Marine snow
Encyclopedia
In the deep ocean, marine snow is a continuous shower of mostly organic detritus
falling from the upper layers of the water column. It is a significant means of exporting energy
from the light
-rich photic zone
to the aphotic zone
below. The term was first coined by the explorer William Beebe
as he observed it from his bathysphere. As the origin of marine snow lies in activities within the productive photic zone, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic
activity and ocean current
s.
), protist
s (diatom
s), fecal matter, sand, soot and other inorganic dust. Most trapped particles are more vulnerable to grazers than they would be as free floating individuals and can be classified as "olive green" or "gray body" cells, which are plant parts and degrading plant material. A majority of marine snow composition is actually made up of aggregates of smaller particles held together by a sugary mucus, transparent extracellular polysaccharides (TEPs). These are natural polymer
s exuded as waste products mostly by phytoplankton
and bacteria. Mucus
secreted by zooplankton
(mostly salps, appendicularians, and pteropods
) also contribute to the constituents of marine snow aggregates. These aggregates grow over time and may reach several centimeters in diameter, traveling for weeks before reaching the ocean floor.
Marine snow often forms during algal bloom
s. As algae accumulate, they aggregate or get captured in other aggregates, both of which accelerate the sinking rate. Aggregation and sinking is actually thought to be a large component of sources for algae loss from surface water. Most organic components of marine snow are consumed by microbes, zooplankton
and other filter-feeding animals within the first 1,000 metres of their journey. In this way marine snow may be considered the foundation of deep-sea mesopelagic
and benthic ecosystem
s: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. The small percentage of material not consumed in shallower waters becomes incorporated into the muddy "ooze" blanketing the ocean floor, where it is further decomposed through biological activity.
Marine snow aggregates exhibit characteristics that fit Goldman's "aggregate spinning wheel hypothesis". This hypothesis states that phytoplankton, microorganisms and bacteria live attached to aggregate surfaces and are involved in rapid nutrient recycling. Phytoplankton have been shown to be able to take up nutrients from small local concentrations of organic material (e.g. fecal matter from an individual zooplankton cell, regenerated nutrients from organic decomposition by bacteria. As the aggregates slowly sink to the bottom of the ocean, the many microorganisms residing on them are constantly respiring and contribute greatly to the microbial loop
.
. Aggregation theory outlines main mechanisms by which marine aggregates may form and are as follows:
Aggregation theory represents a two state system. At low cell concentrations aggregation is relatively unimportant and somewhat more unlikely. However, at higher cell concentrations it becomes increasingly important. A model has been proposed to characterize the formation of marine aggregates and the loss due to sinking:
Thus, aggregation of marine particles is more prevalent when cell and particle concentration is higher (e.g. algal blooms)
produced in the ocean
by primary production
that is not recycled (remineralised
) before it sinks into the aphotic zone
. Because of the role of export production in the ocean's biological pump
, it is typically measured in units of carbon
(e.g. mg C m-2
d-1
).
The fraction of primary production that is exported to the aphotic zone is generally higher when primary production occurs in short (seasonal) bursts, than when it occurs more evenly spread out across a year.
Because of the relatively long residence time of the ocean's thermohaline circulation
, carbon transported as marine snow into the deep ocean by the biological pump
can remain out of contact with the atmosphere for more than 1000 years. That is, when the marine snow is finally decomposed to inorganic nutrients and dissolved carbon dioxide
, these are effectively isolated from the surface ocean for relatively long time-scales related to ocean circulation. Consequently, enhancing the quantity of marine snow that reaches the deep ocean is the basis of several geoengineering
schemes to enhance carbon sequestration by the ocean. Ocean nourishment
and iron fertilisation seek to boost the production of organic material in the surface ocean, with a concomitant rise in marine snow reaching the deep ocean. As of yet, these efforts have not produced a sustainable fertilization that effectively transports carbon out of the system.
Increases in ocean temperatures, a projected indicator of climate change
, may result in a decrease in the production of marine snow due to the enhanced stratification
of the water column. Increasing stratification decreases the availability of phytoplankton nutrients such as nitrate
, phosphate
and silicic acid
, and could lead to a decrease in primary production and, thus, marine snow.
Marine snow has also begun to garner interest from microbiologists
, owing to the microbial communities associated with it. Recent research indicates transported bacteria may exchange gene
s with previously thought to be isolated populations of bacteria inhabiting the breadth of the ocean floor. In such an immense area there may be as yet undiscovered species tolerant of high pressures and extreme cold, perhaps finding use in bioengineering and pharmacy
.
Detritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
falling from the upper layers of the water column. It is a significant means of exporting energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
from the light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
-rich photic zone
Photic zone
The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...
to the aphotic zone
Aphotic zone
The aphotic zone 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% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone...
below. The term was first coined by the explorer William Beebe
William Beebe
William Beebe, born Charles William Beebe was an American naturalist, ornithologist, marine biologist, entomologist, explorer, and author...
as he observed it from his bathysphere. As the origin of marine snow lies in activities within the productive photic zone, the prevalence of marine snow changes with seasonal fluctuations in photosynthetic
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
activity and ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s.
Composition
Marine snow is made up of a variety of mostly organic matter, including dead or dying animals and plants (planktonPlankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
), protist
Protist
Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s (diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s), fecal matter, sand, soot and other inorganic dust. Most trapped particles are more vulnerable to grazers than they would be as free floating individuals and can be classified as "olive green" or "gray body" cells, which are plant parts and degrading plant material. A majority of marine snow composition is actually made up of aggregates of smaller particles held together by a sugary mucus, transparent extracellular polysaccharides (TEPs). These are natural polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s exuded as waste products mostly by phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
and bacteria. Mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
secreted by zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
(mostly salps, appendicularians, and pteropods
Pteropoda
Pteropoda, common name the pteropods, from the Greek meaning "wing-foot" is a term applied to what are now considered to be two separate taxonomic groups of specialized free-swimming pelagic sea snails and sea slugs, marine opisthobranch gastropods...
) also contribute to the constituents of marine snow aggregates. These aggregates grow over time and may reach several centimeters in diameter, traveling for weeks before reaching the ocean floor.
Marine snow often forms during algal bloom
Algal bloom
An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...
s. As algae accumulate, they aggregate or get captured in other aggregates, both of which accelerate the sinking rate. Aggregation and sinking is actually thought to be a large component of sources for algae loss from surface water. Most organic components of marine snow are consumed by microbes, zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
and other filter-feeding animals within the first 1,000 metres of their journey. In this way marine snow may be considered the foundation of deep-sea mesopelagic
Mesopelagic
The mesopelagic is that part of the pelagic zone that extends from a depth of 200 to 1000 metres below the ocean surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, where there is no light at all...
and benthic ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s: As sunlight cannot reach them, deep-sea organisms rely heavily on marine snow as an energy source. The small percentage of material not consumed in shallower waters becomes incorporated into the muddy "ooze" blanketing the ocean floor, where it is further decomposed through biological activity.
Marine snow aggregates exhibit characteristics that fit Goldman's "aggregate spinning wheel hypothesis". This hypothesis states that phytoplankton, microorganisms and bacteria live attached to aggregate surfaces and are involved in rapid nutrient recycling. Phytoplankton have been shown to be able to take up nutrients from small local concentrations of organic material (e.g. fecal matter from an individual zooplankton cell, regenerated nutrients from organic decomposition by bacteria. As the aggregates slowly sink to the bottom of the ocean, the many microorganisms residing on them are constantly respiring and contribute greatly to the microbial loop
Microbial loop
The microbial loop describes a trophic pathway in the marine microbial food web where dissolved organic carbon is returned to higher trophic levels via the incorporation into bacterial biomass, and coupled with the classic food chain formed by phytoplankton-zooplankton-nekton. The term microbial...
.
Aggregation
Aggregates begin as the colloidal fraction, which typically contains particles sized between 1 nm and several micrometers. The colloidal fraction of the ocean contains a large amount of organic matter unavailable to grazers. This fraction has a much higher total mass than either phytoplankton or bacteria but is not readily available due to size characteristics of the particles in relation to potential consumers. The colloidal fraction must aggregate in order to be more bioavailableBioavailability
In pharmacology, bioavailability is a subcategory of absorption and is used to describe the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs. By definition, when a medication is administered...
. Aggregation theory outlines main mechanisms by which marine aggregates may form and are as follows:
- Brownian motionBrownian motionBrownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...
- Brownian motion, also known as the Brownian Pump in this case, describes the interaction between individual particles in the colloid fraction. Particles in Brownian motion interact randomly due to the impact of solute molecules. These interactions lead to collision and aggregation of these small particles. The small aggregates then collide and aggregate with other aggregates and particles until the aggregate in question is several micrometers in diameter.
- Shear
- Once particles have aggregated to several micrometers in diameter, they begin to accumulate bacteria, since there is sufficient site space for feeding and reproduction. At this size it is large enough to undergo sinking. It also has the components necessary to fit the "aggregate spinning wheel hypothesis." Evidence for this has been found by Alldredge and Cohen (1987) who found evidence of both respiration and photosynthesis within aggregates, suggesting the presence of both autotrophic and heterotrophic organisms.
- Differential SettlingSettlingSettling is the process by which particulates settle to the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction exerted by that force...
- This form of aggregation involves particles sinking at different rates and their collision to form aggregates.
- DiffusiveDiffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
Capture- Diffusive capture describes a particle that is advected into the diffusion-limited boundary layer of another particle and is eventually captured/collides with that particle.
- Surface CoagulationCoagulationCoagulation is a complex process by which blood forms clots. It is an important part of hemostasis, the cessation of blood loss from a damaged vessel, wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
- Aggregates may also form from colloids trapped on the surface of rising bubbleBubble-Physical bubbles:* Liquid bubble, globule of one substance encased in another, usually air in a liquid* Soap bubble, a bubble formed by soapy water * Antibubble, a droplet of liquid surrounded by a thin film of gas-Arts and literature:...
s. For example, Kepkay et al. found that bubble coagulation leads to an increase in bacterial respiration since more food is available to them. In fact, a storm at sea could increase bacterial respiration by up to 36 fold for two to four hours.
- Aggregates may also form from colloids trapped on the surface of rising bubble
- FiltrationFiltrationFiltration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
- Particles and small organisms floating through the water column can become trapped within aggregates. Marine snow aggregates are porous, however, and some particles are able to pass through them.
- Bacterial Motility
- It is unclear how relative this mode of aggregation is to marine snow, but there have been limited observations of bacteria moving fast and far enough to capture colloidal particles.
Aggregation theory represents a two state system. At low cell concentrations aggregation is relatively unimportant and somewhat more unlikely. However, at higher cell concentrations it becomes increasingly important. A model has been proposed to characterize the formation of marine aggregates and the loss due to sinking:
- C1 is the concentration of the cells
- r is the radius of each cell
- G is the shearing rate
- α is the stickiness coefficient
- g is the growth rate.
Thus, aggregation of marine particles is more prevalent when cell and particle concentration is higher (e.g. algal blooms)
Study
Export production is the amount of organic matterOrganic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
produced in the ocean
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
by primary production
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
that is not recycled (remineralised
Remineralisation
In biogeochemistry, remineralisation refers to the transformation of organic molecules to inorganic forms, typically mediated by biological activity....
) before it sinks into the aphotic zone
Aphotic zone
The aphotic zone 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% of sunlight penetrates. Consequently, bioluminescence is essentially the only light found in this zone...
. Because of the role of export production in the ocean's biological pump
Biological pump
In oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.-Overview:...
, it is typically measured in units of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
(e.g. mg C m-2
Square metre
The square metre or square meter is the SI derived unit of area, with symbol m2 . It is defined as the area of a square whose sides measure exactly one metre...
d-1
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
).
The fraction of primary production that is exported to the aphotic zone is generally higher when primary production occurs in short (seasonal) bursts, than when it occurs more evenly spread out across a year.
Because of the relatively long residence time of the ocean's thermohaline circulation
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
, carbon transported as marine snow into the deep ocean by the biological pump
Biological pump
In oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.-Overview:...
can remain out of contact with the atmosphere for more than 1000 years. That is, when the marine snow is finally decomposed to inorganic nutrients and dissolved carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, these are effectively isolated from the surface ocean for relatively long time-scales related to ocean circulation. Consequently, enhancing the quantity of marine snow that reaches the deep ocean is the basis of several geoengineering
Geoengineering
The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...
schemes to enhance carbon sequestration by the ocean. Ocean nourishment
Ocean Nourishment
Ocean Nourishment is a type of geoengineering based on the purposeful introduction of nutrients to the upper ocean to increase marine food production and to sequester carbon dioxide from the atmosphere. Fertilization can also potentially create sulfur aerosols which reduce the rate of global warming...
and iron fertilisation seek to boost the production of organic material in the surface ocean, with a concomitant rise in marine snow reaching the deep ocean. As of yet, these efforts have not produced a sustainable fertilization that effectively transports carbon out of the system.
Increases in ocean temperatures, a projected indicator of climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
, may result in a decrease in the production of marine snow due to the enhanced stratification
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
of the water column. Increasing stratification decreases the availability of phytoplankton nutrients such as nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
, phosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
and silicic acid
Silicic acid
Silicic acid is a general name for a family of chemical compounds of the element silicon, hydrogen, and oxygen, with the general formula [SiOx4-2x]n...
, and could lead to a decrease in primary production and, thus, marine snow.
Marine snow has also begun to garner interest from microbiologists
Microbiology
Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...
, owing to the microbial communities associated with it. Recent research indicates transported bacteria may exchange gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s with previously thought to be isolated populations of bacteria inhabiting the breadth of the ocean floor. In such an immense area there may be as yet undiscovered species tolerant of high pressures and extreme cold, perhaps finding use in bioengineering and pharmacy
Pharmacy
Pharmacy is the health profession that links the health sciences with the chemical sciences and it is charged with ensuring the safe and effective use of pharmaceutical drugs...
.
See also
- Biological pumpBiological pumpIn oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.-Overview:...
- DetritivoreDetritivoreDetritivores, also known as detritophages or detritus feeders or detritus eaters or saprophages, are heterotrophs that obtain nutrients by consuming detritus . By doing so, they contribute to decomposition and the nutrient cycles...
- f-ratioF-ratioIn oceanic biogeochemistry, the f-ratio is the fraction of total primary production fuelled by nitrate . This fraction is significant because it is assumed to be directly related to the sinking flux of organic marine snow from the surface ocean by the biological pump...
- Marine mucilageMarine mucilageSea snot or marine mucilage is a collection of mucus-like organic matter found in the sea. Sea snot forms when globs of marine snow coagulate into large globs that can span distances as large as 124 miles. A recent increase in the amount of sea snot in the Mediterranean and other seas may be the...
- Sediment trapSediment trapSediment traps are instruments used in oceanography to measure the quantity of sinking particulate organic material in aquatic systems, usually oceans...
- Whale fallWhale fallWhale fall is the term used for a whale carcass that has fallen to the ocean floor. Whale falls were first observed in the 1980s, with the advent of deep-sea robotic exploration....