Tonya DelSontro, Ph.D.

This project was funded by the European Commission as a Marie Skłodowska-Curie Action Individual Fellowship. I worked at the Department F.-A. Forel for Environmental and Aquatic Sciences at the University of Geneva, Switzerland with the Aquatic Physics group led by Dr. Daniel F. McGinnis.

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Human activities like agricultural and urban runoff have drastically increased nutrient loading to freshwaters, namely of phosphorus (P) and nitrogen (N). Such nutrient loading influences the trophic state of a system and can result in eutrophication, which degrades water quality and reduces overall ecosystem health. Eutrophication manifests itself by enhancing primary production, often causing toxic blooms, and depleting oxygen (O2) from bottom waters during the decomposition of the newly produced organic matter (OM). Human population pressures are expected to enhance eutrophication globally and the Horizon 2020 program (H2020) highlights the importance of minimizing the impact land use change will have on the environment, particularly on aquatic and marine resources. Also, the impact eutrophication has on OM and O2 will influence aquatic GHG emissions as they are key biogeochemical variables related to GHG production. Currently, little is known about the relationship between trophic state and GHG emissions, which hampers our ability to predict how aquatic GHG balances will be impacted by future environmental changes or how to mitigate them. Therefore, the overall goals of this project were 1) to quantify how the aquatic GHG emission balance varies with trophic state and 2) to develop a model describing the main drivers of this variability to aid in predicting the response of aquatic systems to environmental change.

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Seven lakes were surveyed in Switzerland three times over summer: Soppen, Hallwil and Baldegg in the lowlands and Lioson, Chavonnes, Bretaye and Noir in the Alps (Fig.1). Complete budgets for CH4 and CO2 were measured, including sediment production, water column accumulation, and atmospheric flux. Fluxes for N2O were also measured. Various physical, chemical and biological variables were also measured, including light extinction, stability, nutrient and carbon concentrations, and chlorophyll and algal concentrations.

The findings of this project encompass two themes: (1) the balance of GHG emissions according to trophic state; and (2) trophic state interactions with potential drivers of aquatic CH4 dynamics. To the first, we found that indeed the balance of GHG emissions shift with trophic state, such that more eutrophic systems emit more GHGs, particularly CH4. The meso-oligotrophic systems, however, tend to emit more N2O than the eutrophic ones. We found that the highest areal fluxes can come from the smallest lakes, despite their small size and shallow depth, but in terms of total carbon emission that a large mesotrophic lake will dominate over a small eutrophic system. In addition, we found that Alpine lakes, which were thought to be pristine systems, can also be eutrophic and significant carbon emitters. If these mountainous regions are to experience climate change going forward, then they may become eutrophic and negatively feedback on the climate as they also become significant carbon emitters.

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Secondly, this project has provided a more in-depth understanding of the trophic state interactions of potential drivers of aquatic CH4 dynamics in lakes. Our work has shown that eutrophic systems are higher in overall CH4 concentrations and significant CH4 emitters. Instead of finding a relationship between CH4 and chlorophyll and/or P as we expected, we found a negative correlation between N and CH4 variables. This is likely because methanogens that produce CH4 cannot outcompete denitrifiers for organic matter when N is present. Regardless of the mechanism, the relationship between N and CH4 is rather understudied but may be important in aquatic systems. We also found positive correlations between CH4 and some hydrodynamic variables. Essentially, lakes with stronger stratification and less light penetration had higher concentrations of CH4 throughout. While these variables may impact the transport of CH4 throughout the water column or the production of biomass that could eventually lead to CH4 production, they are likely indirect variables because they have little to do with the actual production of CH4. It is more likely that the link between hydrodynamics and CH4 is mediated by trophic state.

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Stay tuned for work published from this multi-lake survey!