Lakes play an important role in the global carbon cycle by receiving, transforming, storing, emitting, and transporting carbon. The carbon is emitted as carbon dioxide (CO2) and methane (CH4), which are both potent GHGs. Lakes also emit nitrous oxide (N2O), another GHG. Scientists currently do not possess a detailed understanding of GHG dynamics in aquatic systems. This limits their ability to predict how these emissions will be altered in response to global environmental changes, be they climate-related, or land use change pressure-related. One consequence of both climate and land use change for surface waters is eutrophication, whereby water quality is degraded because of excess nutrient loading, namely phosphorus and/or nitrogen.
Trophic state and GHG emissions
The TRIAGE project addressed this knowledge gap by quantifying how the aquatic GHG emission balance varies with trophic state and by developing a model describing the main drivers of this variability to help in predicting the response of aquatic systems to environmental change. Trophic state is the total biomass in the water body and ranges from oligotrophic (least productive) to eutrophic/hyper eutrophic (most productive). This research was undertaken with the support of the Marie Skłodowska-Curie Actions programme and involved surveys of seven Swiss lakes: Baldegg, Hallwil and Soppen in the lowlands and Bretaye, Chavonnes, Lioson and Noir in the Alpine foothills known as the Prealps.
“These seven were chosen because they constitute a climate, size, and trophic gradient. Three lakes are in the lowlands and are relatively large, while the other four are small prealpine lakes about 1 800 metres above sea level. A trophic gradient was present in both sets of lakes,” states MSCA research fellow Tonya DelSontro.
Scientists quantified the total CH4 and CO2, including sediment production, water column accumulation, and atmospheric flux, and fluxes for N2O. They also measured, physical, chemical, and biological variables such as light extinction, water column stability, and concentrations of nutrients, carbon, chlorophyll, and algae. Results indicate that the balance of GHG emissions shift with trophic state, so that more eutrophic systems emit more GHGs, particularly CH4. In addition, prealpine lakes, which were thought to be pristine systems, can also be eutrophic and significant carbon emitters.
Better water management
TRIAGE conducted some of the most detailed studies of GHG balances of prealpine lakes to date, demonstrating their huge carbon-emitting capacity.
“These findings will likely change the perception of all prealpine lakes as pristine systems and alter the way land and water are managed in the Alps, particularly in a warming climate. Moreover, our findings that nitrogen plays a more important role than phosphorus in determining CH4 emissions from systems suggests that land and water managers should reconsider how they evaluate potential eutrophication in Swiss lakes,” notes DelSontro.
The trophic state of a lake dictates its CH4 production and cycling. Hence, CH4 could be used as a variable to describe the trophic state of a system.
“Trophic state indices used in the literature can present significantly different results within a system and over time, most likely due to varying nutrients and biomass over time. The overall CH4 level of a lake is consistent across systems, particularly in summer, and could provide a more robust approximation of the trophic state of a lake. This change in thinking could have significant implications for current water management policies,” DelSontro concludes.