How does oxygen naturally enter an aquatic ecosystem?
Remembering that water is one part Oxygen and two parts Hydrogen, there is an abundance of Oxygen in aquatic ecosystems to begin with. Abundant, but not inexhaustible.
Oceans, lakes and rivers are full of plant life, with algae, reeds and other plants using normal processes of photosynthesis to grown, absorbing Carbon Dioxide in the process and giving off Oxygen into the ecosystem.
In addition, gases exchange on the surface of the water, similar to how, by swirling a glass of wine, it oxygenates. This is a continual process of Oxygen exchange.
One of the major environmental problems we have is that there are large swaths of ocean that are saturated with CO2, and unable to sustain plant life. This has led to large "dead zones" in the ocean, and contributed to the increase in greenhouse gases since C02 can no longer be absorbed.