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EOSC 475 Group 5: We'll use this sandbox to draft our article!

Actual wiki page: Deep chlorophyll maximum

Deep Chlorophyll Maximum
The deep chlorophyll maximum (DCM), also called the subsurface chlorophyll maximum, is the region below the surface of a body of water with the maximum concentration of chlorophyll. 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, especially in regions of strong thermal stratification. The depth, thickness, intensity, composition, and persistence of DCMs vary widely. The DCM generally exists at the same depth as the nutricline, the region of the ocean where the greatest change in the nutrient concentration occurs with depth.

A common way of determining the DCM is through the use of a CTD rosette, and underwater vehicle that measures various parameters of water at specific depths. The location and formation of the DCM depends on multiple factors, such as the resident organisms' nutritional needs and light availability. Some organisms have adapted to lower levels of light through increasing its cellular chlorophyll amounts, and others have adapted by migrating vertically with varying nutrient and light levels. The DCM species composition vary with water chemistry, location, seasonality, and depth. Not only is there a difference in DCM species composition between oceans and lakes, variation is also present within different oceans and lakes. Because the DCM holds much of the world's primary productivity, it plays a significant role in nutrient cycling, the flow of energy, and biogeochemical cycles.

Measurements
The DCM is often located tens of meters below the surface, and cannot be observed by using traditional satellite remote sensing methods. As estimates of primary productivity are often made via remote sensing, calculations may not have included the those occurring in the DCM. Instead, the DCM of a study area can be determined in-situ through the use of an underwater vehicle (CTD rosette with niskin bottles) to measure various parameters such as temperature, salinity (including dissolved nutrients), 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.

Location and Formation
Since its initial discovery, oceanographers have presented various theories to explain the formation of deep chlorophyll maxima.

Biological Approach
The formation of a DCM correlates with a number of biological processes, affecting nutrient cycling for local heterotrophic bacteria and generation of specialized phytoplankton.

Adaptations to light levels
Certain phytoplankton communities have adapted to functioning in low light conditions and reside at greater depths, increasing overall phytoplankton counts. The accumulation of a large phytoplankton body is coupled to the abundance of chlorophyll, such that photosynthesis would generate an influx in biomass present. Light attenuation factors prove to be quite predictive of the DCM depth, such that the phytoplankton present in the region require sufficient sunlight for growth, resulting in a DCM that is generally found in the euphotic zone. However, if the phytoplankton population has adapted to lower light environments, the DCM can also be located in the aphotic zone.

Some phytoplankton populations have been found to have increased amounts of chlorophyll counts per cell , resulting in the formation of the DCM region. Rather than an increase of overall cell numbers, seasonal light limitation or low irradiance levels can raise the individual cellular chlorophyll content. As depth increases within the mixing zone, phytoplankton must rely on having higher pigment counts (chlorophyll) to capture photic energy.

In addition, compared to shallower regions of the mixing zone, the DCM has high nutrient concentrations and/or lower respiratory, grazing, and death rates which further promote phytoplankton cell production.

Vertical migration
Vertical migration, or movement of phytoplankton within the water column, establishes the DCM due to the diversity in which resources are needed. Dependent on factors like nutrients and available light, some phytoplankton species will intentionally move to greater depths, in order to fulfill their physiological requirements. Other adaptations of phytoplankton, such as certain species of diatoms and cyanobacteria, regulate their own buoyancy and move throughout the water column. This contributes to the formation of the DCM in areas where these species make up a significant proportion of the phytoplankton community. Generally these species are larger in size and are not found in significant abundance in nutrient poor regions, so these physiological aspects of phytoplankton contribute less to DCM formation in oligotrophic waters.

In lakes, the thickness of the DCM shows positive correlations with lake sizes. However, due to the great diversity of lakes, there is no constant factor for light and temperature that can be utilized to predict DCM depths across different lake types.

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, even in areas of low light penetration. To optimize light energy collection, phytoplankton will move to specific depths to access different wavelengths of visible 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.

In the Indian Ocean, the most abundant microorganisms present in the DCM are cyanobacteria such as prochlorophytes, coccolithophorids, dinoflagellates and diatoms.

In the North Sea, dinoflagellates are the main phytoplankton species present in the DCM at and below the pycnocline. In shallower parts of the DCM - above the pycnocline, dinoflagellates are also present, as well as nanoflagellates.

Lake Superior
The DCM of clear, stratified water is commonly found below the epilimnion. Lake Superior is the world’s largest freshwater lake, and in the summer, its DCM ranges approximately from 20 m and 35 m 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. These differences include the presence of less centric diatoms, more pennate diatoms, cryptophytes, and pyrrophytes at the DCM compared to the epilimnion layer. Additionally, the most significant difference in between these two layers is the abundance of Cyclotella comta, which occurs much less in the DCM. The most abundant species (********where?) are of the picocyanobacteria category.

Much more of the DCM community are flagellated (e.g. cryptophytes and pyrrophytes) compared to those of the epilimnion. As flagellated species are better swimmers, this could explain how they are able to reside at their desired depth, the DCM. Another factor for the development of the DCM is nutrient availability. It has been found that the DCM had a lower ratio of particulate organic carbon (POC) to phosphorous (P) than the epilimnion. Since phosphorous is one of the limiting factors for growth, especially in Lake Superior during stratified times , this phenomenon may indicate that phytoplankton at 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. On the other hand, the fact that the DCM has lower light availability could be used to argue that the lower POC:P ratio is due to light limitation rather than increased nutrient (P) concentrations.

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. Lake Tahoe represents a chlorophyll gradient similar to that of oligotrophic areas, such that the depth of the region is dependent on seasonal fluctuations. Due to these temporal shifts, especially between spring and summer, Lake Tahoe's phytoplankton communities undergo distinct changes. During the spring months, the DCM coincides with the upper surface of the nitracline, making the water nutrient-rich for diatoms Cyclotella striata and crysophytes Dinobryon bavaricum to thrive in. During the summer months, the DCM deepens, and productivity within the layer almost becomes entirely light dependent. Similar to the chlorophyll structures found in oceans, the DCM becomes incredibly fluid and variable, such that certain phytoplankton species (diatoms Synedra ulna, Cyclcotella comta and green flagellates) begin to dominate, despite being absent during the spring productivity period.

Overall, the phytoplankton community between the epilimnion and the DCM in Lake Tahoe differ with size. Both regions are abundant with diatoms; small diatoms (cell volume = 30.5μm³) such as Cyclotella stelligera, Synedra radians make up the majority in the epilimnion, while larger diatoms (cell volume = 416.3μm³) such as C.ocellata, Stephanodiscus alpinus, Fragilaria crotonensis, dominate the DCM.

Ecological Implications
The DCM plays an important ecological role in harbouring much of the world’s primary production. In oligotrophic waters, like the North Sea, the DCM is where over half of the overall primary production occurs due to phytoplankton growth. The DCM also facilitates nutrient cycling, such that during the summer, the nutricline deepens to a depth below the DCM. This means that only the phytoplankton in the DCM have access to nutrients, which eventually recycles back and provides nutrients for heterotrophs in the mixed layer. Significant export of organic material from the water column also occurs, as heterotrophs consume phytoplankton and their fecal matter sinks down the column.

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.