In coastal estuaries, where fresh and saltwater mix, they ... 17 Related Question Answers Found. What are the three major categories of aquatic ecosystem? There are three basic types of freshwater ecosystems: ... Section 4.4 Aquatic Ecosystems. A B; An ecosystem in which water either covers the soil or is present at or near the surface of the soil for at least part of the year is called a(n) _____.