What organisms would you use to create a self-sustaining ecosystem

You can create other self-sustaining ecosystems including those in water. Include a producer (i.e., algae), a consumer (e.g., water flea) and a decomposer (e.g., protozoans) and plenty of sunlight.

don't know sorry

Explanation:

parasites can detrimentally affect their host, but also must not allow their host to die before they can complete their life cycle. some parasites have complex life cycles with many different hosts, and have had to find ways of persisting in many different environments during the change from one host to another. ectoparasites live outside the body of the host, and can more easily  disperse, but are  more exposed to predators, parasites, parasitoids. endoparasites live inside the body of the host, and are therefore protected from the outside environment. but they can be  attacked by the host's immune system. they have therefore developed  mechanisms for dispersal such as  complex life cycles.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."