Introduction

The open ocean is vertically divided into four zones. The Photic zone is one of them. And others are the twilight zone, midnight zone, and abyssal zone. Oceanographers discuss different ocean regions when studying individual regions of the ocean (1).

The vertical zones of the ocean are the epipelagic, mesopelagic, and bathypelagic zones. The epipelagic zone is the photic zone. These vertical zones of the ocean start at sea level and end at the deepest points of the ocean. All these vertical zones are based on the amount of light that penetrates the seawater.

One of the most important vertical zones in the oceans is the small surface area with light, the photic zone. This zone is the top 200 meters of water. The open ocean begins at the edge of the continental shelf and extends outward. More than 90% of the world’s ocean area is considered the Open Ocean. The open ocean is divided into two zones. These are photic and aphotic zones. Now here we discuss the photic zone (2) & (5).

Definition

The zones of the ocean are the upper region of the water mass that receives the maximum light from the sun, this zone is known as the photic zone (4).

Characteristics

• Photic word is a derivative of ‘photon’, the word for a particle of light.
• It is the surface layer of the ocean that receives sunlight.
• This zone is the portion of the ocean where sunlight penetrates.
• It is also known as the sunlight zone.
• The diversity of various organisms is very high here.
• Pressure is low in this zone.
• Photosynthetic organisms are found here because photosynthesis occurs here.
• About 90% of marine organisms exist in this zone (3) & (4).

Importance and Role of the Photic Zone

• One of the most important features is that it is the uppermost layer of water that receives sunlight, allowing phytoplankton to photosynthesize.
• It undergoes a series of physical, chemical, and biological processes that deliver nutrients to the upper water column.
• This zone is home to most aquatic organisms due to the activity of phytoplankton.
• Most of the biomass (80%) lives in this area.
• A great variety of marine plants and their types and all kinds of other plants can grow because of proper sunlight.
• It provides important information about the health of the ocean (2) & (3).

Depth and Extent of the Photic Zone

It extends to a maximum depth of 200 meters below the surface of the water. The depth of this zone depends on the transparency of the water. If the water is too clear, the photic zone is too deep. If it is very turbid, it may only be fifty feet (1).

Factors Affecting the Photic Zone

1. Temperature

The temperature of this zone depends on regions, water turbulence, and cloud cover. Temperature levels play an important role in regulating water density and the functioning of many chemical and physical processes in marine organisms.

2. Salinity of the water

Salinity is also an important factor that affects the photic zone. The higher salinity means more density thus preventing ocean water from freezing. And all the minerals in water help to provide calcium for marine organisms.

3. Sunlight

Sunlight plays an important role in the organisms which are present in the photic zone. Phytoplankton carry out photosynthesis by absorbing sunlight. The intensity of light decreases as it goes deeper.

4. Transparency of water

If the transparency of water is very clear the photic zone can become very deep (about 100 to 200m). But if the water is turbid, then the photic zone is not so deep (about 15m).

5. Nutrients

Some nutrients like phosphorus, nitrate, silicates, sulfates, etc. are essential for the growth of life. These elements of the photic zone help in the growth of animals living in the photic zone (2) & (5).

Sunlight and Photosynthesis in the Photic Zone

The rate of photosynthesis is high. This is due to the large amount of solar energy that is used as an energy source for photosynthesis by primary producers such as phytoplankton. All the phytoplankton grow extremely fast due to the influence of sunlight. In fact, 90% of photosynthesis in the ocean occurs here.

Phytoplankton are microscopic plants. These phytoplankton are the source of energy for various animals living in water. They absorb the rays coming from the sun and produce their own food through the process of photosynthesis. Energy transfer occurs when these phytoplankton are consumed as food by aquatic zooplankton and other organisms. Thus energy is transformed through the food chain in aquatic ecosystems (3).

Organisms and Ecosystems in the Photic Zone

Various organisms are present in the ocean water. But all these organisms are divided according to the level of seawater. It is home to 90% of marine life due to sunlight. There are various types of plants and organisms present. These are phytoplankton, including dinoflagellates, diatoms, cryptomonads, etc. This zone also includes zooplankton called the consumers. All these zooplankton survive by feeding on phytoplankton thus building an ecosystem (1).

Adaptations of Organisms in the Photic Zone

• Some plants like Seaweeds, grasses, algae, plankton, etc. present in the photic zone have a body structure and surface area that easily support their weight and float around the ocean.
• Their bodies have cells that undergo photosynthesis, which makes it easier for them to survive and get the nutrients that they need.
• A larger number of species of fish, sharks, whales, etc. live in the photic zone.
• These animals have dark-colored skin on top and light-colored underneath. This is because, when the predator is viewed from above, they can see the black skin as the dark part of the sea. And when the predator sees from below, they will see a light shadow like sunlight water above.
• All animals in the photic zone also have very good smell, sound, eyesight, and a good sense of navigation. These characteristics help them to adapt to life underwater. All these features make these animals adapted to living in the photic zone (2) & (4).

Threats and Conservation of the Photic Zone

Due to climate change, and human activities, the photic zone is declining at an alarming rate. Combined with the effects of overfishing, pollution, and habitat destruction, this is leading to the loss of marine biodiversity.

1. Pollution

Currently, plastics are responsible for the pollution. As a result, various marine species in all photic zones are endangered.

2. Chemical substance

The ecosystem of the photic zone is being destroyed due to the pollutants released from different types of factories along the sea, the movement of ships in the sea, etc. The species of the photic zone are endangered due to various pollutants and radioactive substances, pharmaceutical waste materials, other chemical liquids, etc.

3. Overfishing

Overfishing is a major threat as it depletes resources and destabilizes the ecosystem of the photic zone.  Also damages the seabed and marine ecosystems.

4. Habitat destruction

Every year, the oceans absorb 23% of human-caused carbon dioxide emissions. Due to global warming and human activities, temperatures are rising rapidly, and the oceans are becoming cloudy. This is causing the waters to become more acidic and the death of aquatic life is increasing. If this continues, more than half of the world’s marine species will go extinct by 2100.

Conservation

Regulatory measures on waste discharge into water bodies should be adopted to preserve the biodiversity of the photic zone. Raising public awareness is one of the most important ways of conserving this ecosystem. And this process is mainly completed through educational programs, volunteer monitoring programs, etc. (5).

Research and Exploration of the Photic Zone

• Research is mainly based on three features. These are the eutrophication of water bodies, fundamental food web research, and the understanding of nutrient movement and cycling. Eutrophication is a process that occurs when nutrients such as nitrogen, and phosphorus accumulate in bodies of water, leading to the growth of algae and other aquatic plants (phytoplankton).
• A third area of research involves the movement and cycling of nutrients within the watershed. The movement of particles and nutrients by water currents in the ocean is not well understood. All life ultimately depends on the continued productivity of the world’s photic zone.

Q&A

1. What is the photic zone?

It is the layer at the top of the ocean that is illuminated by sunlight.

2. Photic zone depth

The depth is 200 meters.

3. How deep is the photic zone?

About 200 meters deep.

4. Where is the photic zone?

It is the uppermost layer of the ocean.

5. Photic zone characteristics

• Is the region of the ocean where sunlight penetrates.
• It is also known as the sunlight zone.
• The diversity of various organisms is very high here.
• Pressure is low in this zone.
• Photosynthetic organisms are found here because photosynthesis occurs here.
• About 90% of marine organisms exist in this zone.
•  High intensity of sunlight, enabling photosynthesis and the production of organic compounds.

Written By: Manisha Bharati