Which of these factors distinguishes the two main types of aquatic habitats?

The two main types of aquatic ecosystems are the freshwater type (rivers, lakes, ponds etc) and marine type (seas, oceans).

The main difference between these two types of aquatic habitats is the concentration of salts in the water, called salinity. The most abundant salt in seawater is NaCl, and its ions Na + and Cl-, but there are also carbonate and sulfate salts present.