Freshwater zooplankton under elevated pCO2: from individual to community effects

Economic development and human population growth have been accompanied by increasing emissions of greenhouse gasses since the industrial revolution. It has been well documented that high carbon dioxide (CO2) levels will acidify oceans and affect marine life. Although evidence is growing that CO2 levels are also rising in freshwater, the impact of rising CO2 levels and associated acidification on freshwater ecosystems has been largely overlooked. In this thesis we explored the impact of elevated dissolved CO2 in freshwater, indicated by the partial pressure (pCO2), on zooplankton. This group plays a crucial role in food webs of ponds and shallow lakes as primary consumers of algae and prey for higher trophic levels. We studied effects on individual, population and community level using an experimental approach.

Our results show strong direct effects of elevated pCO2 that differed between species. Effects include reduced survival, smaller body sizes and delayed maturation in a water flea (Cladocera: Daphnia magna) and a seed shrimp (Ostracoda: Heterocypris incongruens) and delayed population growth in a rotifer (Rotifera: Brachionus calyciflorus). We identified a reduced tissue calcium content in the water flea suggesting problems with calcification and we showed that these responses are not simply the result effect of acidification. Both elevated pCO2 as well as the combination of pCO2 and climate warming resulted in changes in the zooplankton community composition, with differential sensitivity zooplankton species. Finally, we detected potential adaptation to elevated pCO2 in a related water flea (Simocephalus vetulus). Despite a slower heart rate and reduced body size, this species was able to develop large populations when exposed to elevated pCO2. We also showed that these responses can impact the species composition of a zooplankton community via an eco-evolutionary feedback.

Rising pCO2 and related acidification is a newly identified stressor and potential threat to inhabitants of freshwater ponds and shallow lakes. It has the potential to change ecosystem functioning by reducing efficient control of algal growth and changing available biomass for animals that depend on zooplankton for food such as fish. While some species show higher tolerance or the capacity to acclimate or adapt to these changes, communities will very likely change since other stressors such as warming and pollution might constrain adaptation ability  even in the most tolerant species. The main conclusion of this thesis is that elevated pCO2 and related acidification must not be overlooked and should be considered in predictions of species responses to global change.

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