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TRophic state Interactions with drivers of Aquatic greenhouse Gas Emissions

Periodic Reporting for period 1 - TRIAGE (TRophic state Interactions with drivers of Aquatic greenhouse Gas Emissions)

Reporting period: 2019-03-01 to 2021-02-28

Lakes are known for being oversaturated (in respect to the atmosphere) with two important greenhouse gases (GHG) – carbon dioxide (CO2) and methane (CH4) - and thus emit substantial quantities of both. Lakes can also act as either sources or sinks of nitrous oxide (N2O), a much more potent GHG in terms of atmospheric radiative forcing. Over a 100-yr timescale, CH4 has a global warming potential (GWP) 34 times that of CO2, while N2O has a GWP of 265 times that of CO2. The latest Intergovernmental Panel on Climate Change (IPCC) assessment reports that inland water CO2 emissions are similar to CO2 from land use change and 1/8 the CO2 from fossil fuel use, while aquatic CH4 emissions are close to fossil fuel CH4 emissions. Despite the increasing awareness that lakes are important GHG sources and play an important role in the carbon and climate cycle, aquatic GHG dynamics are still poorly understood.

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.
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. 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 (i.e. productive) systems emit more GHGs, particularly CH4. The meso-oligotrophic (i.e. less productive) systems, however, tend to emit more N2O than the eutrophic ones. 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.
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. 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 physical variables such that lakes with stronger stratification and less light penetration had higher concentrations of CH4 throughout. But the relationships with nutrients was stronger than physical variables, so the CH4 dynamics in these lakes are indeed driven more by trophic state.
I started as tenure track faculty in the Earth and Environmental Sciences Department on 1 July 2020 at the University of Waterloo, Canada and am not yet finished exploiting these results. Two articles are in preparation – one regarding overall carbon emissions with trophic state and the other focused on trophic state drivers of CH4 emissions - and I will present results at future conferences. A third paper is in preparation and led by a PhD student.

As for potential impacts, these are some of the most detailed studies of GHG balances of Alpine lakes that demonstrate their huge carbon-emitting capacity. These results will likely change the pristine perception of all Alpine systems and alter the way land and water are managed in the Alps, particularly in a warming climate. In addition, our findings that N plays a more important role than P in determining CH4 emissions from systems suggests that land and water managers may want to reconsider how they evaluate potential eutrophication in at least Swiss lakes. Finally, since the trophic state of a lake dictates its CH4 state, then perhaps CH4 could be used as a variable to describe trophic state in a system. Trophic state indices used in the literature can present significantly different results within a system and over time, likely due to varying nutrients and biomass over time. The overall CH4 level of a lake is rather consistent across systems, particularly in summer, and could provide a more robust approximation of the trophic state of a system. This change in thinking could have significant implications for current water management policies.