Global change effects on zooplankton body size: a range of experimental approaches

It is a major challenge to understand the impacts of recent climate change on zooplankton communities. The impacts of global warming are manifold and multiple factors, which drive ecological changes in zooplankton communities have to be understood. Increasing sea surface temperature is likely to alter zooplankton phenology and community structure. Recent studies on the global scale showed a decline in size and productivity of zooplankton, which was related to climate change. Reorganization of zooplankton community with warming can change community interactions and energy flow through whole aquatic food webs. The aim of this thesis was to examine the effects of temperature, phosphorus limitation and acidification on copepods and disentangle direct and indirect effects of warming on zooplankton and how the observed changes can alter the metabolic fluxes in food webs. In the first chapter, I tested the effect of three temperatures on copepod communities in a mesocosm experiment. The second chapter presents results of a monoculture experiment with the copepod species Acartia tonsa, where phosphorus concentration in food algae and temperature effects were combined. In my third experiment, a second mesocosm study, I show effects of the combined factors temperature and ocean acidification, to understand single and interactive effects on the copepod community. With respect to trophic chain length, I demonstrate in chapter 4, that total and mass-specific ecosystem primary production and respiration are differently temperature sensitive, and that bloom dynamics and non- bloom dynamics act differently on ecosystem oxygen fluxes. In the first chapter of this thesis, I describe the results of a performed mesocosm experiment, which allowed me to identify copepod responses to temperature. Body size of adult copepods and of all Acartia sp. developmental stages was smaller at higher temperatures. Total zooplankton, nauplii and adult copepod abundance was lower at higher temperatures. Additionally, a stage shift from older, at lower temperatures, to younger developmental stages occurred at higher temperatures. My experimental work, presented in chapter 2, focused on the identification of drivers that lead to smaller body sizes. A monoculture experiment was performed with the copepod species Acartia tonsa grown under five temperature steps (10 – 20 °C) and were fed with Rhodomonas salina, which had three different carbon-to-phosphorus ratios (phosphorus replete, limited, and pulsed). An increase in temperature significantly reduced the individual body size of the herbivore consumer, whereas phosphorus limitation had no influence. Phosphorus limitation counteracts the temperature effect by decreasing developmental rates. I concluded that phosphorus limitation and increasing temperature might accelerate growth rates because temperature has a stronger effect on Acartia tonsa than phosphorus limitation. In chapter 3, I show that warming has a stronger impact on copepod body size, abundance, biomass, and fatty acid composition, whereas acidification did not show a significant effect on copepods. I could identify trends of acidification effects on copepods, with the result that copepods are more positively affected by the fertilizing effect on phytoplankton biomass, as a proxy for food biomass. It seems that copepods were able to partially compensate the negative temperature effects by higher food uptake. Fatty acid composition was significantly affected by warming. Total fatty acid amount did not change with temperature or acidification, but ratios of single essentially polyunsaturated fatty acids to total fatty acid content changed significantly. I concluded that acidification has the potential to dampen temperature effects on copepod body size, abundance, and biomass by a higher availability of food sources. Copepod populations might be more affected by warming than by ocean acidification alone. My experimental work, presented in chapter 4, focused on a multi-generational mesocosm study assessing how warming affects ecosystem metabolic rates of net primary production and respiration. A decrease of phytoplankton biomass, zooplankton body size and abundance increase the temperature driven increase of metabolic rates. The results indicate that future warmer aquatic ecosystems are more affected at higher trophic levels, because of changing food web structure to lower phytoplankton biomass, zooplankton abundance with smaller sized individuals. I suggest that biomass changes have to be included into ecosystem flux analyses. To summarize the results of my experimental studies, I developed a schematic of temperature impacts on biotic interactions in aquatic plankton communities. In this schematic temperature can directly act on copepod metabolic rates, which directly lead to smaller sized individuals and lower abundances, and consequently lower biomass of copepods. Higher metabolic rates lead to higher energy demands and increase the grazing rates on phytoplankton. The increased grazing rates indirectly affect phytoplankton communities. My work highlights the importance of complex community studies including the interactions with different trophic levels for understanding ecological processes in aquatic ecosystems and their responses to predicted global change scenarios. Testing multiple factors and also further effects on the organism’s physiological level, which might result in changes of whole food web efficiency, might achieve the elucidation of the complexity of responses.