Dörner, Isabel (2016) Does ocean acidification influence biomass, community composition and nutritional quality of phytoplankton? A mesocosm approach. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 46 pp.

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Abstract

Next to global warming, ocean acidification (OA) is a consequence of increased anthropogenic CO2 emissions in the last decades. The ocean takes up a lot of the CO2 emitted to the atmosphere and in consequence the water turns more acidic, decreasing the pH. This phenomenon is a particular thread to the ocean and its ecosystems. Up to now, the pH already dropped about ~0.1 units compared to pre-industrial values. To investigate the effects of OA on a natural plankton community in coastal waters off Norway, a mesocosm approach was used. Eight enclosures were deployed in the Raunefjord, near Bergen and two treatment levels were applied (ambient and high CO2 partial pressure (pCO2)). The experiment ran for two month in late spring of 2015. My Master thesis focused on the development of the microphytoplankton community. Former studies already reported effects of OA on phytoplankton. In those experiments, calcareous species were mostly negatively affected, whereas other species did show a positive effect, or were not affected by the simulated OA. The potential effect of OA on primary producers is of acute interest, since they could translate through the food web and consequently affect higher trophic levels such as copepods, pteropods and fish larvae. To investigate whether the phytoplankton community was influenced by the OA treatment, I followed the biomass development and the fatty acid (FA) content. In my study, I found that phytoplankton biomass decreased in the high pCO2 treatments whereas it increased in the ambient ones. In both treatments, the community was dominated by Ceratium longipes. The mixotrophy of this species combined with low nutrient concentrations most likely caused the difference in treatments through a difference in the abundance of their diet, ciliates. In a size comparison of C. longipes between treatments, individuals in the high pCO2 treatment were significantly larger. Fatty acid analysis revealed a decreased polyunsaturated fatty acid (PUFA) to saturated fatty acids (SFA) ratio at elevated pCO2. Especially docosahexaenoic acid (DHA, C 22:6n3c), important for development and reproduction of copepods and other higher trophic levels, was low in the high pCO2 treatment compared to the ambient treatment. Whereas by the quality or quantity of their food, higher trophic levels experienced worse conditions in a community exposed to an increased pCO2, with potentially severe consequences for commercially important fish species such as herring.

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