What are the differences between saltwater and freshwater ecosystems?

Freshwater ecosystems are the ecosystems in freshwater bodies, including rivers, lakes, ponds, streams, etc. In comparison, saltwater ecosystems are the marine ecosystems or ecosystems in oceans and seas. There are a number of differences between these ecosystems:
1) Salt content: The most significant difference between the freshwater and the saltwater ecosystems is the salt content difference between the two. The freshwater ecosystem has less than 0.05% salt content; saltwater ecosystems, on the other hand, are characterized by about 3.5% salt content. The life in these two ecosystems has adapted to these different salt contents.
2) Physiology of organisms: The organisms that live in freshwater ecosystems have adapted to lower salt content, as compared to the saltwater organisms. Freshwater organisms exist in a hypotonic environment since the salt content inside the organisms is more as compared to their surroundings. Hence these organisms have to overcome the osmotic pressure caused by the salt content difference between the inside and outside of their bodies. In comparison, the saltwater environment is isotonic and the saltwater organisms have to exert very little effort to overcome the osmotic pressure. Hence the organisms need to have a different physiology to successfully survive in these two environments.
3) Temperature: The ocean and sea are relatively large bodies and have a uniform temperature throughout the year. In comparison, freshwater bodies may have more variations in the temperature through the year. Depending on the size, some freshwater bodies may have low water content during the summer months and hence the survival of organisms may become tough.
4) Species variability: Freshwater organisms can be classified as either tropical or cold water; in comparison, the saltwater organisms are only coldwater organisms.
These are some of the key differences between the two ecosystems.
Hope this helps.