A review of around 100 scientific publications concludes that eutrophication significantly increases greenhouse gas emissions from freshwaters (CO2, methane, N2O). An increase of 5 µg/l of chlorophyll-a in lakes and reservoirs worldwide would result in an increase of GHG emissions equivalent to >6% of fossil fuel CO2.
The current GHG emissions from freshwaters worldwide are estimated to be equivalent to >30% of global fossil fuel CO2 emissions (56% from freshwater CO2 release, 40% from methane, 4% from N2O).
Eutrophic shallow lakes are estimated to emit nearly 50% more methane than comparable non-eutrophic lakes. Eutrophication increases organic matter production in fresh waters, but it is unclear whether the resulting net CO2 uptake will compensate for increased methane production, because the organic matter produced is readily degradable. Increased nitrogen loading to surface waters can cause them to shift from being N2O sinks to net N2O emitters. Eutrophication also increases freshwater GHG emissions indirectly, for example, by shifting from vegetation dominated by macrophytes to algae, whereas macrophyte roots tend to reduce methane production by moving oxygen to sediments. Also, cyanobacteria readily produce methane even in the oxic water zone, both at day and at night.
The review also shows that climate change is expected to significantly increase freshwater GHG emissions and eutrophication (see also ESPP SCOPE Newsletter n°137 on climate change and eutrophication), with positive feedback loops. Increasing temperatures will increase release of nutrients from sediments (accelerated mineralisation), as will extreme climate events (remobilisation of sediments). Both will also lead to increased nutrient losses from land to freshwaters. Increased temperatures may also favour methane production in freshwaters, rather than methane consumption.
This review confirms that policy makers need to further reduce nutrient inputs to surface waters, both because climate change will increase eutrophication risks, and because freshwater eutrophication contributes significantly to greenhouse gas emissions.
“The role of freshwater eutrophication in greenhouse gas emissions: A review”, Y. Li et al., Science of the Total Environment 768 (2021) 144582 https://doi.org/10.1016/j.scitotenv.2020.144582