Hypoxic to anoxic and even sulfidic conditions have already been reported for various aquatic systems, from lakes, estuaries and coastal areas to off-shore regions of the ocean, where oxygen supply does not compensate for its consumption in various redox processes, including respiration of organic matter. In the coastal ocean, oxygen minimum zones (OMZs) have spread exponentially since the 1960s with potential catastrophic biological losses for the benthic communities living on the bottom. In the open ocean, eastern boundary upwelling systems (EBUSs) are characterized by high primary and export production that, in combination with weak ventilation, cause oxygen depletion and the development of OMZs in sub-surface waters.

The purpose of the 46th International Liege Colloquium organized in May 2014 on "Low Oxygen Environments in marine, fresh and estuarine waters" was to generate an overview of the current knowledge on deoxygenation, its monitoring, modelling, impact on ecosystems and biogeochemical cycles as well as the potential evolution of low oxygen conditions in a changing climate (http://modb.oce.ulg.ac.be/?page=colloquium&year=2014). Thematic sessions have been organized on the following:

• 1. deoxygenation, marine resources, ecosystem functioning and structure of the food web;
• 2. deoxygenation and biogeochemical cycles;
• 3. life and processes in redox gradients;
• 4. palaeoproxies of hypoxia;
• 5. modelling hypoxia;
• 6. oxygen time series and instrumental developments;
• 7. eastern boundary upwelling systems (EBUS) as natural SOLAS laboratories;
• 8. deoxygenation in a global change context.