Benthic and Pelagic zones

Any body of water, a lake or ocean, can be divided into two main zones: the benthic and pelagic zone. The zones are categorized according to depth within a waterbody. The benthic zone refers to the very bottom layer of water in contact with the ocean floor, and the pelagic zone refers to the upper layers of water including the layer in contact with the atmosphere. The location of the benthic zone varies according to the depth of the water body: in deep oceans, it will be thousands of metres below the surface, but along continental shelves it will be just a few metres below. Both zones have distinct characteristics. The benthic zone is usually so deep that there is no sunlight, cold temperatures, low levels of oxygen and a characteristically high level of pressure. With these specific conditions, a unique community of organisms have evolved, called benthic flora and fauna (see definition for benthic flora and fauna). The benthic zone makes up about half of the Earth’s surface and is full of life, but it remains largely unexplored. Deep-sea mining presents a significant threat to life in the benthic zone by, for example, smothering and burying organisms where the sediment plume settles (see definition for sediment plumes), and crushing or dispersing them with mining devices. The pelagic zone has important functions such as oxygen absorption, heat absorption and food production. Most of life present in the ocean can be found in the pelagic zone. Pelagic communities are also threatened by deep-sea mining, by the formation of near-surface plumes (see definition of sediment plumes), which risk clogging the filter feeding apparatus of zooplankton (see definition of zooplankton), as well as blocking sunlight from photosynthetic organisms.