Spisla, Carsten (2021) Marine Zooplankton Community Responses to Anthropogenic Influences. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 162 pp.

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Abstract

Continued anthropogenic carbon dioxide (CO2) emissions lead to a persistent intensification of current levels of ocean acidification (OA) and have already had measurable impacts on life in the world’s oceans. Even if greenhouse gas emissions, including those of CO2, immediately ceased, a deliberate anthropogenic removal of CO2 from the atmosphere via so-called ‘negative emission technologies’ (NETs) is inevitable in order to reach the 2015 Paris Agreement goal of maximum 2°C global warming. Little is known, however, about possible side-effects of particularly ocean based NETs and potential interdependencies with ongoing climate change, which could affect plankton communities, marine ecosystems, and ecosystem services like fisheries. In the present thesis, the response of pelagic ecosystems was investigated under the influence of the environmental stressor OA and one of the ocean-based NETs that is under consideration, artificial upwelling. In this NET nutrient rich deep-water is admixed into nutrient depleted surface waters in order to enhance productivity and sequester carbon via the biological carbon pump. My work focused on the responses of plankton community composition, trophic level interactions, and productivity of the impacted zooplankton community. All of which are regulating factors that are important for the overall production of organic matter in an ecosystem. In a first step to assess these factors, I evaluated in a large scale mesocosm experiment, how the overall function and structure of a natural plankton community is affected by xtreme OA conditions. The experiment revealed pronounced positive and negative treatment effects on the species composition, abundance and biomass of various species within the plankton community in the mesocosms, emerging shortly after the CO2 manipulation. These OA effects were visible in all trophic levels of the planktonic food web as well as the elemental stoichiometry of organic matter. The results imply that a variety of indirect and direct OA effects led to an increase in secondary consumer biomass and enhanced top-down control in the food web, yet with unknown consequences for the productivity of the ecosystem. Intrigued by the clear effects of OA on higher trophic levels (fish larvae and hydrozoans) in the aforementioned mesocosm study, a more detailed analysis was carried out in a separate study. Here, I focused on the drivers and implications of the observed changes on the top predators in the enclosed plankton community. The findings of this study indicate that an observed decrease in Hydrozoa predation pressure and higher fish larvae survival at the top of the food chain were indirectly mediated by OA affecting lower trophic levels (phyto-, micro-, mesoplankton) as well as the predator - prey relationship between fish larvae and hydrozoans. The immediate consequences these indirect OA effects had on the top predators could entail extensive alterations also for ecosystem services. In a final step, I assessed the possible application of artificial upwelling for enhancing productivity and the efficiency of carbon transfer in an oligotrophic plankton community. A large scale mesocosm experiment simulating different upwelling modes and intensities was conducted. The results revealed an increase in primary and secondary production as well as the carbon transfer efficiency in the food web. The corresponding effect size of this increase was closely linked to temporal frequency and intensity of upwelling as well as the species and size composition of the zooplankton/ copepod community and its access to food in different qualities. Altogether, this dissertation revealed two distinct findings on the stressor and application of potential NET of artificial upwelling. Firstly, intensifying OA under a business-as-usual scenario could lead not only to a pronounced restructuring of zooplankton populations, but also entire plankton communities, including substantial changes in the predator-prey coupling of higher trophic levels, e.g. Hydrozoa and fish larvae. On the other hand, the application of artificial upwelling caused distinct changes in the productivity of the affected zooplankton community, which, under certain circumstances, supports the potential of this technique to create efficient food webs with considerable biomass output. In conclusion, these findings make clear that more focus has to be put on ocean-based climate change solutions like artificial upwelling in order to counteract direct effects of future environmental stressors to plankton communities.

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