Scientists divide the world's oceans into several zones, based on a variety of characteristics. The oceans can be divided into four zones based on the amount of sunlight that is available. The amount of light reaching different levels of the ocean has a profound affect on the types of animal and plant life that can live there. Without adequate sunlight, photosynthesis cannot occur. Sunlight is quickly absorbed and/or scattered by the water, which rapidly changes light levels with depth in the ocean. This makes light a limiting factor for ocean ecosystems. The four zones, from the top of the water column to the ocean floor are as follows:

1. The euphotic zone receives the most sunlight, and is found at shallow depths in the ocean. In this zone, underwater vegetation such as seaweed and algae can do photosynthesis to provide food and energy for aquatic ecosystems.
2. The photic zone is located beneath the euphotic zone. The photic zone still receives abundant sunlight, and therefore supports photosynthetic vegetation. The lowest levels of the photic zone change globally, depending on the amount of sediment and organic material in the water column.
3. The disphotic zone is located beneath the photic zone. This zone receives some sunlight, but it is very dim at this depth in the ocean. Some animals can survive here, but no vegetation can grow in the twilight conditions that characterize this zone.
4. The aphotic zone is found on the ocean floor and receives no sunlight. Organisms that live in the aphotic zone consume decaying matter and sediments that trickle down through the water column from above. Organisms in this zone rely on chemosynthesis, rather than photosynthesis, as an energy source for their ecosystems.
   

Scientists can also divide the oceans into habitat zones, based on water depth. The two major habitat zones are the pelagic (shallow water) and benthic (deep water) zones. Each of these can be further sub-divided. The following zones are listed according to depth, from shallowest to deepest:

1. The neritic zone is located in the shallowest waters above the continental shelf. In this zone, sunlight is plentiful and photosynthesis occurs. This zone receives nutrient runoff from land, which makes animal and plant life abundant and diverse in this zone.
2. The oceanic zone is found in the open ocean waters. Plankton, seaweed, and other photosynthetic vegetation is abundant at shallow depths.
3. The bathal zone is found on the continental slope. Temperature and food availability decrease rapidly with depth in this zone.
4. The abyssal zone is found on the flat, muddy, open plains of the seafloor. Life is sparse in this zone, but sometimes predators and scavengers such as sharks can be found here.
5. The hadal zone is located in the deep ocean trenches. Little is known about life in this zone, due to the extremely low temperatures and high-pressure.

Organisms that live in different habitat and light zones behave and move differently. Therefore, scientists can also divide up the oceans based on differing lifestyles of organisms.

1. Plankton float in open waters. Examples include algae, seaweed, and the larvae of some organisms.
2. Nekton are organisms that can propel themselves through the water column. Examples include fish, jellyfish, whales, sharks, etc.
3. Infauna are benthic organisms that bury themselves in the mud and sediments on the seafloor. Examples include some worms, clams, insects, and bacteria.
4. Epifauna live on top of the seafloor. They attach themselves to rocks or lie quietly on the bottom of the ocean floor. Examples include crabs, corals, sponges, sea urchins, and star fish.
   

The interactive lesson on the next page allows you to explore each of these categories of the ocean. See if you can correctly identify and label the various light, habitat, and lifestyle zones of the world's oceans!