What distinguishes marine ecosystems from freshwater ecosystems?

The distinguishing feature of marine ecosystems in comparison to freshwater ecosystems is the presence of saltwater. Marine environments, such as oceans, seas, and estuaries, have a significant concentration of dissolved salts, primarily sodium chloride. This salinity impacts the types of organisms that can thrive within these ecosystems, as many species are specifically adapted to deal with saline conditions. In contrast, freshwater ecosystems, which include rivers, lakes, and ponds, have a much lower concentration of salt.

The salinity affects various aspects of the ecosystem, including buoyancy, osmoregulation in organisms, and the overall biodiversity. While other factors like temperature, current strength, and depth can vary widely within both marine and freshwater environments, it is the salinity that fundamentally differentiates these two types of ecosystems.