QUESTIONS (C): 6. What areas of the ocean are the most productive? 7. How does the nature of the substrate affect the kinds of organism found at the shore? 8. What is the role of each of the following organisms in a marine ecosystem: phytoplankton, zooplankton, algae, coral animals, and fish? 9. List three differences between freshwater and marine ecosystems. 10. What is an estuary? Why are estuaries important?