Aa Aa Aa. Reconstructions of ocean productivity using sediment records typically involve the accumulation of biogenic matter organics or mineral hard parts in the sediment; therefore, these studies speak to export production rather than NPP. However, if export production is reconstructed, it is at least a fair assumption that NPP would have changed in the same direction.
Moreover, as described below, the export of organic matter out of the surface ocean can have broad biogeochemical and climate implications, so reconstructing export production is valuable in itself. One of the greatest challenges for reconstructing ocean productivity is the potential for changes in the fraction of export production that reaches the seabed and is preserved into the sediments, which could be misinterpreted as changes in productivity.
The development and improvement of such reconstructions is an active area of research. There is evidence from the geologic record that ocean productivity has changed in response to ocean circulation. As a prominent example, over the course of the last 35 million years, the Southern Ocean has developed into a cold, highly productive region Kennett These changes suggest the development of circulation upwelling and other processes that today imports new nutrients into the euphotic zone.
In contrast, during the last ice age, export production was reduced relative to interglacial levels in the Antarctic Zone of the Southern Ocean and in the similar environment of the subarctic North Pacific Jaccard et al.
While explanations have been proposed for these changes Sigman et al. Deep water is upwelled into the Southern Ocean surface, from which this nutrient-bearing water is pumped by the winds into the mid-depth ocean interior that supplies nutrients to the low latitude surface ocean Palter et al. As a result, Southern Ocean circulation changes can affect ocean productivity on a global basis. Much of the ongoing discussion regarding the stability or variability of ocean productivity through Earth history focuses on the potential for changes in the ocean's concentrations of different nutrients, with a higher mean ocean concentration of a common limiting nutrient possibly leading to higher productivity.
Central to this question is the concept of "residence time. The shorter the residence time of a chemical, the faster its reservoir size can change because of an imbalance between inputs and outputs. The ocean's P budget is largely controlled by geological and geochemical processes. P enters the ocean by weathering, and it is removed through the sedimentary burial of organic P, P adsorbed onto iron oxides, phosphatic fossil material such as fish debris and shark teeth, and authigenic P minerals Froelich et al.
The residence time of P in the ocean has been estimated as 20—40 thousand years Ruttenberg , which indicates that the ocean P reservoir could change greatly over millions of years. Given the potential for changes in the P reservoir, it is a mystery why there aren't more clear signs of dramatic variation in global ocean productivity over Earth's history.
Figure 4. Composite global ocean maps of concentrations of satellite-derived chlorophyll and ship-sampled nitrate NO 3 - ; the dominant N-containing nutrient. Figure 5. Illustration of the coupled biogeochemical cycles of the "major" nutrients N and P, the trace nutrient iron and CO 2 sequestered by the biological pump.
Figure 6. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Earth's Climate: Past, Present, and Future. Marine Geosystems. The distribution of chlorophyll is shown in the figure above.
You can see the highest abundance close to the coastlines where nutrients from the land are fed in by rivers. The other location where chlorophyll levels are high is in upwelling zones where nutrients are brought to the surface ocean from depth by the upwelling process. Another critical element for the health of the oceans is the dissolved oxygen content.
Oxygen in the surface ocean is continuously added across the air-sea interface as well as by photosynthesis; it is used up in respiration by marine organisms and during the decay or oxidation of organic material that rains down in the ocean and is deposited on the ocean bottom.
Most organisms require oxygen, thus its depletion has adverse effects for marine populations. Deep water ocean currents are formed when cold, nutrient-rich water sinks and flows away from the surface. There are sources of deep water currents in the northern and southern hemispheres. Deep water currents return nutrients to the surface by a process known as upwelling. Upwelling brings nutrients back into sunlight, where plankton can use the nutrients to provide energy that drives an ocean's ecosystem.
The oceans consist of layers of water with different qualities. The energy for ecosystems is generated by plankton, mostly in the upper layer known as the photic zone the region where light penetrates the ocean. Plankton use light and nutrients in the water to generate food energy. Larger organisms feed on the plankton, providing the basis for a food chain or ecosystem that includes invertebrates like shrimp and krill, larger fish, sharks, marine birds and marine mammals.
In particular, the cyanobacteria, which are prokaryotes lacking a nucleus and most other organelles found in eukaryotes , are now known to be important among the phytoplankton.
Initially, the cyanobacteria were identified largely with colonial forms such as Trichodesmium that play the critical role of "fixing" nitrogen see below. It is now recognized that two cyanobacterial genera — Synechoccocus and Prochlorococcus — dominate phytoplankton numbers and biomass in the nutrient-poor tropical and subtropical ocean Waterbury et al. In addition, new methods, both microscopic and genetic, are revealing a previously unappreciated diversity of smaller eukaryotes in the open ocean.
Mapping ecological and biogeochemical functions onto the genetic diversity of the phytoplankton is an active area in biological and chemical oceanography. Based on observations as well as theory, the smaller phytoplankton such as the unicellular cyanobacteria are thought to dominate regenerated production in many systems, whereas the larger eukaryotes appear to play a more important role in new production i.
The food source of a given form of zooplankton is typically driven by its own size, with microzooplankton grazing on the prokaryotes and smaller eukaryotes and multicellular zooplankton grazing on larger eukaryotes, both phytoplankton and microzooplankton. Because of their relative physiological simplicity, microzooplankton are thought to be highly efficient grazers that strongly limit the biomass accumulation of their prey. In contrast, the multicellular zooplankton , because they typically have more complex life histories, can lag behind the proliferation of their prey, allowing them to bloom and sometimes avoid predation altogether and sink directly.
The multicellular zooplankton also often facilitate the production of sinking organic matter, for example, through the production of fecal pellets by copepods. In the nutrient-poor tropical and subtropical ocean, the small cyanobacteria tend to be numerically dominant, perhaps because they specialize in taking up nutrients at low concentrations. Small phytoplankton have a greater surface area-to-volume ratio than do large phytoplankton. A greater proportional surface area promotes the uptake of nutrients across the cell boundary, a critical process when nutrients are scarce, likely explaining why small phytoplankton dominate the biomass in the nutrient-poor ocean.
The microzooplankton effectively graze these small cells, preventing their biomass from accumulating and sinking directly. Moreover, these single-celled microzooplankton lack a digestive tract, so they do not produce the fecal pellets that represent a major mechanism of export.
Instead, any residual organic matter remains in the upper ocean, to be degraded by bacteria. All told, microzooplankton grazing of phytoplankton biomass leads to the remineralization of most of its contained nutrients and carbon in the surface ocean, and thus increases recycling relative to organic matter export.
In contrast, larger phytoplankton , such as diatoms, often dominate the nutrient-rich polar ocean, and these can be grazed directly by multicellular zooplankton. By growing adequately rapidly to outstrip the grazing rates of these zooplankton , the diatoms can sometimes accumulate to high concentrations and produce abundant sinking material. In addition, the zooplankton export organic matter as fecal pellets. Figure 3 The most broadly accepted paradigm for the controls on surface nutrient recycling efficiency.
NPP is supported by both new nutrient supply from the deep ocean and nutrients regenerated within the surface ocean. In the nutrient-poor tropical and subtropical ocean a , the small cyanobacteria tend to be numerically dominant.
The microzooplankton that graze these small cells do so effectively, preventing phytoplankton from sinking directly. Moreover, these single-celled microzooplankton do not produce sinking fecal pellets. Instead, any residual organic matter remains to be degraded by bacteria.
In nutrient-rich regions b , large phytoplankton are more important, and these can be grazed directly by multicellular zooplankton. By growing adequately rapidly to outstrip the grazing rates of zooplankton, the large phytoplankton can sometimes accumulate to high concentrations and produce abundant sinking material.
The relationships between nutrient supply, phytoplankton size, and sinking thus dominate this view of upper ocean nutrient cycling. Satellites can measure the color of the surface ocean in order to track the concentration of the green pigment chlorophyll that is used to harvest light in photosynthesis Figure 4. Higher chlorophyll concentrations and in general higher productivity are observed on the equator, along the coasts especially eastern margins , and in the high latitude ocean Figure 4a and b.
Figure 4 Composite global ocean maps of concentrations of satellite-derived chlorophyll and ship-sampled nitrate NO 3 - ; the dominant N-containing nutrient. Northern hemisphere summer is shown in the left panels and southern hemisphere summer on the right. In the vast unproductive low- and mid-latitude ocean, warm and sunlit surface water is separated from cold, nutrient-rich interior water by a strong density difference that restricts mixing of water and thereby reduces nutrient supply, which becomes the limiting factor for productivity.
These "ocean deserts" are dissected by areas, mainly at the equator and the eastern margins of ocean basins, where the wind pushes aside the buoyant, warm surface lid and allows nutrient-rich deeper water to be upwelled. In the high latitude ocean, surface water is cold and therefore the vertical density gradient is weak, which allows for vertical mixing of water to depths much greater than the sunlit "euphotic zone" as a result, the nutrient supply is greater than the phytoplankton can consume, given the available light and iron, see text.
Sea ice cover impedes measurement of ocean color from space, reducing the apparent areas of the polar oceans in the winter hemisphere upper panels. There are caveats regarding the use of satellite-derived chlorophyll maps to deduce productivity, phytoplankton abundance, and their variation.
Second, chlorophyll concentration speaks more directly to the rate of photosynthesis i. Fourth, the depth range sensed by the satellite ocean color measurements extends only to the uppermost ten's of meters, much shallower than the base of the euphotic zone Figure 2. Compared to nutrient-bearing regions, nutrient-deplete regions e.
Thus, satellite chlorophyll observations tend to over-accentuate the productivity differences between nutrient-bearing and -depleted regions. Despite these caveats, satellite-derived ocean color observations have transformed our view of ocean productivity.
In some temperate and subpolar regions, productivity reaches a maximum during the spring as the phytoplankton transition from light to nutrient limitation. In the highest latitude settings, while the "major nutrients" N and P remain at substantial concentrations, the trace metal iron can become limiting into the summer Boyd et al. In at least some of these polar systems, it appears that light and iron can "co-limit" summertime photosynthesis Maldonado et al.
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