Ecosystem of the North Pacific Subtropical Gyre
The
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
The life processes in open-ocean ecosystems are a
The NPSG is not only a sink for CO2 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
History of discovery
The NPSG is not often sampled because of its distance from the coast and its shortage of
During the early days of marine exploration,
Physical characteristics
The NPSG is the largest of the open ocean habitats and is considered to be the Earth’s largest contiguous biome.[5] This great anticyclonic circulation feature extends from 15°N to 35°N latitude and from 135°E to 135°W longitude. Its surface area spans approximately 2 x 107 km2. Its western portion, west of 180° 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.[2]
This large variability is caused by discrete eddies, near-inertial motions, and internal tides.[2] 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.[3] DiLorenzo et al., 2008 These conditions can have profound effects on biological processes within this habitat,[2] 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.
Nutrient cycling
Low nutrient concentrations and thus a low density of living organisms 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.[4]
In open-ocean systems, biological production depends on intense nutrient recycling within the
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.[8] 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.[9] 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 tens of centimeters in size.[9] 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.[7] 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 ecosystems, nitrogen fixation is a common source of nitrogen.
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[9] 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.[9] 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”.[2]
Microbial community
Before 1978, scientists hypothesized that diatoms dominated plankton populations in the NPSG. The primary consumers were expected to be relatively large mesozooplankton.
Eukaryotic plankton community
This interannual variability has been attributed to alterations in upper ocean nutrient supply stemming from physical variations due to ENSO and PDO.[3] 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.[2] 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.[4] 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.[3] Summer blooms have been observed in these waters as long as research vessels have been frequenting them. All of these blooms have been seen in the eastern part of the NSPG with none reported west of 160° W.[4] 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.[4]
Variations in primary production in the NPSG can significantly affect nutrient cycling,
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 diversity (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.[2]
Studies of mesopelagic fishes of central subtropical waters are scarce. The few studies that do exist found that mesopelagic fish species are not
Benthic community
The deepest community in the NPSG is the
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 cycle and potentially atmospheric CO2 and climate, because such variations can change the amount of carbon that is stored in the subsurface layers of the oceans.[12] 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.
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.[13] 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 has gained 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.
See also
References
- ^ Poretsky, 2009
- ^ a b c d e f g h i j k l m n o Karl, D. 1999
- ^ a b c d e f Corno et al, 2007
- ^ a b c d e f Dore et al., 2008
- ^ (Karl et al., 2002)
- ^ a b Hannides et al., 2009
- ^ a b Nicholson et al., 2008
- ^ a b (Shulenberger and Hessler, 1974)
- ^ a b c d Pilskaln et al., 2005
- ^ Barnett, 1984
- ^ a b Smith Jr. et al., 2002
- ^ Brix et al., 2006
- ^ Moore et al., 2001
Sources
- Barnett, M. A. (1984). "Mesopelagic fish zoogeography in the central tropical and subtropical Pacific Ocean: Species composition and structure at representative locations in three ecosystems". Marine Biology. 82 (2): 199–208. S2CID 86221172.
- Brix, H., Gruber, N., Karl, D., and N. Bates. 2006. "On the relationships between primary, net community, and export production in subtropical gyres". Deep-Sea Research Part II. (53) 698–717.
- Corno, G., Karl, D., Church, M., Letelier, R., Lukas, R., Bidigare, R., and M. Abbott. 2007. "Impact of climate forcing on ecosystem processes in the North Pacific Subtropical Gyre". Journal of Geophysical Research. (112) 1–14.
- DiLorenzo E., Schneider, N., Cobb, K., Franks, P., Chhak, K., Miller, A., McWilliams, J., Bograd, S., Arango, H., Curchitser, E., Powell, T., and P. Riviere. 2008. North "Pacific Gyre Oscillations links ocean climate and ecosystem change". Geophysical Research Letters. (35) 1–6.
- Dore, J. E.; Letelier, R. M.; Church, M. J.; Lukas, R.; Karl, D. M. (2008). "Summer phytoplankton blooms in the oligotrophic North Pacific Subtropical Gyre: Historical perspective and recent observations". Progress in Oceanography. 76 (1): 2–38. .
- Dunning, Brian (16 December 2008). "Skeptoid #132: The Sargasso Sea and the Pacific Garbage Patch". Skeptoid.
- Hannides, C. C. S.; Landry, M. R.; .
- Karl, D. M. (1999). "Minireviews: A Sea of Change: Biogeochemical Variability in the North Pacific Subtropical Gyre" (PDF). Ecosystems. 2 (3): 181–214. S2CID 46309501.
- Karl, D. M.; Lukas, R. (1996). "The Hawaii Ocean Time-series (HOT) program: Background, rationale and field implementation". Deep-Sea Research Part II: Topical Studies in Oceanography. 43 (2–3): 129. .
- Karl, D. M.; Bidigare, R. R.; Letelier, R. M. (2002). "Sustained and Aperiodic Variability in Organic Matter Production and Phototrophic Microbial Community Structure in the North Pacific Subtropical Gyre". Phytoplankton Productivity. p. 222. ISBN 978-0470995204.
- Moore, C., Moore, S., Leecaster, M., and S. Weisberg. 2001. "A comparison of plastic and plankton in the North Pacific central gyre". Marine Pollution Bulletin. (42) page numbers.
- Nicholson, David; Emerson, Steven; Eriksen, Charles C. (2008). "Net community production in the deep euphotic zone of the subtropical North Pacific gyre from glider surveys" (PDF). Limnology and Oceanography. 53 (5 Part 2): 2226–2236. . Retrieved 11 November 2017.
- Pilskaln, C. H.; Villareal, T. A.; Dennett, M.; Darkangelo-Wood, C.; Meadows, G. (2005). "High concentrations of marine snow and diatom algal mats in the North Pacific Subtropical Gyre: Implications for carbon and nitrogen cycles in the oligotrophic ocean". Deep-Sea Research Part I: Oceanographic Research Papers. 52 (12): 2315. hdl:1912/404.
- Poretsky, R. S.; Hewson, I.; Sun, S.; Allen, A. E.; Zehr, J. P.; Moran, M. A. (2009). "Comparative day/night metatranscriptomic analysis of microbial communities in the North Pacific subtropical gyre". Environmental Microbiology. 11 (6): 1358–1375. PMID 19207571.
- Shulenberger, E.; Hessler, R. R. (1974). "Scavenging abyssal benthic amphipods trapped under oligotrophic central North Pacific Gyre waters". Marine Biology. 28 (3): 185. S2CID 84095171.
- Smith, K. L.; Baldwin, R. J.; Karl, D. M.; Boetius, A. (2002). "Benthic community responses to pulses in pelagic food supply: North Pacific Subtropical Gyre". Deep-Sea Research Part I: Oceanographic Research Papers. 49 (6): 971. .