There are many types of ‘black’ waters around the world: places, wastes, natural waters, and… hypoxic blackwater.
Natural waters are a simple enough thing in Australia at least: water stained to a dark brown by tannins in the barks, woods, and leaves that end up in pools. Best observed in rock pools or sandy pools and generally not harmful (though it can stain your clothes).
Hypoxic blackwater is another thing entirely. It’s a description of waters that lack enough oxygen for the organisms in them to survive, generally due to a massive nutrient load and warm conditions. Microbes are so keen on eating that they’ll consume everything available, and, when there’s more food than oxygen, that means they’ll take up all the oxygen. And then die because they’ve no oxygen and become more food for other microbes. And so the cycle continues until everyone relying on dissolved oxygen has died.
Why are warm conditions important? Two reasons: firstly, microbes grow faster in warm water than cold; secondly, warm water contains less oxygen than cold. Put them together and you have a huge demand for oxygen in a system that can’t hold much.
Obviously, there are plenty of shallow lakes and streams around the world that aren’t on the brink of hypoxia all the time – so what drives the event? Easy answer, and one seen frequently in agricultural areas, is that low water flow rates + high nutrient loads = eutrophication. Eutrophication + hot weather = overly happy microbes. Counterintuitively, summer floods can also cause hypoxic blackwater events by suddenly inundating land that is usually dry with a shallow layer of warm water.
Blackwater events are another example of the delicate balance required of land managers around the world. Organic matter (i.e. litter) is good for soil and provides habitat for little animals, but ‘too much’ organic matter under the right conditions and you might cause a blackwater event. Nutrients are required for agriculture, but nutrient runoff is a stream disaster. Livestock need to drink, but animal manure in streams and pools adds to nutrient loads… It’s a difficult business.