I lived on the water in an estuary for nine years. The flow of food into the water may not make it clean, but it sure as heck supports more life. People are capable of managing even that system on a spot by spot basis as a service to maintain those productive spawning and rearing systems, complete with metrology, telemetry, information systems, validation, and risk management architectures.
THAT is a 21st Century idea. "Preservation" per se, has become a demonstrably backward and destructive model.