Deep chlorophyll maximum

The DCM is often located tens of meters below the surface, and cannot be observed by using traditional satellite remote sensing methods. Estimates of primary productivity are often made via these remote sensing methods coupled with statistical models, though these statistical calculations may not have accurately included production in the DCM. The DCM of a study area can be determined in-situ through the use of an underwater instrument (CTD rosette with niskin bottles) to measure various parameters such as salinity (including dissolved nutrients), temperature, pressure, and chlorophyll fluorescence. Collected water samples can be used to determine phytoplankton cell counts. These measurements can then be converted into chlorophyll concentrations, phytoplankton biomass, and phytoplankton productivity. Another way to estimate primary productivity in the DCM is to create a simulation of the DCM formation in a region by making a 3D model of the region. This can be done if sufficient hydrodynamic and biogeochemical data exists for that ocean region. == Location and formation ==

Since its initial discovery, oceanographers have presented various theories to explain the formation of deep chlorophyll maxima. Abiotic factors In-situ studies have determined that the depth of DCM formation is primarily dependent on light attenuation levels, and the depth of the nutricline, internal tides and entrainment through the deepening of the surface mixed layer. In shallow seasonally stratified seas boundary layer processes can also drive mixing of limiting nutrients across the thermocline. Biotic factors The formation of a DCM correlates with a number of biological processes, and composition of specialized phytoplankton. Adaptations to light levels Light attenuation factors have been shown to be quite predictive of the DCM depth, since the phytoplankton present in the region require sufficient sunlight for growth, Vertical migration Vertical migration, or movement of phytoplankton within the water column, contributes to the establishment of the DCM due to the diversity of resources required by the phytoplankton. Dependent on factors like nutrients and available light, some phytoplankton species will intentionally move to different depths to fulfill their physiological requirements. == Composition ==

The composition of microorganisms present in the DCM varies significantly with geographical location, season, and depth. The species of phytoplankton present in the DCM varies with depth due to varying accessory pigmentation. Some phytoplankton species have accessory pigments, compounds that have adapted them to gather light energy from certain wavelengths of light, The difference in phytoplankton composition between the epilimnion layer and the DCM are consistent throughout several bodies of water. The DCM tends to harbour more flagellated organisms and cryptophytes, whereas the epilimnion layer tends to have a larger centric diatom abundance. Oceans In the Northwestern Mediterranean, the most abundant phytoplankton present are coccolithophorids, flagellates, and dinoflagellates. The Southeastern Mediterranean has a similar composition, where coccolithophorids and monads (nano- and picoplankton) make up the majority of the phytoplankton community in the DCM. below the surface. Although the epilimnion and DCM are neighbouring layers of water, the species composition of the epilimnion and the DCM differ almost entirely. especially in Lake Superior during stratified times, this phenomenon may indicate that phytoplankton in the DCM is more enriched with phosphorus than in the epilimnion. The higher availability of phosphorus may have allowed more phytoplankton to prefer the DCM even with the lower amount of light compared to the epilimnion. Lake Tahoe In Lake Tahoe, the DCM is unique, as the depth of the region is much lower than normal, present at around 90–110 metres below the surface. Typically, DCM's are found closely below the thermocline, which is present at around 30–40 metres. making the water nutrient-rich for diatoms Cyclotella striata and chrysophytes Dinobryon bavaricum to thrive in. During the summer months, the DCM deepens, and productivity within the layer almost becomes entirely light dependent. Unlike the homogenous thermal profile of the shallower lakes, deeper lakes undergo strong thermal stratification during the late spring and summer. The two lake types also differ in light attenuation coefficients: it is lower in the transparent deeper lakes, which means more light is able to penetrate though. As such, the main difference between the two lake types that was found to contribute to the DCM community is the light climate. Shallow lakes were found to contain greater concentrations of dissolved yellow particles than the deeper lakes. As a result, for deeper lakes, maximum absorption wavelengths were mainly at the red end of the spectra, whereas shallow lakes exhibited green and blue absorption wavelengths in addition to red. At the DCM region of the large deep lakes, the mixotrophic ciliate Ophrydium naumanni were dominant. Their phototrophic abilities come from their endosymbiotic algae Chlorella, which are strong competitors in poor light conditions. In addition, the ciliates can undergo phagotrophy to obtain other necessary elements. In shallower lakes, O. naumanni were found to be absent, likely due to higher levels of competition with phytoplankton and turbulence intensity. == Ecological implications ==

The DCM plays an important ecological role in harbouring much of the world's primary production, and in nutrient cycling. In oligotrophic waters, like the North Sea and the Mediterranean Sea, the DCM is where over half of the overall primary production occurs due to phytoplankton growth. The high rate of primary production in the DCM facilitates nutrient cycling to higher trophic levels in the mixed layer. The DCM forms at the same depth as the nuricline, so phytoplankton in the DCM can access nutrients coming up from the deep ocean. The phytoplankton in the DCM can then cycle back up the water column providing nutrients for heterotrophs in the mixed layer. Since the DCM environment plays a fundamental role in primary productivity, it can be associated with many aspects of aquatic predator-prey interactions, energy and biomass flow, and biogeochemical cycles. Significant export of organic material from the water column occurs due to the DCM, as heterotrophs consume phytoplankton in the DCM and the fecal matter of grazers sinks to the deep ocean. The DCM is an important food source for secondary producers as it has a relatively high concentration of primary producers at one region of the water column. This makes it easier and faster for grazers to find and consume phytoplankton which in turn increases the rate of movement of energy through the trophic levels. == References ==