The benthic zone is essentially the bottom of the ocean or of any waterway. It is the soil, rocks, outcroppings and other structures of the bottom. In some arguments, wherever there is a bottom in the ocean it is the benthic zone, even in the deepest parts of the ocean: the Hadal, or trenches.
In most arguments, the benthic zone is water that is shallow enough to allow light to penetrate, thus supporting aquatic plants which contribute oxygen to the waters.
The shallower, light penetrable benthic zones support the vast majority of life in the oceans, much of which provides food for humans and produces oxygen for the atmosphere. There are now great areas of "dead zones", where there is not enough oxygen to support the vast array of oxygen dependent life that is coral, that depends on coral and aquatic plants, the aquatic plants, themselves, and the entire food chains, from microscopic to giant aquatic vertibrates and invertebrates, and finally, birds.
The life forms that live on and that burrow under the benthic soil in the shallower ocean are also dependent on a fairly oxygen rich environment.
The dead zones around developed countries are, mostly there because of human or anthropogenic activity or from natural washing out of silt. But the introduction of human wastes and fertilizer, along with industrial runoff, into the shallower waters of the oceans are considered to be the major cause of dead zones around the world.
The wastes and fertilizers cause aquatic plants algae to grow out of control, increasing nitrogen levels and reducing oxygen levels.
In other dead zones, the destruction of coral reefs, where much of nutrition, especially calcium, and hospitality goes on has created non viable areas for entire segments of the food chain. This is from human waste, destructive fishing techniques and other pollution, including oil spills and unregulated ship's waste.
Silt that is washed via rivers and streams into the brackish,intertidal and ocean benthic zone can affect not only the ocean benthic zones, but the brackish and freshwater zones as water becomes cloudy and light is cut off, making it difficult for plants to synthesize light and to produce oxygen. Also, the silt can suffocate eggs, larvae and mature creatures that live on or under the benthic soil.
Temperature is an issue in releasing water from dams and industrial operations,. Eggs and larvae, as well as mature benthic dwellers might be sensitive to the release of water that is too cold or too hot. Measures to temper the water before release allow for less disruption of the living situation for benthic creatures.
From the Gulf of Mexico to the Caribbean and Pacific waters and to the Black Sea, the effects of industrial and fertilizer runoff and human waste runoff are being curtailed and better managed in order to allow the restoration of a more oxygen rich environment that will allow corals and other lives to return to a fuller balance and greater populations.
In many places, waste settling ponds and various filters are being used to ensure that animal wastes and fertilizers are reduced before excess water enters the water systems that feed the oceans. Many times, these wastes and fertilizers are very far inland, but can cause freshwater benthic and riparian damage or carry for great distances until they reach the oceans.
Riparian restoration, where there are practices, programs and techniques to prevent soil from eroding and from washing into waterways, is another way of preventing soil damage inland that can eventually introduce silt and soil to the ocean.
The latest issue in benthic zone preservation is one of disaster that will be felt for the long term: The 2010 BP oil spill in the Northeastern Gulf of Mexico, which is already considered to be a dead zone because of fertilizer and other runoff from the Mississippi River basin.
Finally, introduction of alien species might have an impact on life in shallow ocean benthic zones, especially when aggressive species of mollusks and aquatic plants are capable of taking out indigenous competitors who have no natural defenses against the intruders.