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Striebel, Maren (2008): PLANKTON DYNAMICS: THE INFLUENCE OF LIGHT, NUTRIENTS AND DIVERSITY. Dissertation, LMU München: Faculty of Biology
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Abstract

Phytoplankton growth is controlled by the balance between reproduction and mortality. Phytoplankton reproduction is determined by environmental factors (such as temperature and pH) and by essential resources (such as light and nutrients). In my thesis, I investigated the importance of the essential resources light and nutrients for phytoplankton dynamics in laboratory and field experiments. Research questions involved topics such as: the resource use efficiency of phytoplankton communities, the role of resources for phytoplankton stoichiometry, aspects of phytoplankton food quality and grazing by zooplankton, costs of behavioural strategies of mobile phytoplankton species and the establishment of new methods to quantify growth and loss processes of phytoplankton in situ. EFFECTS OF DIVERSITY ON PHYTOPLANKTON RESOURCE UPTAKE AND GROWTH The resource use efficiency of terrestrial plant communities has been related to taxonomic diversity and a recent metaanalysis of freshwater and brackish phytoplankton communities shows that this relationship also exists in phytoplankton communities. Our experiments with natural and assembled phytoplankton communities showed a clear effect of phytoplankton biodiversity on carbon incorporation. Phytoplankton functional groups differ in their resource use attributes and exhibit different constituents of photosynthetic active pigments. We have shown that the diversity of wavelength specific photosynthetically active pigments was a function of the taxonomic diversity of the phytoplankton communities. The effect of biodiversity on carbon incorporation was related to the functional (biochemical) diversity of phytoplankton communities (Paper 1). Increasing biodiversity and thereby increasing pigment diversity resulted in a higher absorbance of light within the photosynthetic active radiation spectrum and thereby higher carbon assimilation. EFFECTS OF DIVERSITY ON PHYTOPLANKTON RESOURCE UPTAKE AND BIOMASS COMPOSITION (STOICHIOMETRY) Phytoplankton carbon assimilation and nutrient uptake are not tightly coupled. As a result of fluctuating resources, autotrophs can exhibit variable biomass compositions (biomass carbon to nutrient ratios). The increased efficiency of resource use in highly diverse phytoplankton communities (Paper 1) also has consequences for the biomass composition of those communities (Paper 2). Increasing biodiversity resulted in increasing carbon assimilation, but not in a comparable increase of phosphorus uptake. This resulted in increasing biomass carbon to phosphorous ratios. Phytoplankton with high biomass carbon to phosphorus ratios are considered to be low quality food for cladoceran zooplankton such as Daphnia. Although the stoichiometry of Daphnia varies somewhat with algae and diet, they maintain a relatively homeostatic composition with low carbon to nutrient (phosphorus) biomass composition compared to their food. Phytoplankton biodiversity could therefore also have consequences for freshwater phytoplankton-zooplankton interactions. The mismatch in the biomass composition between phytoplankton and Daphnia could lead to changed trophic transfer efficiencies between phytoplankton and zooplankton and hence affect the entire pelagic food web. THE SUPPLY OF LIGHT AND NUTRIENTS AND ITS CONSEQUENCES FOR PHYTOPLANKTON-ZOOPLANKTON INTERACTIONS Both, low and high light to nutrient (phosphorus) ratios in the environment can restrict herbivore growth rates by either the quantity (photosynthetically fixed carbon) of phytoplankton at low light to nutrient ratios or the nutritional quality (biomass carbon to phosphorus ratios) of phytoplankton at high light to nutrient ratios. This can result in an unimodal relationship between light intensity and zooplankton growth. In mesocosm experiments with natural phytoplankton communities from different lakes, we established gradients of light to nutrient ratios by manipulating the light availability for phytoplankton. After two weeks we added the herbivorous zooplankter Daphnia magna to the mesocosms. Indeed, in treatments from phosphorus limited oligotrophic and mesotrophic lakes we found unimodal relationships between light intensity and Daphnia growth rates (Paper 3). At low light levels Daphnia growth rates were limited by food quantity and at high light levels they were limited by food quality. Light dependent variations of natural phytoplankton biomass carbon to phosphorus ratios can effect zooplankton growth. COSTS OF BEHAVIOURAL STRATEGIES FOR PHYTOPLANKTON RESOURCES UPTAKE In pelagic environments, light and nutrients are not equally distributed within the water column and show vertical gradients of availability. While light intensity is higher in upper water layers, nutrient concentrations are, during periods of stratification, generally higher in deeper water layers. A possibility for phytoplankton species to optimize resource uptake is mobility. Mobile species can (at least to a certain degree) migrate within the water column to choose an optimal position for nutrient uptake and photosynthesis. Mobility involves costs in terms of energy to develop, maintain and operate mobility structures. We conducted laboratory growth experiments with mobile and non-mobile green algal species along a gradient of light availability (Paper 4). Phytoplankton biomass (determined as particulate organic carbon) and biomass carbon to phosphorus ratios of non-mobile species were higher than those of mobile species. This indicates that the efficiency of resource use of mobile species was worse than that of non-mobile species. Mobile species had higher energy requirements to balance the costs of basic metabolism. Thus, the advantages of mobility are restricted to specific environmental conditions. NEW METHODS TO ESTIMATE GROWTH AND MORTALITY OF PHYTOPLANKTON COMMUNITIES It is difficult to measure phytoplankton growth and mortality (grazing by micro- and mesozooplankton) in situ in natural phytoplankton communities. However, these are important parameters to understand the dynamics of natural phytoplankton communities. We established a new method to estimate phytoplankton growth and mortality by combining existing dilution (to measure mortality) and dialysis (to measure growth) techniques (Paper 5). Experiments showed that the combination of these methods can be successfully used to quantify phytoplankton gross growth rates and micro- and mesozooplankton grazing in situ.