their role in the pelagic food web at Helgoland is unclear. In this study, we hypothesize that: (1) the microzooplankton with its various feeding modes can control phytoplankton spring blooms, (2) selective grazing by microzooplankton leads to blooms of less-favoured phytoplankton species and (3) microzooplankton succession in spring can be directly linked to the availability of different prey. We conducted a mesocosm experiment and simulated a natural spring bloom using in situ plankton communities from Helgoland Roads. Copepod grazing on microzooplankton can be severe, especially in a restricted mesocosm environment, and can cause strong trophic cascade effects (Sommer et al., 2003, Sommer & Sommer, 2006, Zöllner et al., 2009) thus hampering investigations on the effects of microzooplankton grazing on phytoplankton. By excluding mesozooplankton grazers from the incubations, microzooplankton was relaxed from the grazing of mesozooplankton and we could explicitly examine the role of microzooplankton grazing on phytoplankton communities during the bloom. Grazing experiments for detailed investigations on microzooplankton grazing and selectivity were conducted at four defined points of the phytoplankton spring bloom: Pre-bloom (exponential growth phase = Experiment 1), bloom peak (biomass maximum = Experiment 2), early postbloom (one week after biomass maximum = Experiment 3) and later post-bloom (two weeks after biomass maximum = Experiment 4). The role of copepods in structuring the spring phyto- and microzooplankton community was also examined via measuring copepod grazing and selectivity during these distinct bloom phases. 62
CHAPTER III MATERIAL AND METHODS Sampling site Helgoland is located in the German Bight (Southern North Sea). It is subject to both coastal influences from the shallow Wadden Sea as well as marine influences from the open North Sea. Since 1962 water samples are taken as part of a long term monitoring for plankton and nutrients at the “Kabeltonne” site at Helgoland Roads (54°11.3’N; 7°54.0’E) (Wiltshire et al., 2008). Water for the mesocosm experiment was taken at this site. Set up The aim of the mesocosm experiment was to follow a typical spring plankton succession under near-natural conditions. The mesocosm experiment took place from the middle of March until mid-April 2009 in a constant temperature room with a starting temperature of 4.2°C and a quick rise towards the end temperature of ~6.8°C within a few days. Start and end temperature were similar to in situ conditions (4.2°C/6.7°C), but the rise in temperature in the mesocosms was somewhat faster than in the field. Three cylindrical mesocosms with a volume of 750 L each were filled by gravity feed with natural seawater from Helgoland Roads. Pumps were not used to ensure the survival of the whole plankton community and particularly delicate organisms (Löder et al., 2010). Water was first repeatedly scooped from the water surface using an open 850 L container suspended from the crane of the research vessel Uthörn and three 1000 L containers were subsequently filled by hose via gravitational power. In order to remove mesozooplankton but to allow for the passage of chain-forming diatoms and microzooplankton the water was screened over the feed using a 200 µm gauze bag connected to the end of the hose which floated in the container during filling. Back on land, this water was transferred to the mesocosms via gravitational feed. The even distribution of the water from each container to the three mesocosms was ensured by an interconnected triple-split hose distributor mounted on the main hose. Thus after the filling each mesocosm contained identical over-wintering/spring populations of bacteria, phytoplankton and zooplankton smaller than 200 µm (microzooplankton). 63
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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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Page 66 and 67: CHAPTER III ABSTRACT Mesocosm exper
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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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