Eggers, Sarah Lena (2013) Effects of community composition and global change on the functioning of experimental marine phytoplankton communities. (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 80 pp.

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Abstract

Humans are altering the composition of biological communities through a variety of activities at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a streng potential to alter ecosystem properties and the goods and services they provide to humanity. Since the industrial revolution, atmospheric carbon dioxide (CO2) increased from 280 to 380 μatm and is expected to further increase to 700 μatm by the year 2100. Ocean acidification is the consequence of increasing atmospheric CO2, which dissolves in seawater and subsequently increases seawater acidity and decreases carbonate ion concentration. Changes in carbonate chemistry can act both as fertilizer in case CO2 is a limiting resource and as stressor, particularly for calcifying organisms. Ocean acidification represents a pervasive environmental change that is predicted to affect a wide range of species, yet our understanding of the emergent ecosystem impacts is very limited. Two most challenging questions largely remain uncertain. Firstly, how much of the expected change in community functioning due to elevated CO2 is owing to either changes in the physiology of individual species or in the relative abundance of species or is there a hint towards evolutionary adaptation? Secondly, how da effects of community composition on ecosystem functioning compare to direct effects of ocean acidification? In chapter 1, I tested whether varying initial dominance scenarios lead to different competitive outcomes and subsequently translate into altered community functioning. I used experimental communities consisting of four naturally co-occuring coccolithophore species and manipulated initial community structure by creating five different dominance scenarios: (1) all species contributing evenly to initial biomass, and (2-5) one of each species contributing 4x that of the remaining three species to total initial biomass. I was able to show that priority effects in the communities caused the initially dominant species to remain dominant during the stationary phase in three out of four cases. However, despite varying carrying capacities when species were grown in monocultures and different dominant species, community functioning was unaffected. I suggest that selective and facilitative effects were responsible for the equalization of community functioning. In chapter II, I used three of the four coccolithophore species used in chapter I and explored the effect of initial community composition in combination with ocean acidification on community biomass. In particular, I tackled the question of how much of the expected change in community functioning due to elevated CO2 is owing to either direct changes in the physiology of species or indirect ecological changes in the relative abundance of species. In order to complete the picture, I additionally indirectly tested for evolutionary adaptation to elevated CO2. Contrary to my expectation I found neither a significant physiological effect nor an ecological effect of elevated CO2 on biomass at bloom peak. 1 concluded that the lacking effect on ecosystem functioning in this particular model system in response to elevated CO2 was likely caused by community reorganization due to evolutionary adaptation. In chapter I and II, community functioning at bloom peak was affected neither by initial community composition nor ocean acidification. The communities in both studies however, consisted only of coccolithophores. In order to overcome this limitation, in chapter III, I used communities harboring a variety of functional groups and tested the hypothesis that initial community composition and elevated C02 are equally important to the regulation of phytoplankton biomass. 1 was able to show that initial community composition had a significantly greater impact than elevated CO2 on phytoplankton biomass, which varied largely among communities. Furthermore, I showed that depending on initial community composition, elevated CO2 selected for larger sized diatoms, which led to increased total phytoplankton biomass. Overall, the results suggest that when looking at more than one functional group, initial community composition can have a much greater effect on biomass than elevated CO2. Consequently, the importance of ocean acidification hitherto appears to be overestimated whereas the effect of community composition has been largely overlooked, although it is among the dominant drivers of changes in ecosystem functioning. Because phytoplankton functioning depends on trait composition, it remains a major challenge to understand how phytoplankton communities will reorganize in response to climate change in order to predict the impact on future oceans' ecosystems. lnherently, using independent natural communities, instead of directly manipulating biodiversity, limits the possibility for mechanistic explanation. For future research I suggest to overcome this problem by using one known source-community in which biodiversity (i.e. the loss or distribution of given traits) is manipulated in a non-random approach.

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