Deep chlorophyll maximum
Chlorophyll maximum layers, referred in the literature as deep chlorophyll maximum layers (DCM) or subsurface chlorophyll maximum layers (SCM), are common features of vertical stratified water columns. The term DCM is broadly used to describe a subsurface maxima of chlorophyll concentration. These are found throughout oceans, lakes, and estuaries around the world at varying depths, thicknesses, intensities, composition, and time of year.
Location and Formation
The DCM depth varies both temporally and spatially; in many tropical gyres, this is a permanent structure within the water column, while in more temperate waters this feature can be seasonally formed. Many subtropical and tropical gyres, with minimal adsorption coefficients, contain strongly shade-adapted flora in the deeper euphotic zones forming the deep chlorophyll maximum. In this zone, generally around 100 m in tropical gyres, the net photosynthetic rate is still positive due to large concentrations of chlorophyll per cell resulting from shade adaptation.
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%. DCM’s can also be formed below the euphotic zone, where light levels are >1% and photosynthesis cannot actually occur. These layers are formed through purely physical processes due to the subduction of surface waters.
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. This flora is not only the shade-acclimated plankton from higher up in the euphotic zone, but also includes widespread and populous species, such as Prochlorococcus. Through liquid chromatographic techniques used to distinguish difference in photosynthetic pigments, it was recently found that more than half of the pigment in DCM is divinyl chlorophyll a, which implies a dominance of Prochlorococcus in primary production.
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