Freshwater and marine ecosystems are both vital components of the Earth's hydrosphere, but they differ primarily in their salinity levels. Freshwater ecosystems, including lakes, rivers, and ponds, have a salt concentration of less than 1%. In contrast, marine ecosystems, which encompass the oceans and seas, have a salt concentration typically around 3.5%. This difference in salinity affects the types of organisms that can thrive in each environment and the physical properties of the water.