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Haupt, Florian (2011): Plankton vertical migrations - Implications for the pelagic ecosystem. Dissertation, LMU München: Faculty of Biology
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Abstract

Habitat selection is an important behavior of many organisms. The direction and strength of this behavior is often characterized as a result of a trade off between predator avoidance and obtaining resources. A characteristic example of this trade off may be seen in organisms in the pelagic ecosystem in the form of vertical migrations. Diel vertical migration (DVM) is a predator avoidance behavior of many zooplankton species, which is marked by a significant shift in the vertical distribution of the zooplankton where night time is spent in the epilimnion and day time in the hypolimnion While the causes of DVM and its ecophysiological consequences for the zooplankton are well studied, little is known about the consequences of DVM for the pelagic food ecosystem. Vertical migrations are not only restricted to zooplankton but are often exhibited by phytoplankton species, which respond to vertical gradients of light and nutrient availability. Many phytoplankton species cope with light and nutrient gradients by changing their position in the water column through active movement or buoyancy adjustment. The costs and consequences of this phytoplankton behavior are hardly studied. In my thesis, I studied the consequences of zooplankton DVM for the pelagic food web and the consequences of phytoplankton vertical migrations on individual growth and biomass composition through both field and laboratory experiments. I, Upward phosphorus transport by Daphnia DVM: During stagnation periods of the water column, physical upward transport processes are very unlikely and nutrients become scarce in the photic zone of many lakes. DVM of zooplankton could be a mechanism of nutrient repletion in the epilimnion. I experimentally examined the upward transport of phosphorus by Daphnia DVM. Results revealed that Daphnia DVM caused an upward nutrient transport. The amount of phosphorus transported and released by Daphnia in my study was within a biologically meaningful range: five percent of the estimated daily maximum phosphorus uptake of the phytoplankton community in the epilimnion. Therefore, nutrient transport by Daphnia DVM could be a significant mechanism in fuelling primary production in the phosphorus limited epilimnion. II, Daphnia DVM: implications beyond zooplankton: DVM creates a temporal and spatial predator-free niche for the phytoplankton, and theoretical models predict that parts of the phytoplankton community could use this niche. I experimentally investigated the influence of Daphnia DVM on the phytoplankton community of an oligotrophic lake in field mesocosms. My results suggest that Daphnia DVM had significant effects on quantitative and qualitative characteristics of the phytoplankton community. Phytoplankton biomass was higher in “no DVM” treatments. DVM also increased diversity in the phytoplankton community. The analyses showed that the gelatinous green algae Planktosphaeria gelatinosa was the main species influencing phytoplankton dynamics in the experiment, and therefore the effects of Daphnia DVM were highly species specific. III, Initial size structure of natural phytoplankton communities determines the response to Daphnia DVM: Previous studies have shown that the direction and strength of phytoplankton responses to zooplankton DVM most likely depends on the size of the phytoplankton species. To examine the influence of DVM on different sized phytoplankton communities, I manipulated the size distribution of a natural phytoplankton community a priori in field mesocosms. The results reveal that DVM oppositely affected the two different phytoplankton communities. A comparison of “DVM” and “no DVM” treatments showed that nutrient availability and total phytoplankton biovolume was higher in “no DVM” treatments of phytoplankton communities consisting mainly of small algae, whereas it was higher in “DVM” treatments of phytoplankton communities with a wide size spectrum of algae. It seemed that two different mechanisms on how DVM can influence the phytoplankton community were at work. In communities of mainly small algae nutrient recycling was important, seemed to be important, whereas in communities with a wide size spectrum of algae the refuge effect played the dominant role. IV, Carbon sequestration and stoichiometry of motile and non-motile green algae: The ability to move actively should entail costs in terms of increased energy expenditure and the provision of specific cell structures for movement. In a laboratory experiment, I studied whether motile, flagellated and non-motile phytoplankton taxa differ with respect to their energetic costs, phosphorus requirements, and structural carbon requirements. The results show that flagellated taxa had higher respiration rates and higher light requirements for growth than non-motile taxa. Accordingly, both short-term photosynthetic rates and long-term biomass accrual were lower for flagellated than for non-motile taxa. My results point at significant costs of motility, which may explain why flagellated taxa are often outcompeted by non-motile taxa in turbulently mixed environments, where active motility is of little use. The data in this study also suggest that motility alone may not be sufficient to explain the lower C: P ratios of flagellates. In summary, my results show that migrating phytoplankton and zooplankton species can act as a vector transporting energy, organic matter and ecological interaction. The complex consequences for the pelagic ecosystem are thereby determined by the organisms´ activity and characterized by their life history.