Marine Ecosystems

The term “marine” refers to ocean ecosystems.

Coastal Zone

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The coastal zone is the area of the ocean close to land, a region that includes the highest point on land that is reached during high tide to the continental shelf. The continental shelf is the shallow seabed surrounding a continent, or the part of the continent land mass that is under water. It slopes down gradually until it reaches a point where the bottom of the ocean drops off steeply; this point marks the end of the continental shelf and the transition from the coastal zone to the open sea. The intertidal zone is a sub-category of the coastal zone. It describes the part of the coast that is submerged during high tide but uncovered during low tide. This area, known for its “tide pools,” contains many unique organisms, since these creatures must adapt to dry and wet conditions, including different current strengths and oxygen and light levels. Both the intertidal zone and the continental shelf can be fragile ecosystems because of their proximity to coasts that are heavily populated by humans.

In the coastal zone, the water tends to be warm and shallow with many nutrients available because of its location. Land has a lower heat capacity and tends to be warmer than the ocean, so the coastal area receives much of this warmth. Shallow water means that more sunlight can penetrate down into the water. Additionally, the runoff of water from inland brings nutrients from soil and plants to the coastal area. These two factors, sunlight and runoff, contribute to the fact that although coastal zone comprise less than 10% of the ocean’s total area, 90% of marine species inhabit this region. Coral reefs, known for their great variety of organisms, can be found in this area.

Open Sea

Ocean zones. (Click to enlarge)

The open sea can be divided into several sections. The euphotic zone is the uppermost one, extending from the surface to about 200 meters below. This zone has the highest levels of sunlight, photosynthesis, and dissolved oxygen. Many large predatory fish live here. The euphotic zone has the lowest amount of nutrients for all of the ocean zones. This can be mitigated by upwellings, cold water from lower zones that flow to the surface. Since the lower levels have more nutrients, upwellings provide a source of nutrients for organisms in the euphotic zone.

Coral reefs often make up part of the euphotic zone. Out of all the ecosystems in the world, coral reefs rank second in terms of biodiversity and they are the most diverse aquatic ecosystem. Because of this, the effects of damage are very severe. Coral reefs grow slowly and are easily disturbed, due in part to their specific conditions; the water temperature is between a range of 18 to 30 degrees Celsius with high salinity. An increase of even 1 degree above this range causes coral bleaching and widespread destruction of the reef. This may not seem serious because of the 12 degree range, but global warming has caused the West Antarctic ice sheet to melt, raising ocean temperatures. In addition, human activities have destroyed 10% of coral reefs and threaten up to 60% of the reefs in the world. These activities include sediment deposition, fertilizer runoff, global warming, ozone depletion (which leads to an increased number of ultraviolet rays penetrating through the atmosphere), harvesting coral and fish, and oil spills.

The bathyal zone is the middle zone. It is dimly-lit so little photosynthesis occurs here and the level of dissolved oxygen is low. Many zooplankton and smaller fish live here. The deepest zone, the abyssal zone, begins at about 1500 meters and continues to the ocean floor. Sunlight does not penetrate this far into the ocean, although there are many more nutrients here than in the euphotic zone. These nutrients often consist of debris and waste that floats down from the upper layers of the ocean. Because the abyssal zone is very difficult to reach for humans, many of the organisms that dwell here are unknown. The organisms that we do know of are unique in their adaptations to the lack of light and other conditions characteristic of this mysterious environment.