The photic zone, also called the euphotic or limnetic zone, is the part of a lake or ocean where the rate of photosynthesis is greater than the rate of respiration by phytoplankton. Phytoplankton are microscopic plants living suspended in the water column that have little or no means of motility. They are primary producers that convert solar energy into energy stored in the chemical bonds of carbohydrates. The compensation point, where photosynthesis equals respiration, defines the lower limit of the photic zone. Above this point, the phytoplankton population grows rapidly because there is abundant sunlight to support fast rates of photosynthesis. Below the compensation point, the intensity of sunlight is too low and the rate of respiration is faster than the rate of photosynthesis. The photic zones of the world’s lakes and oceans are critically important because the primary producers upon which the rest of the food web depends, are concentrated in these zones.
Below the photic zone, in both oceans and lakes, is the profundal or dyspohotic zone. In the profundal zone there is still some light, but not enough to support photosynthesis. In oceans, the even deeper depths are called the abyssal zone. This part of the ocean has virtually no sunlight, and is usually deeper than 6,562 ft (2,000 m). The deepest layer of the ocean, below 19,686 ft (6,000 m), is called the hadal zone. All of these zones receive a constant rain of organic debris and wastes from the photic zone which serves as a organic material for the organisms living in the deeper volumes.
All of these are open-water zones, as compared with the shallow areas near the edges of oceans and lakes, called the coastal and littoral zones, respectively. Most of these smaller, shallow areas receive sufficient sunlight to allow plant productivity to occur down to the lake or ocean bottom.
Primary production in the photic zone is influenced by three major factors—nutrients and light, which are essential for photosynthesis, and grazing pressure, the rate at which the plants are eaten by herbivores. Nutrients, especially phosphate and nitrate, are often scarce in the photic zone because they are used up quickly by plants during photosynthesis. External inputs of nutrients are received through rainfall, river flow, the weathering of rocks and soil and from human activities, such as sewage dumping. Nutrient enrichments also occur through internal physical processes such as mixing and upwelling that resus-pend nutrients from deeper regions.
As plants in the photic zone grow and reproduce, they are consumed by herbivores, which excrete their wastes into the water column. These wastes and other organic particles then rain down into the lower regions and eventually settle into the sediment. During periods of resuspension, such as remixing and upwelling, some of these nutrient-rich materials are brought back up to the photic zone. Remixing refers to processes whereby the water of a lake is thoroughly mixed from top to bottom, usually by the force of wind.
Upwellings can sometimes occur in cool lakes with warm underground springs, but they are much more important in oceans. An upwelling is an area in the ocean where the deeper, nutrient-rich waters are brought to the surface. Oceanic upwellings can be caused when the wind tends to blow in a consistent direction across the surface of the ocean. This causes the water to pile up at the lee end of the wind’s reach and, through the sheer weight of the accumulation, pushes down on the deeper volumes of water at the thick end. This pushing causes the deeper, nutrientrich water to rise to the surface back at the region where the winds began.
Upwellings can also be caused by deep ocean currents that are driven upwards because of differences in water temperatures. Such upwellings tend to be very extensive. Upwellings can also occur on a short-term basis when underwater uplands and seamounts force deep currents to the surface. Regardless of the origin of the resuspension event, these cooler, nutrient-rich waters stimulate the productivity of phytoplankton in the photic zone. Photic zones that are replenished with nutrients by either upwellings and or remixing events tend have very high primary production.
Light is essential to photosynthesis. The depth to which light penetrates a water column can vary substantially in space and time. The depth of the photic zone can vary from a few centimeters to several hundred meters. Sunlight is scattered and absorbed by particles and dissolved organic matter in the water column, and its intensity in water decreases with depth. In some cases, when nutrient concentrations are high, the photic zone becomes shallower. This is because the nutrients stimulate the growth of phytoplankton, and these cells then absorb more of the sunlight entering the water column and shade the layers below. Other areas may have very deep photic zones because the nutrient concentration is very small and therefore, the growth of primary producers is limited.
The ideal convergence of sufficient nutrients and sunlight occurs in relatively few areas of our oceans and lakes. These areas are, however, extremely productive. For example, areas off the coasts of Peru, northern Chile, eastern Canada, and Antarctica are responsible for much of the fish production of the world.
Abyssal zone— Volume of water near the bottom of the ocean where there is no sunlight, usually below 6,562 ft (2,000 m).
Compensation point— The point at which the rate of photosynthesis just equals the rate of respiration by phytoplankton. This is the lower limit of the photic zone.
Eutrophication— The enrichment of natural water bodies through the addition of nutrients, usually phosphate and/or nitrate, leading to an excessive growth of phytoplankton.
Hadal zone— The deepest layer of the ocean, below 19,686 ft (6,000 m).
Phytoplankton— Microscopic plants having no or little ability to move themselves, and therefore are subject to dispersal by water movement.
Primary production— The production of organic matter (biomass) by green plants through photosynthesis.
Profundal zone— Zone below the photic zone where there is some light but not enough to support photosynthesis.
Research in the photic zone is focused on three main priorities: eutrophication of water bodies, fundamental food web research, and the understanding of nutrient cycling. Eutrophication is the enrichment of water bodies through the addition of nutrients, often leading to excessive phytoplankton growth. Eutrophication is a well understood process, but it remains as a serious problem in much of the world.
Another important area is research into basic food webs. Many things are still to be discovered regarding the relative roles of species within aquatic food webs. The recent closure of the fisheries off eastern Canada exemplifies the importance of basic understanding of food webs in productive photic zones.
A third area of research within the photic zone involves nutrient cycling within water bodies. Especially in oceans the movement of particles and nutrients by water currents are not well understood. The connections among wind, ocean currents, and global weather patterns are at the forefront of current research.
See also Ocean zones.
Cousteau, Jacques-Yves. The Ocean World of Jacques Cousteau: Window in the Sea. World Publishing Company, 1973.
Garrison, Tom. Oceanography: An Invitation to Marine Science. 5th ed. Stamford, CT: Thompson/Brooks Cole, 2004.
Thomas, David et al. eds. Phytoplankton Productivity: Carbon Assimilation in Marine and Freshwater Ecology. Oxford, U.K.: Blackwell Science Ltd., 2002.
"Photic Zone." The Gale Encyclopedia of Science. . Encyclopedia.com. (October 18, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/photic-zone-0
"Photic Zone." The Gale Encyclopedia of Science. . Retrieved October 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/photic-zone-0
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