There are different levels of the ocean. The level of the ocean is determined largely by depth, however, it’s also determined by the amount of sunlight in the water and how much sunlight the creatures living in that level of the water receive. Each of these levels is called a life zone. Each life zone has unique attributes, described below in order of depth.
The Pelagic zone: The Pelagic zone is sometimes referred to as the “water column”. It’s the combination of the Neritic and Oceanic zones.
The Neritic zone: This is the shallower part of the ocean right above the continental shelf. The continental shelf is the shallow part of the ocean where the vast majority of commercial fishing takes place right around the beaches of continents and other large landmasses. It’s the part of the Pelagic zone nearer to the surface: between the ocean bottom and the hightide line. It’s less than 600 feet deep.
The Intertidal zone: The Intertidal zone contains algae and blue-green bacteria, the periwinkle snails and limpets that eat the blue-green bacteria and algae, and the ocean moistened rocks that the algae live on. This zone is above the high tide zone and receives wave splashes.
The Upper Intertidal zone: This area is higher on the beach. It’s where beach plants such as bushes, trees, and grasses grow; it receives a light misting from the crashing ocean waves along the shoreline.
The Mid-intertidal zone: This is where barnacles that create a natural glue to attach themselves to rocks live along with mussels that attach themselves to rocks with sticky byssal threads that the mussels produce. During high tide, the barnacles living on the rocks open to filter plankton out of the water to eat it by filter feeding. So as not to dry out in the sun during low tide, however, the barnacles close up. Seaweeds also live in the mid-intertidal zone. Rockweeds attach themselves to ocean rocks using a sort of attachment called a holdfast attachment.
The Lower-intertidal zone: The lower-intertidal zone is where tide pools occur. This zone is primarily populated with seaweeds. At low tide when the tide recedes, the spaces between the rocks in the lower-intertidal zone retain water and the living creatures in the water, forming small aquatic communities. Small fish, algae, and invertebrates live in these tide pools. When the tide comes in and covers these tide pools, sea stars, larger fish, and sea urchins swarm into these areas to feed on the inhabitants of the areas that, were, at low tide, tide pools.
The Subtidal zone: Sea urchins, fish, crabs, and sea stars most often live in this zone. This zone includes an area of wave turbulence and is right below the low-intertidal zone. The area of wave turbulence is called the surf zone.
The Oceanic zone: It includes most of the open ocean and is adjacent to the Neritic zone. All the ocean in this zone is water deeper than 600 feet. The Oceanic and Neritic zones, in combination, comprise the Pelagic zone.
The Benthic zone: This is the name for the total seafloor from the upper intertidal zone, all the way to the oceanic basin of the oceanic zone. Benthos are creatures who live along the ocean floor: in the benthic zone.
The following is a list of the zones of the ocean as defined by the amount of sunlight they receive. Using this system, the ocean can be broken into three zones. These are the euphotic zone, the disphotic zone, and the aphotic zone.
The euphotic zone, or sunlit zone, is nearest the surface of the ocean. Most of the photosynthesis that takes place in the ocean happens here because this is where most of the sunlight in the ocean is. Tiny organisms float around in this zone performing photosynthesis and acting as the basis of the food chain. 90% of all marine life takes place in the euphotic zone because of the photosynthetic organisms that provide the basis of the food chain.
The dysphotic zone: Only a small amount of light can penetrate through the euphotic zone into the dysphotic zone. Greater water pressure than in the euphotic zone, and decreased sunlight prevent many plants from growing here in this zone. It is however home to quite a few animals who are built to live in areas of low light. This murky area of the ocean is commonly referred to as the twilight zone and is typically home to creatures who possess bioluminescent capabilities; their bodies can glow or otherwise make light independently. This zone of the ocean extends from 600 feet to 3000 feet deep.
The aphotic zone: The aphotic zone takes up 90% of the ocean and is also referred to as the midnight zone because no light can filter into it, and as such, it’s 100% pitch black. It is also very cold in this zone. Because the temperature is close to freezing in the aphotic zone, most of the creatures who live there tend to stay near the earth’s crust’s cracks harvesting nutrients from the mineral-rich deposits in the cracks. Various bacterias transform hydrogen sulfide from the deposits for food and energy. These specialized forms of bacteria are utilized by every other creature in the midnight zone for food and nourishment.
Different aquatic zones house different aquatic organisms.
Plankton live near the ocean’s surface. They’re any small weakly swimming or floating animal, bacteria, plant, or another organism that just floats along with the ocean current. Plankton has this quality because of their name; it comes from the Greek word for ‘wanderer’, planktos. There are three types of plankton, phytoplankton, bacterioplankton, and zooplankton. Phytoplankton can create their own food; they are autotropic. Zooplankton are not autotrophic and they rely on other organisms for food. Bacterioplankton recycle minerals in the water column. They also emulate archaea and bacteria. Plankton that live their entire lives as plankton are referred to as holoplankton, whereas meroplankton.
Nekton live in open water and can swim strongly and thus don’t have to follow the current like the plankton these nektonic creatures begin their lives as. Most of these creatures are vertebrates. Vertebrates are any animal with a backbone. The vertebrates of the ocean are mostly marine reptiles, marine mammals, and fish. Squid are also considered nekton even though squid are invertebrates. Nekton are creatures who can all swim strongly and thus freely; they are still confined to certain areas of the ocean, however. This is because different areas of the ocean have different salinities, or salt concentrations in the water. The different temperatures in different areas of the ocean also create boundaries for the nekton, as do density changes in different areas of the ocean.
Benthos live near or on the ocean floor. They are any creature that resides in these regions. While the word benthos means the “depths of the sea” benthos don’t necessarily live deep in the ocean; they live anywhere on the ocean floor. Sometimes they live in the shallower parts of the ocean or even on the floor of tide pools along the continental shelf or along the foreshore. Some benthic creatures however do live near the ocean floor. These however have specific adaptations and special abilities to be able to survive in the harsh depths of the ocean near the ocean floor. Benthic organisms include creatures such as copepods, bivalves, and polychaete worms. Additional examples of benthos are seaweed, coral, and crabs.
Text source: http://cms.gavirtualschool.org/Shared/Science/Oceanography/introduction_to_marine_oceanography/introduction_to_marine_oceanography2.html
Image source: https://en.wikipedia.org/wiki/Oceanic_zone#/media/File:Oceanic_divisions.svg
By: Ellia F.