Supersaturation: the neglected half of natural oxygen fluctuations

Dissolved oxygen is one of the most pervasive and variable environmental factors in aquatic ecosystems. The synergy of stochastic climatic and ecological events with the cyclic production and consumption driven by aquatic life determines enormous fluctuation of oxygen availability. Despite marine life evolved in such heterogeneous environment and regularly copes with extreme variations of dissolved oxygen, today the primary focus is on the climate novelty of global oxygen depletion exacerbated by human activities. Particularly, the recurrent oxygen enrichment of water above the saturation concentration at equilibrium, the supersaturation, represents a fundamental and at the same time poorly investigated aspect of marine environments. Here we present long time-series data of dissolved oxygen and water temperature logged at high temporal resolution in diverse habitats across tropical and temperate latitudes and we show that acute oxygen fluctuations and the alternating occurrence of supersaturation and hypoxia are a prevalent feature in productive coastal habitats. These observations challenge the prevalent concern regarding the effect that the depletion of total oxygen content may exert on marine life, which is primarily focused on trends of average oxygen concentration. We advocate that robust projections of potential impacts of predicted ocean and coastal deoxygenation require an understanding of the variability and ecological relevance of dissolved oxygen at all relevant temporal and spatial scales, including finer ones. We further support these conclusions with laboratory experiments focusing on the effect of recurrent daily oxygen supersaturation on animal physiology. We demonstrate in 12 species of four different phyla that marine fauna can exploit oxygen supersaturation to extend their lethal thermal limit up to several Celsius degrees, and elucidate the mechanistic physiological background of this unexpectedly high heat tolerance. Similarly, we provide evidence that that the exposure to oxygen supersaturation enhances animal tolerance to both acute and prolonged hypoxia exposure. These findings provide an innovative insight into how oxygen supersaturation allows aquatic ectotherms to endure daily thermal and hypoxic stresses and lead to reframe our understanding of marine fauna vulnerability to ocean warming and oxygen depletion.