Some members of this protozoan group cause red tide

Red tide is a common name for algal blooms, which are large concentrations of aquatic microorganisms, such as protozoans and unicellular algae (e.g. dinoflagellates and diatoms). The upwelling of nutrients from the sea floor, often following massive storms, provides for the algae and triggers bloom events.

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