Freshwater and marine ecosystems interact in coastal ecosystems where the two environments intermingle in mangrove swamps, estuaries and bays among other environments. The mingling of freshwater and saltwater give rise to unique ecological spheres and biodiversity that cannot survive in any conditions but those provided by coastal ecosystems.
Mangrove swamps occur where saltwater and freshwater mix, enabling the growth of towering mangrove trees which then form waterways and pools due to the growth of their root systems. These unique swamp-like environments are home to crustaceans, fish and many other types of animal which thrive in the interstice between ecosystems.
Some animals can move freely between freshwater and saltwater. These animals include the saltwater crocodile and several species of fish. Adapted thusly, they can cross in and out of coastal ecosystems without having to worry about damage to their organs because of an inability to process one of the given environments.
Coastal ecosystems are very sensitive to human activity and because of their limited scale and high rates of specialized diversity, they can be damaged grievously by pollution and by boating. Many coastal ecosystems are at risk due to shipping and other human activity in or around their borders, making these ecosystems some of the most fragile in the world.