Natural Lakes Are a Minor Global Source of N₂O to the Atmosphere

Abstract

Natural lakes and reservoirs are important yet not well‐constrained sources of greenhouse gasses to the atmosphere. In particular for N2O emissions, a huge variability is observed in the few, observation‐driven flux estimates that have been published so far. Recently, a process‐based, spatially explicit model has been used to estimate global N2O emissions from more than 6,000 reservoirs based on nitrogen (N) and phosphorous inflows and water residence time. Here we extend the model to a data set of 1.4 million standing water bodies comprising natural lakes and reservoirs. For validation, we normalized the simulated N2O emissions by the surface area of each water body and compared them against regional averages of N2O emission rates taken from the literature or estimated based on observed N2O concentrations. We estimate that natural lakes and reservoirs together emit 4.5 ± 2.9 Gmol N2O‐N year−1 globally. Our global‐scale estimate falls in the far lower end of existing, observation‐driven estimates. Natural lakes contribute only about half of this flux, although they contribute 91% of the total surface area of standing water bodies. Hence, the mean N2O emission rates per surface area are substantially lower for natural lakes than for reservoirs with 0.8 ± 0.5 versus 9.6 ± 6.0 mmol N·m−2·year−1, respectively. This finding can be explained by on average lower external N inputs to natural lakes. We conclude that upscaling‐based estimates, which do not distinguish natural lakes from reservoirs, are prone to important biases.

River dam impacts on biogeochemical cycling

“…which has 298 times the global warming potential of CO 2 . Global reservoir N 2 O emissions are between 20 and 71.5 Gg N year −1 (refs 23,27 ), with higher areal N 2 O emissions rates (0.94-1.6 g N m −2 year −1 ) than lakes 35 , rivers and estuaries (a combined 0.01-0.15 g N m −2 year −1 ), by more than an order of magnitude 27 Young reservoirs are typically characterized by large greenhouse gas (GHG) emissions due to the breakdown of flooded soil and biomass, and tend be dominated by respiration rather than photosynthesis. Nutrients accumulate as the reservoir ages, driving increased photosynthesis and rising autotrophy , which can develop into algal blooms in middle-aged reservoirs.…”

“…the global lake plus reservoir surface area 35 . These high emissions are due in part to the disproportionately high TN load that flows along dammed rivers relative to the load delivered to natural lakes, many of which are located above 50° latitude bands (44%) and tend to be nutrient poor 35 .…”

“…These high emissions are due in part to the disproportionately high TN load that flows along dammed rivers relative to the load delivered to natural lakes, many of which are located above 50° latitude bands (44%) and tend to be nutrient poor 35 . Furthermore, reservoirs have an average upstream watershed area of >12,000 km 2 compared with an average of only 617 km 2 for lakes 135 , enabling the accumulation of larger nutrient loads in the rivers that feed into reservoirs 35 . Relating HRT to denitrification, nitrification and N 2 O emissions is not always straightforward.…”

“…At long enough HRTs, N 2 O produced via denitrification is eventually reduced to N 2 (and not emitted as N 2 O), and in reservoirs with HRTs longer than 6-7 months, more reservoir N 2 O emissions are produced via nitrification than by denitrification 23 . Furthermore, there is a strong inverse relationship between the area-normalized N 2 O emissions rate and the HRT 35 , suggesting that reservoirs with short residence times emit more N 2 O per unit area than reservoirs with long residence times. Thus, while many of the ecological impacts related to nutrient elimination could be minimized in small reservoirs with low HRTs, N 2 O emissions can be higher than in large reservoirs, which are often conventionally considered environmentally problematic.…”

“…Our evaluations are based on the hydraulic residence time (HRT, defined as the volume of water divided by the flow through the water body), as it is typically considered the 'master variable' governing the relative rates of transport versus biogeochemical reactions [30][31][32][33][34] . The sizes of nutrient loads delivered from upstream are also considered, as they strongly influence nutrient-elimination fluxes 31,35 . Finally, we discuss the use of these parameters as simple approaches to enable improved management of biogeochemical processes in dammed river systems.…”