Over 70% of the Earth’s surface is covered by sea and it can take 5,000 years for one drop of seawater to travel through all the world’s oceans. They generate 32% of the world's net primary production. They are distinguished from freshwater ecosystems by the presence of dissolved compounds, especially salts, in the water. Approximately 85% of the dissolved materials in seawater are sodium and chlorine. Seawater has an average salinity of 35 parts per thousand (ppt) of water. Actual salinity varies among different marine ecosystems. The world’s largest mountain chain, the Mid-Atlantic Ridge is in the ocean. It is 4 times longer than the Himalayas, Andes and Rockies combined. Scientists believe that oceans have a significant impact on climate change. Most meteorologists predict the world will warm between 1.4 and 5.8º C by the year 2030, unless we do something drastic to cut down the increase in greenhouse gases.
 Did You Know?
- There are several theories as to why the ocean is usually blue. Some scientists believe that when sunlight hits seawater, part of the white light is absorbed and part is reflected. White light is composed of various shades (all the colors in the rainbow). Seawater does not absorb blue light, so the blue light is reflected outwards. Some scientists believe that suspended matter in the sea is responsible for its blue color.
- All the oceans of the world are connected!
- The ocean contains every known naturally occuring element plus various gases, chemical compounds, and minerals.
 The Coral Reefs
Coral reefs are shallow costal zone marine ecosystems, which form in clear, warm coastal waters of the tropics and subtropics. Next to tropical rainforests, coral reefs are the worlds most biologically diverse biome. High biodiversity results in an ecosystem that is very complex in terms of the biological interactions among its many inhabitants. Typically these inhabitants include hard and soft corals, parrot fish, reef sharks, sergeant major fish, wrasse, moray eels, sponges, sea cucumbers, starfish etc. Reminiscent of tropical rainforest soils, the waters around coral reefs are often poor in nutrients, but have high primary production. The factors that favor this high productivity include the presence of symbiotic zooxanthellae, favorable temperature, rapid nutrient recycling, and abundant sunlight.
Periodic disturbance is characteristic of coral reefs. Abiotc disturbances such as hurricanes can flatten coral reefs. Biotic disturbances such as periodic outbreaks of the predatory animals can devastate large areas of coral reefs. The destruction caused by these periodic disturbances may require years to reestablish the reef. However, periodic disturbance may be beneficial to the long-term survival of some reefs.
 The Estuaries
Estuaries are highly productive areas within the coastal zone. They include river mouths, inlets, bays, sounds, mangroves, and salt marshes. The constant water movement, which stirs up the nutrient-rich silt and nutrient runoff from land, help make estuaries some of the most productive ecosystems on earth. Temperature, oxygen, and salinity levels within estuaries vary widely because of daily tide fluctuations, seasonal changes in fresh water flow, and periodic storms. To survive, estuarine organisms must adapt to these variations. Because of the inability of many organisms to adapt to this environment, species richness is low, but because the nutrient-rich environment provides high enough energy levels species abundance is high. In tropical areas dense stands of mangroves predominate.
 The Intertidal Zone
The intertidal zone is the area of shoreline that falls between low tide and high tide. Because of fluctuating tides, organisms that live in intertidal zones need to be able to keep from being swept away and to be able to withstand immersion during high tide and desiccation during low tide. Because of the high levels of light, nutrients, and oxygen, intertidal zones are very biologically productive. Some coasts have steep rocky shorelines pounded by waves. The numerous pools and other niches among the rocks contain a large variety of species. Other coasts have sandy shores with present different types of niches and adaptations.
 The Open Oceans
Open Oceans start at the edge of the continental shelf, where water depth increases sharply to average depths of 4,000 meters. Under sea mountains (some as high as 5,000 meters) and deep trenches (some as deep as 10,000meters) stud the ocean floor. Open oceans can be divided into three vertical zones. Theses are: euphotic zone, bathyal zone, and abyssal zone.
The euphotic zone consists of top layer of water from the surface to the depth of light penetration (up to 200 meters).The surface zone of the ocean is comprised of upper, well-illuminated waters. Photosynthesis occurs by phytoplankton, but primary production is low due to low nutrient levels. However, because of the oceans vastness, this zone contributes 25% of the earth’s primary production and substantially contributes to the earth’s oxygen and carbon dioxide budget.
The bathyal zone is the dimly lit middle zone. It’s also known as neritic zone Because of the lack of sunlight, no primary production occurs in this zone. The neritic zone is occupied by a large number of species. It is thought that because the intertidal part of the neritic zone is so close to the land, it must have been home for the ancestors of today’s terrestrial organisms. Nutrients in this zone are abundant, and the area has become the site of the world’s greatest fisheries. Unfortunately, this resource has been over-exploited in many areas.
The abyssal zone is dark, cold, and low in oxygen. However, this zone supports a large number of species. The abyssal zone of the ocean lies at depths below 1000 meters, including that of the sea floor. Pressures at this depth are incredibly high, temperature is around 2 to 3 degrees Celsius, food sources are few, and virtually no light reaches the sea floor. The abyssal zone is one of the very few ecosystems on earth that does not depend on the sun’s energy.
Ocean currents play a vital role in the transport of nutrients, oxygen, heat, and organisms across the globe. Wind-driven surface currents move across open oceans creating large circulation systems. These currents have significant effects on global climates. Deepwater currents are produced as cooled, high-density water sinks at the Polar Regions, then moves along the ocean floor. Some of this deep water can be moved to the surface by a process called upwelling. This upwelling brings nutrients to the surface and creates highly productive ecosystems.
- World Resource Institute
- Dynamics of Marine Ecosystems, Kenneth
- Principal of Environment Conservation, Singh
- Marine Protected Areas, IUCN