Deep chlorophyll maximum
The deep chlorophyll maximum, also called the subsurface chlorophyll maximum, is the region below the surface of water with the maximum concentration of chlorophyll. 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 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.
A common way of determining the DCM is through the use of a CTD rosette, an underwater instrument 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. 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 to measure various parameters such as salinity, 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, although thermal stratification also plays a role. In lakes, the thickness of the DCM layer is mainly controlled by the sizes and maximum depths of lakes.The DCM forms near the nutricline and the bottom of the photic zone. Phytoplankton growth in the DCM is limited by both nutrient and light availability, therefore either increased nutrient input, or increased light availability to the DCM can in turn increase the phytoplankton growth rate.
The location and formation of the DCM also depends on season. In the Mediterranean Sea, the DCM is present in the summer due to water stratification, and is rare in the winter due to deep mixing. The DCM can be present at shallower depths in the winter and early spring due to light limitation and higher nutrient availability in shallower regions due to mixing, and at lower depths during the summer and early fall as nutrients in the surface water are depleted by primary producers and stronger irradiance allows light to penetrate to greater depths.
The vertical mixing of limiting nutrients across the thermocline is a key process in supporting the deep water chlorophyll maximum. This mixing can be driven by a number of processes linked to wind driven oscillations, 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, affecting nutrient cycling for local heterotrophic bacteria 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, 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. The high chlorophyll concentration at the DCM is due to the high number of phytoplankton that have adapted to functioning in low light conditions.To adapt to low light conditions, some phytoplankton populations have been found to have increased amounts of chlorophyll counts per cell, which contributes to the formation of the DCM. 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 to capture photic energy. Due to the higher concentration of chlorophyll in the phytoplankton present, the DCM does not predict the depth of the biomass maximum in the same region.
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
, 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. A mechanism employed by certain phytoplankton, such as certain species of diatoms and cyanobacteria, is to regulate their own buoyancy to move through the water column. Other species such as dinoflagellates use their flagella to swim to their desired depth. This intentional movement of phytoplankton 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 make up the majority of the phytoplankton community in the DCM.In the Indian Ocean, the most abundant phytoplankton present in the DCM are cyanobacteria, 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.
Lakes
Lake Superior
The DCM of clear, stratified water is commonly found below the epilimnion. Lake Superior is one of the world's largest freshwater lakes, and in the summer, its DCM ranges from approximately 20 m to 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.Much more of the DCM community are flagellated 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 to phosphorus than the epilimnion. Since phosphorus is one of the limiting factors for growth, 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. 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 concentrations.