The oceans cover 71% of the world's surface and contain 97% of the world's free water (the other 3% is on land or in the atmosphere) and is responsible for generating 29% of the earth's oxygen.

The average depth of the world's oceans is 4 km; the deepest spot is the Mariana Trench in the Pacific Ocean at 11.7 km.

There are four major oceans of the world (Atlantic, Pacific, Indian, and Arctic). Some consider a fifth ocean - the Southern Ocean or Antarctic Ocean.

There are many types of marine habitats. Habitats are classified using a variety of factors such as vegetation, substrate, depth, salinity, and wave action. Below are some of the habitats found in the oceanic environment. These may be useful key words to use when searching for resources listed below.

• Beaches
• Rocky Shores
• Mudflats
• Salt Marsh
• Mangrove Swamp
• Estuaries
• Sea grass beds
• Coral Reefs
• Kelp Forest
• Deep Sea
• Pelagic (Open Ocean)
• Hydrothermal Vents

Dead zones are defined as areas with severely low dissolved oxygen levels or devoid of dissolved oxygen altogether. Such zones have been noted by scientists worldwide since the 1970s and range in size from a square kilometer to about the size of the State of Massachusetts. Currently, the most publicized dead or hypoxic zone is located in the northern Gulf of Mexico just south and west of Louisiana. It is thought to be driven by the outflow of the Mississippi River, which carries sediments and nutrients from agricultural run-off from upstream in the Midwest. The effect of low to no dissolved oxygen on biota has been studied, and it results in lower reproductive capability to a lack of spawning. Obviously, in areas devoid of dissolved oxygen, living organisms that are mobile enough will attempt to escape. This results in vast expanses where organisms are either dead or gone. How does this affect human activities? The primary impact of the dead zone on human activities is a displacement of fishing activities. Fishers who typically fished the area for shrimp are no longer able to catch shrimp there, and as a consequence are forced to travel to other areas to fish. In a climate where expenses often exceed income, this displacement can make or break a fishing operation.

Can dead zones be fixed? Apparently so. A well-known dead zone in the Black Sea largely disappeared between 1991 and 2001 when fertilizer use was severely curtailed due to economic hardship. As a result of reversing the dead zone, fisheries returned as an economic force in the area. What will it take to reverse the dead zone in the northern Gulf of Mexico? Our agricultural belt is quite active, and it isn't likely that the use of fertilizers will cease in the foreseeable future. We may be able to implement better farming practices, such as the establishment of riparian buffers along the river banks that will absorb fertilizer run-off before it can reach the river. Under any circumstance, it will take considerable political will to implement changes that will begin to diminish the Gulf of Mexico dead zone.