Ecosystem of the North Pacific Subtropical Gyre
Encyclopedia
The North Pacific Subtropical Gyre
(NPSG) is the largest contiguous ecosystem on earth. In oceanography
, a subtropical gyre is a ring-like system of ocean current
s rotating clockwise in the Northern Hemisphere
and counterclockwise in the Southern Hemisphere
caused by the Coriolis Effect
. They generally form in large open ocean areas that lie between land masses. The NPSG is the largest of the gyres as well as the largest ecosystem on our planet. Like other subtropical gyres it has a high-pressure zone in its center. Circulation around the center is clockwise around this high-pressure zone. Subtropical gyres make up 40% of the Earth’s surface and play critical roles in carbon fixation
and nutrient cycling . This particular gyre covers most of the Pacific Ocean and comprises four prevailing ocean currents: the North Pacific Current
to the north, the California Current
to the east, the North Equatorial Current
to the south, and the Kuroshio Current
to the west. Its large size and distance from shore has caused the NPSG to be poorly sampled and thus poorly understood.
The life processes in open-ocean ecosystems are a sink for the atmosphere’s increasing . Gyres make up a large proportion, approximately 75%, of what we refer to as the open ocean, or the area of the ocean that does not consist of coastal areas. They are considered oligotrophic, or nutrient poor because they are far from terrestrial runoff. These regions were once thought to be homogenous and static habitats. However, there is increasing evidence that the NPSG exhibits substantial physical, chemical, and biological variability on a variety of time scale
s. Specifically, the NPSG exhibits seasonal and interannual variations in primary productivity
(simply defined as the production of new plant material), which is important for the uptake of . The NPSG is not only a sink for in the atmosphere, but also other pollutants. As a direct result of this circular pattern, gyres act like giant whirlpools and become traps for anthropogenic pollutants, such as marine debris
. The NPSG has become recognized for the large quantity of plastic debris floating just below the surface in the center of the gyre. This area has recently received a lot of media attention and is commonly referred to as the Great Pacific Garbage Patch
.
. These vast and deep ocean waters, far from the influence of land, have historically been considered the oceanic equivalent of terrestrial deserts, with low standing stocks of biomass and low production rates. This perspective is derived from a dearth of comprehensive investigation of central gyre habitats. Over the past two decades these views have been challenged with a newfound understanding of the dynamics of the NPSG.
During the early days of marine exploration, the HMS Challenger
(1872-1876), on its leg from Yokohama to Honolulu, collected plant and animal specimens as well as numerous seawater samples. The goals of this expedition were to determine the chemical composition of seawater and the organic matter in suspension and to study the distribution and abundance of various communities of organisms. The motivation for studying open ocean ecosystems has changed over time, whereas today more modern studies focus on biodiversity and the effects of climate on ecosystem dynamics. Today, the Hawaii Ocean Time-series (HOT) program has assembled the largest and most comprehensive ecological data set
for the NPSG and is scheduled to continue to the next millennium
. Programs like HOT have debunked the hypothesis that this ecosystem is static and homogenous, finding that the NPSG exhibits dynamic seasonal patterns separating it from other open ocean systems.
. This great anticyclonic circulation feature extends from 15oN to 35oN latitude and from 135oE to 135oW longitude. Its surface area spans approximately 2 x 107 km2. Its western portion, west of 180o longitude, has greater physical variability than the eastern portion. This variability, where different weather patterns affect subregions differently, is due to the large dimensions of this gyre.
This large variability is caused by discrete eddies, near-inertial motions, and internal tide
s. Climate patterns such as the North Pacific Gyre Oscillation (NPGO), El Nino/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) affect the interannual variability in primary productivity in the NPSG. DiLorenzo et al., 2008 These conditions can have profound effects on biological process
es within this habitat, they have the ability to shift sea surface temperature
(SST), chlorophyll patterns, nutrient patterns, oxygen concentrations, mixed layer
depths, and thus the carrying capacity
(amount of life this habitat can carry) of the NPSG.
characterize the surface waters of the NPSG. The low biomass
results in clear water, allowing photosynthesis
to occur to a substantial depth. The NPSG is classically described as a two-layered system. The upper, nutrient-limited layer accounts for most of the primary production
, supported primarily by recycled nutrients. The lower layer has nutrients more readily available, but photosynthesis is light-limited.
In open-ocean systems, biological production depends on intense nutrient recycling within the euphotic (sunlit) zone, with only a small fraction supported by the input of “new” nutrients. Previously there was a perception that the NPSG was a marine desert and that “new” nutrients were not commonly added to this system. The outlook has changed, as scientists have begun to have a better understanding of this habitat. Although fairly high rates of primary production are maintained through rapid recycling of nutrients, physical processes such as internal wave
s and tides, cyclonic mesoscale eddies, wind-driven Ekman pumping, and atmospheric storms may carry in new nutrients.
Nutrients that do not get used up on the surface will eventually sink down and nourish the seafloor habitat. The deep benthic habitats
of the ocean gyres have been thought to typically consist of some of the most food-poor regions on the planet. One of the sources of nutrients to this deep ocean habitat is marine snow
. Marine snow consists of detritus, dead organic matter, which falls from the surface waters where productivity is highest and exports carbon and nitrogen from the surface mixed layer to the deep ocean. Data on the abundance of marine snow to the deep ocean floor
is lacking in this large ecosystem. However, Pilskaln et al. found that in the NPSG, marine snow was at a higher abundance than expected and were surprisingly comparable to a deep coastal upwelling system.
Higher nutrient value may be because of Rhizosolenia mats, which also play an important role in contributing to marine snow in subtropical gyres. These are generally multi-species associations of Rhizosolenia species of diatoms. This larger phytoplankton may reach up to 10s of centimeters in size. These mats are particularly abundant in the NPSG. Their abundance in this ecosystem suggests a higher flux of nutrients in the NPSG than was predicted in classic theories.
While N is transported deeper by this mechanism, the surface waters are potentially cut off from this source. Nitrogen must be available for life at the surface. In order to account for this lack of nitrogen to the surface, there are organisms that are capable of nitrogen fixation
in the NPSG. Trichodesmium is one species capable of nitrogen fixation that is found in many surface plankton blooms. Nitrogen fixation is the process where inert N2 is taken from the atmosphere and converted into a nitrogen compound that is available to organisms for use. In many oligotrophic marine ecosystem
s, nitrogen fixation is a common source
of nitrogen.
Vertically migrating zooplankton can also actively transport nutrients to different zones of the water column. Zooplankton
feed in the surface waters at night, and then by day release fecal pellets to the midwaters, which can transport C, N, and P to the deeper waters. In the NPSG the zooplankton community is not static but fluctuates seasonally and is dominated by copepods, euphausiids, and chaetognaths.
Recently, classic theories about the lack of nutrients in the NPSG have been disproven and new theories suggest that the ecosystem actually is dynamic and characterized by strong seasonal, interannual, and even decadal variability It has also been deemed highly sensitive to climate change
, scientists have observed increases in water column stratification and decreased inorganic nutrient availability. These changes are proposed as driving mechanisms that are changing the current trend in phytoplankton community structure from eukaryotic to prokaryotic populations, as these simpler organisms can withstand lower nutrient supply. Zooplankton and phytoplankton represent less than 10% of living organisms in this region, and it is now well documented that the NPSG is a “microbial ecosystem”.
in the NPSG. They are autotrophic, meaning they capture their own “food” from sunlight and chemicals, including . These organisms comprise the base of the food chain
, and thus their presence in an ecosystem is fundamental. In the NPSG, primary productivity is often described as low.
Before 1978, scientists hypothesized that diatoms dominated plankton populations in the NPSG. The primary consumers were expected to be relatively large mesozooplankton. It is now well known that most of the algae in the NPSG are actually bacteria (unicellular organisms), dominated by cyanobacteria, or blue-green algae. These simple organisms make up the majority of the standing stock of photosynthesizing marine life in this ecosystem. Scientists have also recently discovered Archaea (also a single-celled microorganism
, but more similar to a eukaryote than bacteria) genes in the NPSG, suggesting that additional diversity exists in this habitat. Many microorganisms may exist in this gyre because small body size has a competitive advantage
in the ocean for resource (light and nutrients) acquisition. In the contemporary view of the NPSG, the microbial food web is always present, whereas the larger eukaryote-grazer food chain is seasonal and ephemeral.
This interannual variability has been attributed to alterations in upper ocean nutrient supply stemming from physical variations due to ENSO and PDO. Based on new data, it now appears that present rates of primary production in these low nutrient regions are much greater than had been considered, and can vary significantly on time scales ranging from daily to interdecadal. In the spring, rapid increases in surface phytoplankton are occasionally observed in association with cyclonic mesoscale eddies or intense atmospheric disturbances, both physical processes that bring in new nutrients. In the summer, blooms are seen more regularly and are typically dominated by diatoms and cyanobacteria. These regular summer blooms may be caused by variations in the PDO. Summer blooms have been observed in these waters as long as research vessels have been frequenting them. Interestingly, all of these blooms have been seen in the eastern part of the NSPG with none reported west of 160o W . Hypotheses to explain this phenomenon are that the gyre is characterized by low phosphate, but that the bloom region of the eastern NPSG has considerably higher phosphate concentrations than the western.
Variations in primary production in the NPSG can significantly affect nutrient cycling, food-web dynamics, and global elemental fluxes. The size distribution of pelagic primary producers determines both the composition and magnitude of the exported nutrients to the deeper waters. This in turn affects the communities that live in the deeper waters of this system.
(or high number of species) and high equitability (meaning relatively equal numbers of each exist). There is also a low degree of seasonal variability of densities of zooplankton.
Studies of mesopelagic fishes of central subtropical waters are scarce. The few studies that do exist found that mesopelagic fish species are not uniformly distributed throughout the subtropical Pacific Ocean. Their geographic ranges conform to patterns shown by zooplankton. Some of the species found are restricted to these low-productivity central gyres. Some of the families of fish that are highly represented are Mytophids, Gonostomatids, Photichthyids, Sternoptychids, and Melamphaids. Our understanding of the mesopelagic community of the NPSG suffers from an insufficiency of data due to the difficulty of accessing the deeper zones of this system.
clay sediments. This sediment is home to a community of organisms, which generally receive their nutrients as a “rain” of productivity sinking from above. At depth under the gyre lies one of the most food-poor areas on the planet, which therefore supports very low densities and biomass of benthic infauna, or animals residing in the sediment. In the sediment itself, nutrients generally decline with depth, including C, chlorophyll and N. Density of the benthic infauna is consistent with this nutrient pattern. Infauna are typically found in the shallower layers of sediments where the sediment-water interface
lies and generally decrease in number with increasing depth in the sediment. Bacteria in the sediment show this pattern as well as macrofauna (infaunal organisms >0.5mm), which are dominated by agglutinating (soft-bodied) foraminifera and nematodes. Other prominent macrofauna found in the sediment are calcareous foraminifera, copepods, polychaetes, and bivalves. These benthic organisms rely heavily on the supply of nutrients that settle to the sea floor. Any change in primary production at the surface could pose a major threat to these organisms, as well as cause other potential negative outcomes to other parts of the NPSG.
and potentially atmospheric and climate, because such variations can change the amount of carbon that is stored in the subsurface layers of the oceans. Because the NPSG is the largest contiguous biome on earth, it is not only important to a community of organisms, but also the rest of the planet.
Recently the NPSG has received copious attention because of another issue it currently faces. The eddy effects of the gyre serve to retain pollutants in its center. If a pollutant gets trapped in a current that is headed toward a gyre, it will stay there indefinitely or as long as the life of the pollutant. One such pollutant that is persistent and common in the NPSG is plastic debris. The NPSG forces debris into its central area. This phenomenon has recently given this gyre the nickname, “The Pacific Garbage Patch.” The mean abundance and weight of plastic pieces in this area are currently the largest observed in the Pacific Ocean. It is rumored that this plastic “soup” is anywhere from the size of Texas to the size of the US. With increasing interest in pollution and climate change, the NPSG gains more attention. It is important that our knowledge of this system continue to flourish for these reasons, as well as solely for the understanding of the world’s largest ecosystem.
North Pacific Gyre
The North Pacific Gyre, located in the northern Pacific Ocean, is one of the five major oceanic gyres. This gyre comprises most of the northern Pacific Ocean. It is the largest ecosystem on our planet...
(NPSG) is the largest contiguous ecosystem on earth. In oceanography
Oceanography
Oceanography , also called oceanology or marine science, is the branch of Earth science that studies the ocean...
, a subtropical gyre is a ring-like system of 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 rotating clockwise in the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
and counterclockwise in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
caused by the Coriolis Effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
. They generally form in large open ocean areas that lie between land masses. The NPSG is the largest of the gyres as well as the largest ecosystem on our planet. Like other subtropical gyres it has a high-pressure zone in its center. Circulation around the center is clockwise around this high-pressure zone. Subtropical gyres make up 40% of the Earth’s surface and play critical roles in carbon fixation
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...
and nutrient cycling . This particular gyre covers most of the Pacific Ocean and comprises four prevailing ocean currents: the North Pacific Current
North Pacific Current
The North Pacific Current is a slow warm water current that flows west-to-east between 40 and 50 degrees north in the Pacific Ocean. The current forms the southern part of the North Pacific Subpolar Gyre...
to the north, the California Current
California Current
The California Current is a Pacific Ocean current that moves south along the western coast of North America, beginning off southern British Columbia, and ending off southern Baja California. There are five major coastal currents affiliated with upwelling zones...
to the east, the North Equatorial Current
North Equatorial Current
The North Equatorial Current is a significant Pacific and Atlantic Ocean current that flows east-to-west between about 10° north and 20° north. It is the southern side of a clockwise subtropical gyre. Despite its name, the North Equatorial Current is not connected to the equator...
to the south, and the Kuroshio Current
Kuroshio Current
The Kuroshio is a north-flowing ocean current on the west side of the North Pacific Ocean. It is similar to the Gulf Stream in the North Atlantic and is part of the North Pacific ocean gyre...
to the west. Its large size and distance from shore has caused the NPSG to be poorly sampled and thus poorly understood.
The life processes in open-ocean ecosystems are a sink for the atmosphere’s increasing . Gyres make up a large proportion, approximately 75%, of what we refer to as the open ocean, or the area of the ocean that does not consist of coastal areas. They are considered oligotrophic, or nutrient poor because they are far from terrestrial runoff. These regions were once thought to be homogenous and static habitats. However, there is increasing evidence that the NPSG exhibits substantial physical, chemical, and biological variability on a variety of time scale
Time scale
A time scale specifies divisions of time.*Time standard, a specification of either the rate at which time passes, or points in time, or both*Duration , a quantity of time...
s. Specifically, the NPSG exhibits seasonal and interannual variations in primary productivity
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...
(simply defined as the production of new plant material), which is important for the uptake of . The NPSG is not only a sink for in the atmosphere, but also other pollutants. As a direct result of this circular pattern, gyres act like giant whirlpools and become traps for anthropogenic pollutants, such as marine debris
Marine debris
Marine debris, also known as marine litter, is human created waste that has deliberately or accidentally become afloat in a lake, sea, ocean or waterway. Oceanic debris tends to accumulate at the centre of gyres and on coastlines, frequently washing aground, when it is known as beach litter or...
. The NPSG has become recognized for the large quantity of plastic debris floating just below the surface in the center of the gyre. This area has recently received a lot of media attention and is commonly referred to as the Great Pacific Garbage Patch
Great Pacific Garbage Patch
The Great Pacific Garbage Patch, also described as the Pacific Trash Vortex, is a gyre of marine litter in the central North Pacific Ocean located roughly between 135°W to 155°W and 35°N to 42°N...
.
History of discovery
The NPSG is not often sampled because of its distance from the coast and its shortage of marine lifeMarine biology
Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather...
. These vast and deep ocean waters, far from the influence of land, have historically been considered the oceanic equivalent of terrestrial deserts, with low standing stocks of biomass and low production rates. This perspective is derived from a dearth of comprehensive investigation of central gyre habitats. Over the past two decades these views have been challenged with a newfound understanding of the dynamics of the NPSG.
During the early days of marine exploration, the HMS Challenger
HMS Challenger
Eight ships of the Royal Navy have been named HMS Challenger, most famously the survey vessel Challenger that carried the Challenger expedition from 1872 to 1876....
(1872-1876), on its leg from Yokohama to Honolulu, collected plant and animal specimens as well as numerous seawater samples. The goals of this expedition were to determine the chemical composition of seawater and the organic matter in suspension and to study the distribution and abundance of various communities of organisms. The motivation for studying open ocean ecosystems has changed over time, whereas today more modern studies focus on biodiversity and the effects of climate on ecosystem dynamics. Today, the Hawaii Ocean Time-series (HOT) program has assembled the largest and most comprehensive ecological data set
Data set
A data set is a collection of data, usually presented in tabular form. Each column represents a particular variable. Each row corresponds to a given member of the data set in question. Its values for each of the variables, such as height and weight of an object or values of random numbers. Each...
for the NPSG and is scheduled to continue to the next millennium
3rd millennium
In contemporary history, the third millennium is a period of time that commenced on January 1, 2001, and will end on December 31, 3000, of the Gregorian calendar. This is the third period of one thousand years in the Anno Domini...
. Programs like HOT have debunked the hypothesis that this ecosystem is static and homogenous, finding that the NPSG exhibits dynamic seasonal patterns separating it from other open ocean systems.
Physical characteristics
The NPSG is the largest of the open ocean habitats and is considered to be the Earth’s largest contiguous biomeBiome
Biomes are climatically and geographically defined as similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, and are often referred to as ecosystems. Some parts of the earth have more or less the same kind of abiotic and biotic factors spread over a...
. This great anticyclonic circulation feature extends from 15oN to 35oN latitude and from 135oE to 135oW longitude. Its surface area spans approximately 2 x 107 km2. Its western portion, west of 180o longitude, has greater physical variability than the eastern portion. This variability, where different weather patterns affect subregions differently, is due to the large dimensions of this gyre.
This large variability is caused by discrete eddies, near-inertial motions, and internal tide
Internal tide
Internal tides are generated as the surface tides move stratified water up and down sloping topography, which produces a wave in the ocean interior. So internal tides are internal waves at a tidal frequency. The other major source of internal waves is the wind which produces internal waves near...
s. Climate patterns such as the North Pacific Gyre Oscillation (NPGO), El Nino/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) affect the interannual variability in primary productivity in the NPSG. DiLorenzo et al., 2008 These conditions can have profound effects on biological process
Biological process
A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation....
es within this habitat, they have the ability to shift sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
(SST), chlorophyll patterns, nutrient patterns, oxygen concentrations, mixed layer
Mixed layer
The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, cooling, or processes such as evaporation or sea ice formation which result in an increase in salinity...
depths, and thus the carrying capacity
Carrying capacity
The carrying capacity of a biological species in an environment is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water and other necessities available in the environment...
(amount of life this habitat can carry) of the NPSG.
Nutrient cycling
Low nutrient concentrations and thus a low density of living organismsOrganism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
characterize the surface waters of the NPSG. The low biomass
Biomass (ecology)
Biomass, in ecology, is the mass of living biological organisms in a given area or ecosystem at a given time. Biomass can refer to species biomass, which is the mass of one or more species, or to community biomass, which is the mass of all species in the community. It can include microorganisms,...
results in clear water, allowing photosynthesis
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...
to occur to a substantial depth. The NPSG is classically described as a two-layered system. The upper, nutrient-limited layer accounts for most of the 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...
, supported primarily by recycled nutrients. The lower layer has nutrients more readily available, but photosynthesis is light-limited.
In open-ocean systems, biological production depends on intense nutrient recycling within the euphotic (sunlit) zone, with only a small fraction supported by the input of “new” nutrients. Previously there was a perception that the NPSG was a marine desert and that “new” nutrients were not commonly added to this system. The outlook has changed, as scientists have begun to have a better understanding of this habitat. Although fairly high rates of primary production are maintained through rapid recycling of nutrients, physical processes such as internal wave
Internal wave
Internal waves are gravity waves that oscillate within, rather than on the surface of, a fluid medium. They are one of many types of wave motion in stratified fluids . A simple example is a wave propagating on the interface between two fluids of different densities, such as oil and water...
s and tides, cyclonic mesoscale eddies, wind-driven Ekman pumping, and atmospheric storms may carry in new nutrients.
Nutrients that do not get used up on the surface will eventually sink down and nourish the seafloor habitat. The deep benthic habitats
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
of the ocean gyres have been thought to typically consist of some of the most food-poor regions on the planet. One of the sources of nutrients to this deep ocean habitat is marine snow
Marine snow
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...
. Marine snow consists of detritus, dead organic matter, which falls from the surface waters where productivity is highest and exports carbon and nitrogen from the surface mixed layer to the deep ocean. Data on the abundance of marine snow to the deep ocean floor
Seabed
The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...
is lacking in this large ecosystem. However, Pilskaln et al. found that in the NPSG, marine snow was at a higher abundance than expected and were surprisingly comparable to a deep coastal upwelling system.
Higher nutrient value may be because of Rhizosolenia mats, which also play an important role in contributing to marine snow in subtropical gyres. These are generally multi-species associations of Rhizosolenia species of diatoms. This larger phytoplankton may reach up to 10s of centimeters in size. These mats are particularly abundant in the NPSG. Their abundance in this ecosystem suggests a higher flux of nutrients in the NPSG than was predicted in classic theories.
While N is transported deeper by this mechanism, the surface waters are potentially cut off from this source. Nitrogen must be available for life at the surface. In order to account for this lack of nitrogen to the surface, there are organisms that are capable of nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
in the NPSG. Trichodesmium is one species capable of nitrogen fixation that is found in many surface plankton blooms. Nitrogen fixation is the process where inert N2 is taken from the atmosphere and converted into a nitrogen compound that is available to organisms for use. In many oligotrophic marine ecosystem
Marine ecosystem
Marine ecosystems are among the largest of Earth's aquatic ecosystems. They include oceans, salt marsh and intertidal ecology, estuaries and lagoons, mangroves and coral reefs, the deep sea and the sea floor. They can be contrasted with freshwater ecosystems, which have a lower salt content. Marine...
s, nitrogen fixation is a common source
Common source
In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal...
of nitrogen.
Vertically migrating zooplankton can also actively transport nutrients to different zones of the water column. 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"...
feed in the surface waters at night, and then by day release fecal pellets to the midwaters, which can transport C, N, and P to the deeper waters. In the NPSG the zooplankton community is not static but fluctuates seasonally and is dominated by copepods, euphausiids, and chaetognaths.
Recently, classic theories about the lack of nutrients in the NPSG have been disproven and new theories suggest that the ecosystem actually is dynamic and characterized by strong seasonal, interannual, and even decadal variability It has also been deemed highly sensitive to 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...
, scientists have observed increases in water column stratification and decreased inorganic nutrient availability. These changes are proposed as driving mechanisms that are changing the current trend in phytoplankton community structure from eukaryotic to prokaryotic populations, as these simpler organisms can withstand lower nutrient supply. Zooplankton and phytoplankton represent less than 10% of living organisms in this region, and it is now well documented that the NPSG is a “microbial ecosystem”.
Microbial community
Microbial organisms make up the majority of the primary producersPrimary producers
Primary producers are those organisms in an ecosystem that produce biomass from inorganic compounds . In almost all cases these are photosynthetically active organisms...
in the NPSG. They are autotrophic, meaning they capture their own “food” from sunlight and chemicals, including . These organisms comprise the base of the food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
, and thus their presence in an ecosystem is fundamental. In the NPSG, primary productivity is often described as low.
Before 1978, scientists hypothesized that diatoms dominated plankton populations in the NPSG. The primary consumers were expected to be relatively large mesozooplankton. It is now well known that most of the algae in the NPSG are actually bacteria (unicellular organisms), dominated by cyanobacteria, or blue-green algae. These simple organisms make up the majority of the standing stock of photosynthesizing marine life in this ecosystem. Scientists have also recently discovered Archaea (also a single-celled microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
, but more similar to a eukaryote than bacteria) genes in the NPSG, suggesting that additional diversity exists in this habitat. Many microorganisms may exist in this gyre because small body size has a competitive advantage
Sustainable competitive advantage
Competitive advantage is defined as the strategic advantage one business entity has over its rival entities within its competitive industry. Achieving competitive advantage strengthens and positions a business better within the business environment....
in the ocean for resource (light and nutrients) acquisition. In the contemporary view of the NPSG, the microbial food web is always present, whereas the larger eukaryote-grazer food chain is seasonal and ephemeral.
Eukaryotic plankton community
Eukaryotic plankton in the gyre is dependent on “new” nutrients coming in from physical weather patterns. The classic two-layered model discussed in previous sections considers the upper layer to be equivalent to a “spinning wheel,” with little export of nutrients because they are constantly recycled. This model does not allow for the input of new nutrients, which is problematic because this would make any rapid increase, or bloom of phytoplankton impossible. Despite ever-present nutrient limitation in the upper portion, plankton biomass and rates of primary production have considerable temporal variability and do produce blooms in the NPSG.This interannual variability has been attributed to alterations in upper ocean nutrient supply stemming from physical variations due to ENSO and PDO. Based on new data, it now appears that present rates of primary production in these low nutrient regions are much greater than had been considered, and can vary significantly on time scales ranging from daily to interdecadal. In the spring, rapid increases in surface phytoplankton are occasionally observed in association with cyclonic mesoscale eddies or intense atmospheric disturbances, both physical processes that bring in new nutrients. In the summer, blooms are seen more regularly and are typically dominated by diatoms and cyanobacteria. These regular summer blooms may be caused by variations in the PDO. Summer blooms have been observed in these waters as long as research vessels have been frequenting them. Interestingly, all of these blooms have been seen in the eastern part of the NSPG with none reported west of 160o W . Hypotheses to explain this phenomenon are that the gyre is characterized by low phosphate, but that the bloom region of the eastern NPSG has considerably higher phosphate concentrations than the western.
Variations in primary production in the NPSG can significantly affect nutrient cycling, food-web dynamics, and global elemental fluxes. The size distribution of pelagic primary producers determines both the composition and magnitude of the exported nutrients to the deeper waters. This in turn affects the communities that live in the deeper waters of this system.
Mesopelagic community
The mesopelagic zone is sometimes referred to as the twilight zone; it extends from 200m to around 1000m. In the deeper layers of the NPSG, species higher up on the food chain will migrate vertically or horizontally within or in and out of the gyre. Based on analyses of the zooplankton community, the Central North Pacific has a high species diversitySpecies richness
Species richness is the number of different species in a given area. It is represented in equation form as S.Species richness is the fundamental unit in which to assess the homogeneity of an environment. Typically, species richness is used in conservation studies to determine the sensitivity of...
(or high number of species) and high equitability (meaning relatively equal numbers of each exist). There is also a low degree of seasonal variability of densities of zooplankton.
Studies of mesopelagic fishes of central subtropical waters are scarce. The few studies that do exist found that mesopelagic fish species are not uniformly distributed throughout the subtropical Pacific Ocean. Their geographic ranges conform to patterns shown by zooplankton. Some of the species found are restricted to these low-productivity central gyres. Some of the families of fish that are highly represented are Mytophids, Gonostomatids, Photichthyids, Sternoptychids, and Melamphaids. Our understanding of the mesopelagic community of the NPSG suffers from an insufficiency of data due to the difficulty of accessing the deeper zones of this system.
Benthic community
The deepest community in the NPSG is the benthic community. At the depths of the gyre lies a sea floor of fine-grainedGranularity
Granularity is the extent to which a system is broken down into small parts, either the system itself or its description or observation. It is the "extent to which a larger entity is subdivided...
clay sediments. This sediment is home to a community of organisms, which generally receive their nutrients as a “rain” of productivity sinking from above. At depth under the gyre lies one of the most food-poor areas on the planet, which therefore supports very low densities and biomass of benthic infauna, or animals residing in the sediment. In the sediment itself, nutrients generally decline with depth, including C, chlorophyll and N. Density of the benthic infauna is consistent with this nutrient pattern. Infauna are typically found in the shallower layers of sediments where the sediment-water interface
Sediment-water interface
In oceanography and limnology, the sediment-water interface is the boundary between bed sediment and the overlying water column. The topography of this interface is often dynamic, as it is affected by physical processes and biological processes ....
lies and generally decrease in number with increasing depth in the sediment. Bacteria in the sediment show this pattern as well as macrofauna (infaunal organisms >0.5mm), which are dominated by agglutinating (soft-bodied) foraminifera and nematodes. Other prominent macrofauna found in the sediment are calcareous foraminifera, copepods, polychaetes, and bivalves. These benthic organisms rely heavily on the supply of nutrients that settle to the sea floor. Any change in primary production at the surface could pose a major threat to these organisms, as well as cause other potential negative outcomes to other parts of the NPSG.
Future and importance of the NPSG
Until recently the NPSG was considered to be a static part of a vast global marine desert. Recent discoveries have proved that this system is dynamic and contains physical, chemical, and biological variability on a variety of time scales. With the current changing climate, patterns in the atmosphere are shifting and causing changes in primary production in the NPSG. Variations in primary productivity can affect the ocean carbon cycleCarbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
and potentially atmospheric and climate, because such variations can change the amount of carbon that is stored in the subsurface layers of the oceans. Because the NPSG is the largest contiguous biome on earth, it is not only important to a community of organisms, but also the rest of the planet.
Recently the NPSG has received copious attention because of another issue it currently faces. The eddy effects of the gyre serve to retain pollutants in its center. If a pollutant gets trapped in a current that is headed toward a gyre, it will stay there indefinitely or as long as the life of the pollutant. One such pollutant that is persistent and common in the NPSG is plastic debris. The NPSG forces debris into its central area. This phenomenon has recently given this gyre the nickname, “The Pacific Garbage Patch.” The mean abundance and weight of plastic pieces in this area are currently the largest observed in the Pacific Ocean. It is rumored that this plastic “soup” is anywhere from the size of Texas to the size of the US. With increasing interest in pollution and climate change, the NPSG gains more attention. It is important that our knowledge of this system continue to flourish for these reasons, as well as solely for the understanding of the world’s largest ecosystem.