Answer:
d. salinity
Step-by-step explanation:
The aquatic ecosystems are simply divided into two branches, marine ecosystems and freshwater ecosystems. This division is based upon the salinity of the water where certain ecosystem is found. If the water is classified as salty water, then the ecosystem will be a marine one, if the water is classified as freshwater, then the ecosystem will be a freshwater one. The marine ecosystems are found in the oceans, seas, salty marshes, and salty lakes, while the freshwater ecosystems are found in the streams, rivers, freshwater lakes, marshes, ponds, swamps.