That can be used as a proxy for phytoplankton population in a given area, since the tiny organisms live close to the ocean's surface, where they are exposed to sunlight they use to produce energy.
Data gathered with a Secchi disk are roughly as accurate as observations collected by satellites, Boyce said, although satellites have greater global reach.
The researchers found the most notable phytoplankton declines in waters near the poles and in the tropics, as well as the open ocean. They believe that rising sea temperatures are driving the decline. As surface water warms, it tends to form a distinct layer that does not mix well with cooler, nutrient-rich water below, depriving phytoplankton of some of the materials they need to turn CO2 and sunlight into energy.
Lauren Morello works for Nature magazine. Already a subscriber? Sign in. Thanks for reading Scientific American. With warm, buoyant water on top and cold, dense water below, the water column doesn't mix easily. Phytoplankton use up the nutrients available, and growth falls off until winter storms kick-start mixing. In lower-latitude areas, including the Arabian Sea and the waters around Indonesia, seasonal blooms are often linked to monsoon-related changes in winds.
As the winds reverse direction offshore versus onshore , they alternately enhance or suppress upwelling, which changes nutrient concentrations. In the equatorial upwelling zone, there is very little seasonal change in phytoplankton productivity. In spring and summer, phytoplankton bloom at high latitudes and decline in subtropical latitudes. These maps show average chlorophyll concentration in May — left and November — right in the Pacific Ocean.
The organisms in the coldest waters are at the greatest risk. Researchers believe that phytoplankton could evolve to alter their body chemistry or migrate, but such a change could mean that species higher up the food chain will be unable to feed themselves.
At higher latitudes, higher temperatures and less mixing could force phytoplankton to stay closer to the surface. More sunlight in that top layer may result in changes in the mix of microorganisms, once again affecting the creatures that eat phytoplankton.
Researchers believe that this warming — along with other factors such as changing levels of nitrogen and iron and ocean acidification — is also affecting the phytoplankton. In the summer of — as the young salmon swam to their ocean feeding grounds — a volcano erupted in the Aleutian Islands of Alaska. For a few days, the erupting volcano spewed a vast cloud of volcanic ash into the air.
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. Our planet's climate has changed throughout its long history among various extremes and on different time scales, ranging from millions of years, to just a few millennia, to just a few centuries. Discover oceanic processes, productivity of life in the ocean, and how ocean organisms and circulation respond to climate change. Our planet's surface is created by tectonic processes, but later molded into shape by water, wind, and ice.
Discover the many terrestrial landscapes Earth contains and the processes that create them. Citation: Sigman, D. Nature Education Knowledge 3 10 Productivity fuels life in the ocean, drives its chemical cycles, and lowers atmospheric carbon dioxide.
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