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The photic zone or sunlight (or sunlit) zone is the uppermost layer of the ocean that receives sunlight enabling it to perform photosynthesis. It undergoes a series of physical, chemical, and biological processes that supply nutrients into the upper water column. The photic zone is home to the majority of marine life due to its location.
Photosynthesis in photic zone
In the photic zone, the photosynthesis rate exceeds to respiration rate it is due to the abundant solar energy which is used as a food source for primary producers such as phytoplankton. These phytoplankton grow extremely quickly because of sunlight's heavy influence enabling it to be produced at a fast rate. In fact, ninety five percent of photosynthesis occurs in the photic zone. Therefore if we go deeper beyond the photic zone such as into the compensation point, there is little to no phytoplankton because of the insufficiency of sunlight. The zone which extends from the base of the euphotic zone to about 200 metres is sometimes called the disphotic zone.
Life in photic zone
Ninety percent of marine life lives in the photic zone which is approximately two hundred meters deep. This includes phytoplankton which include dinoflagellates, diatoms, cyanobacteria, coccolithophorids, and cryptomonads. Others feature zooplankton, the consumers of the photic zone. They are carnivorous meat eaters and herbivorous plant eaters. Next, copepods are the small crustaceans distributed everywhere in the photic zone and are actually the biggest group of animals on the planet. Finally there are nekton, which are the largest and the most obvious animals in the photic zone, but their quantity is the smallest among all the groups. The depth of the photic zone depends on the transparency of water. If the water is on the clearer side, the photic zone can become very deep. If it is very murky, it can be only fifty feet deep.
Photic zone depth
The depth is roughly equivalent to one percent of the surface incident radiant energy penetrates. Accordingly, its thickness depends on the extent of light attenuation in the water column but as incoming light at the surface can vary widely, this says little about the net growth of phytoplankton. Typical euphotic depths vary from only a few centimetres in highly turbid eutrophic lakes, to around 200 meters in the open ocean. It also varies with seasonal changes in turbidity, which can be strongly driven by phytoplankton concentrations such that the depth of the photic zone often decreases as primary production increases. Moreover, the respiration rate is actually greater than the photosynthesis rate. The reason why phytoplankton production is so important is because it plays a prominent role when interwoven with other food webs.
Nutrients uptake in the photic zone
Due to biological uptake, the photic zone has relatively low layers of nutrient concentrations. As a result, phytoplankton don’t receive enough nutrients due to high water-column stability. The spatial distribution of organisms can be controlled by a number of factors including physical: temperature, hydrostatic pressure, turbulent mixing such as the upward turbulent flux of inorganic nitrogen across the nutricline, as well as chemical: oxygen, trace elements, as well as biological: grazing, migrations. Upwelling carries nutrients from the deep waters into the photic zone to strengthen phytoplankton growth. Processes including resuspension; the remixing and upwelling eventually bring nutrient rich wastes back into the photic zone. The Ekman transport additionally brings more nutrients to the photic zone. Nutrient pulse frequency affects the phytoplankton competition. Photosynthesis produces ninety percent of Earth’s gaseous oxygen and phytoplankton, playing a major role, produces more of it. Being the first link in the food chain, what happens to phytoplankton creates a rippling effect to other species. Besides phytoplankton, many other animals also take home in this zone and utilize these nutrients. Majority of ocean life occur in the photic zone, the smallest ocean zone in water volume. It can be seen that the photic zone, although small, has a large impact to those who reside in it.
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