Anoxic age as a new tool to predict biogeochemical consequences of oxygen depletion in lakes

Lake deoxygenation is of growing concern because it threatens ecosystem services delivery. Complete deoxygenation, anoxia, is projected to prolong and expand in lakes, promoting the production or release of nutrients, greenhouse gases and metals from water column and the sediments. Accumulation of these compounds cannot be easily predicted thus hindering our capacity to forecast the ecological consequences of global changes on aquatic ecosystems. Here, we used lakes Arendsee and Mendota monitoring data to develop a novel metric, anoxic age, characterizing lake hypolimnetic anoxia. Anoxic age explained, as a single predictor, 44% to 58% of the variation for ammonium, soluble reactive phosphorus and a dissolved organic matter fluorophore. Anoxic age could be modelled using only two oxygen profiles and lake bathymetry, making it an easily applicable tool to interpret and extrapolate biogeochemical data. This novel metric thus has the potential to transform widely available oxygen profiles into an ecologically meaningful variable. Scientific Significance StatementOxygen depletion in deep water layers of lakes is of growing concern as it expands due to eutrophication and climate change. Anoxia is deleterious to benthic invertebrates and fishes, enables the production of potent greenhouse gases and releases stored phosphorus from sediments, among others. However, quantitatively forecasting the consequences of anoxia remains a challenge. Here, we developed a novel metric, anoxic age, which may be derived from oxygen profiles to predict end-of-summer concentration of various water chemical parameters. We argue that all by-products of anaerobic microbial metabolism should be related to anoxic age as they are released or processed continuously during anoxia. We believe that anoxic age can be used to predict the ecological consequences of temporally and spatially growing anoxia.