The deep chlorophyll maximum (DCM), also called the subsurface chlorophyll maximum,
is the region below the surface of water with the maximum concentration of
chlorophyll
Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
. 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
Stratification may refer to:
Mathematics
* Stratification (mathematics), any consistent assignment of numbers to predicate symbols
* Data stratification in statistics
Earth sciences
* Stable and unstable stratification
* Stratification, or st ...
.
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 cycle
A biogeochemical cycle (or more generally a cycle of matter) is the pathway by which a chemical substance cycles (is turned over or moves through) the biotic and the abiotic compartments of Earth. The biotic compartment is the biosphere and the ...
s.
Measurements
![Ctd hg](https://upload.wikimedia.org/wikipedia/commons/a/a0/Ctd_hg.jpg)
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
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
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
Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
fluorescence. Collected water samples can be used to determine
phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
cell counts. These measurements can then be converted into chlorophyll concentrations, phytoplankton
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
, 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
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 ter ...
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
![Chlorophyll maximum](https://upload.wikimedia.org/wikipedia/commons/2/29/Chlorophyll_maximum.png)
Light attenuation factors have been shown to be quite predictive of the DCM depth, since the
phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
present in the region require sufficient sunlight for growth,
resulting in a DCM that is generally found in the
euphotic zone
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
. However, if the phytoplankton population has adapted to lower light environments, the DCM can also be located in the
aphotic zone
The aphotic zone (aphotic from Greek prefix + "without light") is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. Above the aphot ...
.
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 (chlorophyll) 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
Vertical migration, or movement of phytoplankton within the
water column
A water column is a conceptual column of water from the surface of a sea, river or lake to the bottom sediment.Munson, B.H., Axler, R., Hagley C., Host G., Merrick G., Richards C. (2004).Glossary. ''Water on the Web''. University of Minnesota-D ...
, 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
A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...
and
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
, is to regulate their own buoyancy to move through the water column. Other species such as
dinoflagellates
The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates a ...
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
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates of ...
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
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
present in the DCM varies significantly with geographical location, season, and depth.
The species of
phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
Ph ...
present in the DCM varies with depth due to varying
accessory pigmentation. Some phytoplankton species have
accessory pigments Accessory pigments are light-absorbing compounds, found in photosynthetic organisms, that work in conjunction with chlorophyll ''a''. They include other forms of this pigment, such as chlorophyll ''b'' in green algal and higher plant antennae, w ...
, 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
The epilimnion or surface layer is the top-most layer in a thermally stratified lake. It sits above the deeper metalimnion and hypolimnion. It is typically warmer and has a higher pH and higher dissolved oxygen concentration than the hypolimnion ...
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 Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...
, the most abundant phytoplankton present are
coccolithophorids
Coccolithophores, or coccolithophorids, are single celled organisms which are part of the phytoplankton, the autotrophic (self-feeding) component of the plankton community. They form a group of about 200 species, and belong either to the kingd ...
, flagellates, and
dinoflagellates
The dinoflagellates (Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates a ...
. The Southeastern Mediterranean has a similar composition, where coccolithophorids and monads (nano- and
picoplankton
Picoplankton is the fraction of plankton composed by cells between 0.2 and 2 μm that can be either prokaryotic and eukaryotic phototrophs and heterotrophs:
* photosynthetic
* heterotrophic
They are prevalent amongst microbial plankton communit ...
) make up the majority of the phytoplankton community in the DCM.
In the
Indian Ocean
The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...
, the most abundant phytoplankton present in the DCM are
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
,
prochlorophytes, coccolithophorids, dinoflagellates and diatoms.
In the
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
, dinoflagellates are the main phytoplankton species present in the DCM at and below the
pycnocline
A pycnocline is the Cline (hydrology), cline or layer where the density gradient () is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, temperature and salinity differences, wind, Coriolis effec ...
. 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
The epilimnion or surface layer is the top-most layer in a thermally stratified lake. It sits above the deeper metalimnion and hypolimnion. It is typically warmer and has a higher pH and higher dissolved oxygen concentration than the hypolimnion ...
.
Lake Superior
Lake Superior in central North America is the largest freshwater lake in the world by surface areaThe Caspian Sea is the largest lake, but is saline, not freshwater. and the third-largest by volume, holding 10% of the world's surface fresh wa ...
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
A diatom (New Latin, Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group com ...
, 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
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have fro ...
(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 phosphorus (P) 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 (P) concentrations.
Lake Tahoe
In
Lake Tahoe
Lake Tahoe (; was, Dáʔaw, meaning "the lake") is a Fresh water, freshwater lake in the Sierra Nevada (U.S.), Sierra Nevada of the United States. Lying at , it straddles the state line between California and Nevada, west of Carson City, Nevad ...
, 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, Cyclotella 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.
Lakes of the North Patagonian Andean Region
Lakes of the North Patagonian Andean Region can be divided into larger deeper lakes and smaller shallow lakes. Although these lakes are found in the same region, they exhibit different conditions leading to varying DCM species compositions. The large deep lakes are ultra-oligotrophic, as they are very transparent and contain low DOC and nutrients.
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 A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...
ciliate
The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...
''Ophrydium naumanni'' were dominant. Their phototrophic abilities come from their
endosymbiotic
An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship.
(The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...
algae ''
Chlorella
''Chlorella'' is a genus of about thirteen species of single-celled green algae belonging to the division Chlorophyta. The cells are spherical in shape, about 2 to 10 μm in diameter, and are without flagella. Their chloroplasts contain the ...
'', which are strong competitors in poor light conditions.
In addition, the ciliates can undergo
phagotrophy
Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ca ...
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
Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
role in harbouring much of the world's
primary production
In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...
, and in nutrient cycling. In
oligotrophic
An oligotroph is an organism that can live in an environment that offers very low levels of nutrients. They may be contrasted with copiotrophs, which prefer nutritionally rich environments. Oligotrophs are characterized by slow growth, low rates of ...
waters, like the
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
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
A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
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
Biogeochemistry is the scientific discipline that involves the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment (including the biosphere, the cryosphere, t ...
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
The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it i ...
.
References
{{DEFAULTSORT:Deep Chlorophyll Maximum
Oceanography
Biological oceanography
Limnology