Deep chlorophyll maximum
A deep chlorophyll maximum (DCM) is a subsurface maximum in the concentration of chlorophyll in the ocean or a lake. A DCM is not always present—sometimes there is more chlorophyll at the surface than at any greater depth—but it is a common feature of most aquatic ecosystems. The depth, thickness, intensity, composition, and persistence of DCM's vary widely.
Location and formation
Throughout much of the tropical ocean, the DCM is a permanent structure, while in temperate and polar waters it is a more variable feature. In some cases, the DCM may coincide with a maximum in phytoplankton biomass; in others, it is made up of strongly shade-adapted cells, which contain a high ratio of chlorophyll to biomass, and is well separated vertically from the location of maximum biomass.
In several studies, the DCM layer was found to be located in the thermocline, adjacent to the nutracline, at the bottom of the euphotic layer, where light attenuation ranges from ~1-2% up to ~10% of that at the surface. A DCM can also exist below the euphotic zone, where less than 1% of the surface light remains and little photosynthetic growth is possible; these layers are formed by subduction of surface waters, or sinking of cells.
The plankton community within the DCM is highly variable and diverse. One study in the western pacific gyre found over 223 major taxa, including alga, coccolithophorids, dinoflagellates, silicoflagellates, and diatoms.
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