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Lakes and reservoirs provide multiple essential ecosystem services to humans, and several of these require the presence of a cool, oxygen‐rich hypolimnion. These ecosystem services include supporting habitat for recreational fish species and the maintenance of a non‐eutrophic state by limiting internal nutrient loading. However, changes in land use and global warming are modifying the thermal structure of lakes worldwide, creating episodic or prolonged periods of hypoxia, changing a lakes ability to deliver certain ecosystem services. Here, we used National Lake Assessment data and machine learning approaches to identify which lake or watershed features determined the presence of a hypolimnion and which contributed to the development of hypoxia. A random forest of 1000 trees predicted the presence of a hypolimnion with 85% accuracy using commonly measured morphometric and physicochemical variables as well as land use. Mean and maximum depths had a disproportionate influence, entirely obscuring the influence of variables commonly associated with lake mixing, such as light penetration and fetch. In contrast, depth had a more restrained role to predict the presence of hypoxia; the latter was predicted with an overall accuracy of 85%, from epilimnetic nitrogen and carbon concentrations. Although the nutrient‐color groups had a minor influence in the random forests, there was a clear trend of increased hypoxia with higher turbidity. Our approach allows for the broad‐scale assessment of lakes with hypolimnia and suggests that increasing eutrophication and browning will promote profundal hypoxia, altering the delivery of certain ecosystems services. [ABSTRACT FROM AUTHOR]

The current Intergovernmental Panel on Climate Change (IPCC) method for tabulating agricultural nitrous oxide ([N.sub.2]O) emissions suggests that aquatic ecosystems may be important [N.sub.2]O sources. However, estimates are highly uncertain, and the method to estimate emissions is rarely tested. The default IPCC emission factor for groundwater and surface drainage ([EF.sub.5-g]; defined as [N.sub.2]O-N:nitrate-N) has been lowered recently. Our data support further reduction. Notably, we present the first [EF.sub.5-g] data under ice, which reflects groundwater inputs not confounded by gas exchange. These under- ice data suggest an [EF.sub.5-g] of 0.13%, approximately half the current IPCC default value. Our data from the open-water season also suggest a reduction to 0.11% (based on annual means). The data highlight major problems with the IPCC method. [EF.sub.5-g] was extremely variable, with the highest values observed in a stream that was a net sink of [N.sub.2]O. Testing the overall method for freshwater emissions is more problematic because of the multidecade transit time from headwaters in our study region to the Atlantic Ocean and limited information on emissions from lakes. La methode couramment utilisee par le Groupe d'experts intergouvernemental sur revolution du climat (GIEC-IPCC, Intergovernmental Panel on Climate Change) pour comptabiliser les emissions agricoles d'oxyde nitreux ([N.sub.2]O) indique que les ecosystemes aquatiques peuvent representer des sources significatives de [N.sub.2]O. Cependant, ces estimations sont tres incertaines et la methode pour calculer les emissions est rarement evaluee. Le facteur par defaut des emissions de l'IPCC pour les eaux phreatiques et les ecoulements de surface ([EF.sub.5-g] ; defini comme [N.sub.2]O-N:N-nitrate) a ete abaisse recemment. Nos donnees appuient une reduction additionnelle. En particulier, nous presentons les premieres donnees [EF.sub.5-g] sous la glace qui refletent les apports des eaux souterraines, sans l'effet confondant des echanges gazeux. Ces donnees sous la glace suggerent un [EF.sub.5-g] de 0,13 %, a peu pres la moitie de la valeur par defaut actuelle de l'IPCC. Nos donnees recoltees durant la saison d'eau libre suggerent aussi une reduction a 0,11 % (d'apres les moyennes annuelles). Ces donnees soulignent des problemes significatifs avec la methode de l'IPCC. Les valeurs d'[EF.sub.5-g] sont extremement variables et les valeurs les plus elevees ont ete observees dans un cours d' eau qui represente un puits net de [N.sub.2]O. La verification de la methode globale pour les emissions en eau douce pose des problemes supplementaires a cause du temps de transition de l'ordre de plusieurs decennies depuis les eaux d'amont dans notre region d'etude jusqu'al'Atlantique et du peu d'information disponible sur les emissions par les lacs. [Traduit par la Redaction], Introduction Nitrous oxide ([N.sub.2]O) is a potent greenhouse gas, with 298 times the warming potential of carbon dioxide ([GWP.sub.100]; IPCC 2007, p. 212). It has contributed approximately 6% to increases [...]