But a functional ecosystem keeps the water clean. You can have either a complete system with complex food chains or chlorine.

But after what time is it an ecosystem that has to be protected? We have here the “Lüneburger Heide” with a lot of protected species and take great care to keep it a heathen. It is an artificial landscape and was woodland before all the trees were chopped off to cook salt in Lüneburg. It was kept clear by sheep. Shepherding on an open plain is now unprofitable and if we would stop now to pay the shepherds a complete ecosystem would vanish and be overgrown by bushes and trees. That would hurt biodiversity and tourism. But it would be the natural environment for northern Germany.

Any body of water is an aquatic ecosystem. You’ll have algae after some days and fish after a year or two.