Deep chlorophyll maximum
[3] 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.
[2][5] 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.
[11] 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.
[7][11] The vertical mixing of limiting nutrients across the thermocline is a key process in supporting the deep water chlorophyll maximum.
[16] In shallow seasonally stratified seas boundary layer processes can also drive mixing of limiting nutrients across the thermocline.
[17] The formation of a DCM correlates with a number of biological processes,[6] affecting nutrient cycling for local heterotrophic bacteria[9] and composition of specialized phytoplankton.
[2][6][9] To adapt to low light conditions, some phytoplankton populations have been found to have increased amounts of chlorophyll counts per cell,[2][18][19] which contributes to the formation of the DCM.
[3] Rather than an increase of overall cell numbers, seasonal light limitation or low irradiance levels can raise the individual cellular chlorophyll content.
[6][18] As depth increases within the mixing zone, phytoplankton must rely on having higher pigment counts (chlorophyll) to capture photic energy.
[18] 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.
Dependent on factors like nutrients and available light, some phytoplankton species will intentionally move to different depths to fulfill their physiological requirements.
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.
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.
[5][20] Lake Tahoe represents a chlorophyll gradient similar to that of oligotrophic areas,[21] such that the depth of the region is dependent on seasonal fluctuations.
During the spring months, the DCM coincides with the upper surface of the nitracline,[21][31] making the water nutrient-rich for diatoms Cyclotella striata and chrysophytes Dinobryon bavaricum to thrive in.
[31] Similar to the chlorophyll structures found in oceans,[5] the DCM becomes incredibly fluid and variable, such that certain phytoplankton species (diatoms Synedra ulna, Cyclotella comta and green flagellates) begin to dominate, despite being absent during the spring productivity period.
[4][11] The high rate of primary production in the DCM facilitates nutrient cycling to higher trophic levels in the mixed layer.
[7][9] 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.
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.
