Elements of carbon cycling: primary producers in aquatic systems under global change

Abstract

It is of crucial importance to understand the consequences of climatic changes (e.g. elevated pCO2, warming and eutrophication) on aquatic primary producers, as they play a vital role in the global carbon cycle. Therefore, the main aims of the thesis were (1) to assess the role of primary producer identity (i.e. phytoplankton vs submerged aquatic plants) in aquatic carbon cycling, and (2) to assess the effects of elevated pCO2, warming and eutrophication on aquatic carbon cycling (with the focus on primary producers). The results of this thesis show that global change can significantly alter the role of primary producers in the carbon cycle, but that the direction of this change may depend on the prominent global change stressor. Producer carbon:nutrient stoichiometry can be enhanced by elevated pCO2, but reduced by eutrophication. The effect of warming on carbon:nutrient stoichiometry is context-dependent and may be influenced by abiotic conditions such as nutrient availability. Systems dominated by aquatic plants can have significantly higher sedimentation fluxes of carbon than those dominated by phytoplankton. This difference can be attributed to standing stocks of producer biomass related to producer identity. Additionally, the results of this thesis show that consumer biomass can advance in response to climatic warming, thereby imposing top-down control on primary producers. Thus, processes hampering the build-up of producer biomass may alter the flux of carbon to the sediment. To integratively assess the effect of warming on freshwater carbon cycling, producer biomass, sedimentation and decomposition were measured in a mesocosm experiment. The results of this experiment show that warming can significantly enhance the standing stock of plant biomass, sedimentation and decomposition. When combined into a carbon budget model, these similarly enhanced carbon fluxes cancelled each other out resulting in no net change in carbon burial between temperature treatments. All in all, I conclude that global change can affect the balance of aquatic carbon cycling by altering the biomass and carbon:nutrient stoichiometry of aquatic primary producers as well as their interactions with higher trophic levels. These changes in turn can have potential consequences for our future climate.