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‘Blackwater’ occurs when there is a very high concentration of Dissolved Organic carbon (DOC) in the water column. Usually this happens when large quantities of terrestrial litter (e.g. leaves) are submerged (similar effect to tea bags in water). If conditions are suitable (e.g. high water temperature), consumption of DOC by bacteria can use up large quantities of oxygen, leading to ‘hypoxic conditions’ – where Dissolved oxygen concentration can drop below 2 mg L-1, sometimes leading to fish deaths.