Aquatic systems are of two types: freshwater and marine. Physical factors important to both include depth of sunlight penetration, water temperature, and the concentration of dissolved nutrients. There are linkages between aquatic systems and terrestrial biomes, such as forest litter enriching a stream or salmon returning from the sea to spawn. In tropical rain forests, floods give fish access to seeds of tropical trees which they help disperse. Seabirds breeding in coastal areas transport nitrogen and phosphorus from marine systems back to land by producing guano.