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Description :
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Marine Food Web and the Carbon Cycle
Microscopic plants and other photosynthetic organisms
that drift with ocean currents lie at the heart of
the marine carbon cycle. Sunlit surface waters teem with phytoplankton that
convert inorganic carbon dissolved in surface waters to organic
carbon—which forms the basis of the marine food web—and
account for about half of all primary production on Earth (Field
et al. 1998; Falkowski, Barber, and Smetacek 1998). In contrast
to terrestrial carbon-turnover times that may take months to
years, carbon cycles rapidly in oceans, with the entire phytoplankton
population in some environments replacing itself
weekly (Falkowski 2002).
Phytoplankton, such as those described above, are grazed upon
by marine heterotrophs known as zooplankton. These grazer
species range from microscopic protozoa and copepods to worms, krill, crabs, jellyfish, and the larvae of fish
and other organisms. Comprising most of the animal mass in the ocean, zooplankton serve as the crucial link
between primary producers and the rest of the marine food web. Viruses, which act as predators in oceanic food
chains by infecting and lysing marine bacteria, also play an important but still poorly understood role in marine
carbon turnover.
The overall efficiency with which organic carbon is exported to the deep ocean depends on the type of photoautotrophic
cells that create the organic material and the efficiency with which heterotrophic organisms respire it.
Carbon Flow and Fate
Carbon fixed in phytoplankton eventually enters the water column as either particulate or dissolved organic carbon
through direct exudation, consumption by grazing zooplankton, viral lysis, or cell death. Subsequently, most
of this carbon material is degraded by heterotrophic bacteria, resulting in particulate solubilization and conversion
of organic carbon back to CO2. Some of the organic matter, however, sinks intact to the underlying twilight
zone (the ocean’s barely lit middle layer) and beyond, where lower temperatures, lack of oxygen, and other factors
significantly slow degradation.
CO2 fixed during photosynthesis by phytoplankton in the upper ocean can be transferred to the depths via three
major processes: passive sinking of particles, physical mixing of particulates and dissolved organic matter through
currents, and active transport by zooplankton migrating to deeper waters. Detrital particles and organic matter
associated with mineral structures from phytoplankton, for example, may resist rapid microbial degradation and
sift down as flakes, also called marine snow, becoming platforms for microbes to live on. As this particulate organic
matter falls deeper, it can cluster with other small particles, such as zooplankton fecal pellets, molts, and larvacean
houses, to form larger, heavier aggregates held together by a polysaccharide matrix. The carbon in these particles
can be isolated from exchange with the atmosphere for centuries to millennia before upwelling currents return
it and other nutrients from the deep ocean to warm surface waters. Some carbon is lost at each step of the way,
however, as the organisms involved consume or degrade the organic carbon and remineralize it to CO2 through
respiration.
However, if climate change and ocean acidification significantly alter marine ecosystems’ functions, the efficiency
of this biologically mediated ocean carbon export may change, leading to an indirect effect on the net annual
uptake of carbon.
Major Primary Producers
Coccolithophores (5 to 10 μm in diameter),
single-celled algae prevalent in tropical oceans.
Diatoms (about 30 μm in diameter), prevalent
in temperate and polar oceans.
Dinoflagellates (30 to 2000 μm in diameter),
prevalent in the subtropics and tropics, as well
as in temperate oceans in late summer.
Cyanobacteria (about 1 μm in diameter), the
world’s most abundant phytoplankton.
References:
Falkowski, P. G. 2002. “The Ocean’s Invisible Forest: Marine Phytoplankton Play a Critical Role in Regulating the Earth’s Climate. Could They Also be Used to Combat Global Warming?” Scientific American 287(2), 54–61.
Falkowski, P. G., R. T. Barber, and V. Smetacek. 1998. “Biogeochemical Controls and Feedbacks on Ocean Primary Production,” Science 281(5374), 200–06.
Field, C. B., et al. 1998. “Primary Production of the Biosphere: Integrating Terrestrial and Oceanic Components,” Science 281(5374), 237–40. |
