How can you explain estuary as an ecosystem?

Question

ecosystem

Answer 1

because it provides an environment for organisms to live, may it be fish, algae, birds, frogs all at once. A food chain may exist among them too fish eat algae, burd eats fish, algae graw from sun. etc energy transfer

Answer 2

estuary is formed naturally due to a compulsion that water has to flow that way: so, it becomes part of the eco or natural system! tamper with it, and you are denying that water passage and creating a choke point!