Marine habitats and ecosystems
This article should provide an introduction to the habitat categories identified as forming the key elements in marine biodiversity.
Marine ecosystems are part of the earth’s aquatic ecosystem. The habitats that make up this vast system range from the productive nearshore regions to the barren ocean floor. The marine waters may be fully saline, brackish or nearly fresh. The saline waters have a salinity of 35-50 ppt (= parts per thousand). The freshwater has a salinity of less than 0.5 ppt. The brackish water lies in between these 2. Marine habitats are situated from the coasts, over the continental shelf to the open ocean and deep sea. The ecosystems are sometimes linked with each other and are sometimes replacing each other in other geographical regions. The reason why habitats differ from another is because of the physical factors that influence the functioning and diversity of the habitats. These factors are temperature, salinity, tides, currents, wind, wave action, light and substrate. 
Marine ecosystems are home to a host of different species ranging from planktonic organisms that form the base of the marine food web to large marine mammals. Many species rely on marine ecosystems for both food and shelter from predators. They are very important to the overall health of both marine and terrestrial environments. Coastal habitats are those above the spring high tide limit or above the mean water level in non-tidal waters.  They are close to the sea and include habitats such as coastal dunes and sandy shores, beaches , cliffs and supralittoral habitats. Coastal habitats alone account for approximately 30% of all marine biological productivity. The diversity and productivity are also important for humans. These habitats provide a rich source of food and income. They also support species that serve as animal feed, fertilizers, additives in food and cosmetics. Habitats such as mangroves and seagrasses protect the coastlines from wave action and erosion. Other areas provide sediment sinks or act as filtering systems.
Despite the importance of marine ecosystems, increased human activities have caused significant damage or are serious threats to the marine biodiversity. These activities can be overfishing, pollution, introduction of exotic species or coastal development. For this reason, conservation plans are necessary to save the marine ecosystems from being lost. 
- Levinton J.S. 1995. Marine biology: function, biodiversity, ecology. Oxford University Press. p. 420