Thin layers (oceanography)
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Thin layers are concentrated aggregations 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 ...
and
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
in coastal and offshore waters that are vertically compressed to thicknesses ranging from several centimeters up to a few meters and are horizontally extensive, sometimes for kilometers. Generally, thin layers have three basic criteria: 1) they must be horizontally and temporally persistent; 2) they must not exceed a critical threshold of vertical thickness; and 3) they must exceed a critical threshold of maximum concentration. The precise values for critical thresholds of thin layers has been debated for a long time due to the vast diversity of plankton, instrumentation, and environmental conditions. Thin layers have distinct biological, chemical, optical, and acoustical signatures which are difficult to measure with traditional sampling techniques such as nets and bottles. However, there has been a surge in studies of thin layers within the past two decades due to major advances in technology and instrumentation. Phytoplankton are often measured by optical instruments that can detect fluorescence such as
LIDAR Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
, and zooplankton are often measured by acoustic instruments that can detect acoustic backscattering such a
ABS
These extraordinary concentrations of plankton have important implications for many aspects of
marine ecology Marine ecosystems are the largest of Earth's aquatic ecosystems and exist in waters that have a high salt content. These systems contrast with freshwater ecosystems, which have a lower salt content. Marine waters cover more than 70% of the surf ...
(e.g., phytoplankton growth dynamics, zooplankton grazing, behaviour, environmental effects,
harmful algal bloom A harmful algal bloom (HAB) (or excessive algae growth) is an algal bloom that causes negative impacts to other organisms by production of natural algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are sometimes ...
s), as well as for ocean optics and acoustics. Zooplankton thin layers are often found slightly under phytoplankton layers because many feed on them. Thin layers occur in a wide variety of ocean environments, including estuaries, coastal shelves, fjords, bays, and the open ocean, and they are often associated with some form of vertical structure in the water column, such as
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 ...
s, and in zones of reduced flow.


Criteria


Persistence

Thin layers persist from hours to weeks while other small-scale patches of plankton exist for minutes. The presence of nutrients as well as coastal fronts, eddies, and upwelling zones greatly increase the persistence of thin layers. One of the main criteria for an aggregation of plankton to be considered a thin layer is that the increased concentration at a certain depth of the water column must appear in subsequently measured profiles. However, thin layers are dynamic and horizontally extensive so their persistence cannot be defined using multiple measurements at only one location. A study on the Karenia brevis algae responsible for more recent and increasingly longer
red tide A harmful algal bloom (HAB) (or excessive algae growth) is an algal bloom that causes negative impacts to other organisms by production of natural phycotoxin, algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are ...
blooms shows that the cellular gene expression patterns are extremely diverse which means that this particular species of plankton are more resilient because they adapt well to changing conditions. Studies also indicate that red tide blooms are often terminated by interactions with other microbes such as viruses and bacteria that may either compete for the same nutrients or adversely impact the algal cells.


Thickness

Some studies have considered the maximum critical threshold for vertical thickness of thin layers as three meters, but more recent data has shown that the criteria can be relaxed to five meters. The horizontal extents of thin layers can reach tens of kilometers, and their horizontal to vertical aspect ratio is usually at least 1000:1.


Intensity

The intensity of a thin layer refers to the maximum concentration of the plankton within the layer relative to the background and the water column. Thin layer concentrations can range between three and 100 times more than the background and up to 75% of the total biomass in the water column.


Formation


Buoyancy

Thin layers of non-motile phytoplankton tend to collect at boundaries of strong vertical gradients in salinity (
halocline In oceanography, a halocline (from Greek ''hals'', ''halos'' 'salt' and ''klinein'' 'to slope') is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity (in concert with temp ...
s), temperature (
thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
s), and density (
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 ...
s) which often coincide because they are directly proportional. These particular thin layers are formed by sinking non-motile phytoplankton reaching a neutral buoyancy at a
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 ...
, and the stifling of vertical turbulent dispersion at these depths. Other studies have shown that gradients in nutrients (nutriclines) also contribute to the formation of thin layers.


Vertical Migration

Many zooplankton normally exhibit a diel vertical migration ( DVM) pattern that dictates their depth in the water column based on the time of day. Phytoplankton require sunlight for photosynthesis and protein production, but they are not primarily attracted to light. This is evident by their single move up near the surface prior to sunrise and single move down into deeper waters prior to sunset. Their collective movements may result in the aggregation that form thin layers. These regular movements are thought to be governed by an internal clock in normal nutrient concentrations However, they have also been observed to migrate irregularly when nutrient concentrations are higher or lower than normal.


Chemotaxis

Motile plankton have been observed to be able to detect and swim towards higher nutrient concentrations and/or light intensities. This mechanism is called
chemotaxis Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...
and is partly responsible for the formation of thin layers at depths where nutrients are abundant. Another mechanism specific to dinoflagellates is called helical klinotaxis where the algal cell's ability to respond to both positive and negative chemosensory signals is crucial to their motility. If dinoflagellates were not capable of both positive and negative chemotaxis, they would not navigate successfully due to the nature of the transverse and longitudinal flagella causing rotating and translating motions, respectively.


Eddies, Filaments, and Fronts

Another obvious cause of thin layers is the horizontal transport of waters with high plankton concentration into waters with lower concentrations. In this case, upwelled intrusions of nutrient-rich slope water are suggested to be the cause of algal blooms and some thin layers. However, thin layers have been observed to form at the boundaries of more complex fluid mechanisms such as eddies, filaments, and fronts. These thin layers were located at the transition layer, a region of maximum shear and stratification at the base of the mixed layer.


Straining by Shear

A fluid mechanism that contributes to the formation of thin layers is the straining of fluid by the sheared velocity profile which causes the fluid to tilt and disperse horizontally. If a patch of plankton is located at the fluid being sheared, a thin layer could be formed by the straining of the patch by velocity shear. The four phases of plankton distributions caused by straining are: 1) tilting, 2) shear-thinning, 3) decay, and 4) shear-dispersion (dissipation).


Gyrotactic Trapping

A sharp change in flow velocities can also prevent some motile plankton from orienting themselves or swimming vertically. This fluid mechanism is called gyrotactic trapping.


See also

Critical Scales and Thin Layers
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 ...
Zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
Red tide A harmful algal bloom (HAB) (or excessive algae growth) is an algal bloom that causes negative impacts to other organisms by production of natural phycotoxin, algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are ...
Karenia brevis
Algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompas ...
Dinoflagellate 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 are ...
Halocline In oceanography, a halocline (from Greek ''hals'', ''halos'' 'salt' and ''klinein'' 'to slope') is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity (in concert with temp ...
Thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
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 ...
Chemotaxis Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemica ...


References

{{reflist Biological oceanography Planktology Aquatic ecology