Declining oxygen concentrations in the deep waters of lakes worldwide pose a pressing environmental and societal challenge. Existing theory suggests that low deep-water dissolved oxygen (DO) concentrations could trigger a positive feedback through which anoxia (i.e., very low DO) during a given summer begets increasingly severe occurrences of anoxia in following summers. Specifically, anoxic conditions can promote nutrient release from sediments, thereby stimulating phytoplankton growth, and subsequent phytoplankton decomposition can fuel heterotrophic respiration, resulting in increased spatial extent and duration of anoxia. However, while the individual relationships in this feedback are well established, to our knowledge there has not been a systematic analysis within or across lakes that simultaneously demonstrates all of the mechanisms necessary to produce a positive feedback that reinforces anoxia. Here, we compiled data from 656 widespread temperate lakes and reservoirs to analyze the proposed Anoxia Begets Anoxia (ABA) feedback. Lakes in the dataset span a broad range of surface area (1–126,909 ha), maximum depth (6–370 m), and morphometry, with a median time series duration of 30 years at each lake. Using linear mixed models, we found support for each of the positive feedback relationships between anoxia, phosphorus concentrations, chlorophyll-a concentrations, and oxygen demand across the 656-lake dataset. Likewise, we found further support for these relationships by analyzing time series data from individual lakes. Our results indicate that the strength of these feedback relationships may vary with lake-specific characteristics: for example, we found that surface phosphorus concentrations were more positively associated with chlorophyll-a in high-phosphorus lakes, and oxygen demand had a stronger influence on the extent of anoxia in deep lakes. Taken together, these results support the existence of a positive feedback that could magnify the effects of climate change and other anthropogenic pressures driving the development of anoxia in lakes around the world.

Lakes have a controversial climate footprint. In fact, they are a sink of CO2 but at the same time they can be an important source of CH4. Indeed, a global synthesis of methane emission data (Bastviken et al., 2011) suggests that freshwater ecosystems - in particular lakes - may be much larger sources of methane than previously thought, questioning current methane budgets and the general role of freshwater ecosystems in the greenhouse gas balance. The main objective of this study is to improve data availability and quality regarding methane emissions from lakes in the Alpine region - a region that presently is heavily under-represented in global data sets - in order to allow a robust assessment of their role in the global greenhouse gas balance. This is of fundamental importance for the assessment of CH4 emissions from regions particularly sensitive to an increasing climatic variability. Aiming at spatial and temporal representativeness of flux measurements, we made use of an innovative mobile eddy covariance system. We installed the instruments on a small boat, and we performed measurements while cruising. Meteorological and bio-physical data got recorded simultaneously to investigate drivers of gas fluxes by means of empirical modelling. Additionally, we made use of classical chamber measurements for validating our approach. In this fashion, we investigated a number of natural and man-made lakes across a transect of two degrees of latitude across the Alps. In fact, the alpine region provides a unique opportunity to assess the role of environmental drivers on GHG emissions over a limited latitudinal range in an altitude-for-time substitution manner. We repeatedly visited target lakes across the ice-free season during the years 2018 and 2019. We demonstrated that our method is valid for capturing methane emissions from different pathways (diffusion but also ebullition and transport through vegetation). We found that most of the lakes are supersaturated and the highest emissions were measured in shallow and eutrophic lakes at low altitude. In conclusion, with this study we were able to develop new insights on the role of freshwater ecosystems in the global methane budget. References: Bastviken D, Tranvik LJ, Downing JA, Crill PM, Enrich‐Prast A (2011). Freshwater methane emissions offset the continental carbon sink. Science 331, 50.