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A Helpful Guide to Understand the Photic Zone

The region of a water body that receives enough sunlight for photosynthesis to take place, is called the photic zone. The characteristics of this environment encourages the growth of a wide variety of flora and fauna. This article will provide more information about the photic region and its biome.
ScienceStruck Staff
Did You Know?
Around 90% of the gaseous oxygen in the Earth's atmosphere is created by the photosynthesis of aquatic plants and phytoplankton living in the photic zone of the world's oceans.
The photic zone definition is as follows: "The upper layer of a body of water delineated by the depth to which enough sunlight can penetrate to permit photosynthesis. On an average, the photic zone of an ocean reaches around 200 meters in depth. These waters usually exist till the depth of the continental shelf, and is home to an immense variety of plant and animal life.
Scientifically measured, the intensity of the sunlight in the photic zone should be at least 1% of that at the surface. This area is highly studied by humans due to the ease of access. Any person with basic scuba diving experience and equipment can explore this region well.
The animals living inside the aphotic zone are highly dependent on the dead matter of the photic zone for nutrition needed in sustaining life.
Photic Zone Characteristics
Illustration of the Photic Zone in the Ocean
Factors Affecting the Depth of the Photic Zone
The photic zone, also known as the euphotic zone (Greek for "well-lit": εὖ "well" and φῶς "light") or sunlit zone is the uppermost region of the pelagic zone and occupies around 25% of the ocean's water. Usually, more than half of the light from the Sun is absorbed in the first meter of water, i.e, the entire red-light spectrum is absorbed by this level. By the time the sunlight reaches a 10-meter depth, the entire yellow light spectrum is also absorbed, and only 20% of light is still visible. Most marine beings do not live below this region. Around 50-70 meters depth all the green light is absorbed, and any photosynthesis taking place at this level is only sufficient to keep the organism alive, there is no scope for growth or reproduction using light as the source of energy. From here on, till approximately 100 meters depth, only 0.5% of sunlight is available―comprising blue to violet light spectrum. It is impossible for any photosynthetic beings to survive from here on. By the time the sunlight reaches 200 meters, only 0.005% of surface light is left, which is almost as good as complete darkness.
This depth of the photic zone depends upon the clarity of water―measured by a photometer or a Secchi disk. Factors such as silt, sand, or mud can drastically reduce the depth of this region. If the water is turbid or cloudy, the depth of the photic zone can be as low as 15 meters. Other factors affecting the depth of the photic zone are varying sunlight intensity due to seasonal changes or the angle of the sun, huge groups of marine organisms like algae, or large man-made disasters like oil spills.
Photic Zone Food Chain
The temperature in this region ranges around 100 to 30 degrees Fahrenheit, while the pressure ranges around 15 to 300 pounds per square inch. Such an environment is very conducive for life. Hence, you can find a wide variety of animals and plants in the photic zone biome. We will now categorize these biological beings according to the food chain hierarchy existing in the photic zone.
  1. Phytoplankton: These single-celled photosynthetic autotrophs are the most important aquatic plants, as they are the primary food source for most life in the ocean. They can come in varieties of flagellates or diatoms.
  2. Micro-zooplanktons: These are single-celled, amoeba-like protozoans, they are primary consumers, that trap and feed on phytoplankton.
  3. Macro-zooplankton: These are the most widespread of zooplankton varieites. Usually seen as large groups of tiny shrimp like creatures, that are members of the Phylum Arthropoda.
  4. Mega-zooplankton: Containing krill or other euphasids from the class Crustacea. These animals are usually 2-6 cm long, and are primarily herbivores, feeding on phytoplankton. A smaller group of carnivorous mega-zooplankton, such as arrow worms and various kinds of jellyfish, are found in many parts of the globe.
  5. Planktivores: Varieties of fish and other organisms that sustain themselves on plankton. Usually fish living in the deeper part of the photic zone come in this category.
  6. Piscivores: A wide variety of fish, mammals, and squid that eat fish, come in this category. They are at the top of the food chain in the photic zone.
Photic Zone Adaptations by Plants and Animals
With the evolution of life on Earth, different animals and plants undertook many changes to adapt to their surroundings and thrive. We shall look at the changes undergone by the organisms living in the photic zone, and how this helps them to flourish in this environment.
  • Plants: Seaweeds, grasses, algae, mangroves, kelp, plankton, etc., have developed their body structure and surface area to easily support their weight and float around in the ocean, or to reach areas where they can affix themselves firmly. The most important adaptation is to have body cells that undergo photosynthesis, making it easy for them to survive and get the nutrients they need.
  • Animals: Huge numbers of species of fish, sharks, whales, jellyfish, krill, etc., live in the photic zone. Most animals living in this region have counter shading, i.e., dark-colored skin on top, and light colored on the bottom, which acts as a camouflage from predators. This is because, a predator watching from above will see dark skin similar to the darker part of the ocean beneath, while those hunting from below, will see lighter shades, similar to the sunlit waters above. Also, animals have a good sense of sight, smell, sound, and navigation, suited for a life underwater. The existence of streamlined bodies and collection blubber gives them an added advantage to live in the photic zone. Adaptations by animals, such as whales, to be able to hold their breath even when diving more than a mile deep, or the ability of other marine creatures to extract a high amount of oxygen from the water, exist.
  • Symbiotic and Parasitic Animals: Cleaner shrimp with electric eels, and remora fish with sharks are two examples of symbiotic animals in the photic zone. Cleaning of parasites in return for food is a very effective survival strategy for these beings.
Abiotic Factors That Influence Photic Zone
Like any land biome, the photic zone is also affected by abiotic factors. The abiotic factors that affect the photic zone are availability of light and depth of the photic zone, temperature, stratification, salinity, nutrients, and currents and tides. Let us look at each of them in a little more detail.
  • Light and Depth Levels in the Photic Zone: Sunlight plays two important roles in the functioning of marine life, i.e., it enables vision in animals and photosynthesis in plants. The intensity of light decreases as we go deeper and becomes negligible at around 150 meters. Cloud cover, turbulence of water, angle of sunlight, and particulate materials that can absorb or scatter the light, determines the extent of the photic zone, and separates the region from the aphotic zone.
  • Temperature: Most heat in the water comes from the Sun, and depending on how close a particular region is to the poles or the equator, the temperature decreases or increases. Other factors like water turbulence and cloud cover play a part in how much heat is absorbed by the water. Temperature levels play a critical role in controlling the density of water, and the functioning of many chemical and physical processes of marine creatures.
  • Salinity of the Water: Salinity is another important factor affecting the density of water, i.e., more salinity means more density. Salinity prevents the water from completely freezing in an extremely cold environment, and the minerals also play a part in providing calcium for the shells of marine creatures.
  • Stratification: Temperature and salinity affect the level of density in water. Denser water sinks and lighter water rises, thus circulating the water and controlling stratification, i.e., different temperatures at different depths of the water.
  • Currents and Tides: Wind, gravity, and the rotation of the Earth are the major factors that affect water currents and tides. These movements of water evenly spread nutrients, and other essential compounds throughout the ocean, keeps them from settling on the ocean floor. Also, smaller animals and plants like plankton or weeds use these currents and tides for moving from one place to another.
  • Nutrients: Depending on all the above factors, the concentration of nutrients in the water is determined. Nitrates, phosphates, sulfates, silicates and other compounds, and trace elements are all necessary for growth of life.
Environmental Problems Affecting the Photic Zone
The photic zone is of great interest to marine researchers for conserving the natural state of the oceans. This zone is also important for the fishing business and other ocean-related industries. Most importantly, the survival of the photic zone is necessary for sustaining life all over our planet. Despite its importance, man has used the resources of this region negligently, threatening the state of the photic zone. Destruction of marine habitats, overfishing, oil spills, pollution have resulted in the near extinction of natural predators, dying coral reefs, creation of dead zones, etc. For this reason, organizations like Oceana, Sea Shepherd Conservation Society, Greenpeace, ASOC Southern Ocean Fisheries Campaign, etc., work to protect the ocean.
Even though the photic zone is such a large part of the Earth, we are just scratching the surface in terms of discovering its beauty and richness in resources. Its importance to us cannot be understated.