Please complete the passages about marine ecosystems. A(n)__ is a brackish-water ecosystem located at the mouth of a river. Tides have a heavy influence on this ecosystem. Mangrove trees dominate this ecosystem in the tropics, but a variety of salt-tolerant, herbaceous plants dominate at middle and high latitudes. Primary productivity is high because nutrient-rich sediments fuel plant growth. This ecosystem is also the world's most endangered marine ecosystem due to threats from human settlement. A(n) __ ecosystem is found in temperate and polar waters where the water temperature does not exceed 20 °C (68 °F). Large algae that form "forests" dominate this ecosystem. The algae only grow in water shallower than 50 m (165 ft). Pacific sea otters play a key role in this ecosystem. People often call __ ecosystems the rainforests of the sea because of their extraordinary biodiversity. This ecosystem requires clean, well-lit water between 18 °C and 30 °C (64 °F and 86 °F). Approximately 4,000 species of marine fish use this ecosystem at some point in their life cycles.
Flowering plants that resemble grasses dominate __ ecosystems. This type of ecosystem is most. common in tropical waters, but is found in temperate waters as well. Some animals, such as green sea turtles, manatees, and the dugong graze in this ecosystem.