Unit 3: Oceans // Section 6: Biological Activity in the Upper Ocean
Most living organisms in the ocean, including most that live in the deepest waters, are nourished directly or indirectly by plankton that live at the surface. Marine biologists are still discovering new life forms of all shapes and sizes in the sea, but some of the most important findings in recent decades concern the ocean's tiniest life forms: microbes that produce organic matter (mostly through photosynthesis) and form the base of ocean food chains.
Measured in caloric output per year, the oceans account for nearly half of the world's gross primary production. Nearly all of it takes place in a thin upper layer of the oceans and is done by phytoplankton—microorganisms that typically measure one-twentieth of a millimeter across at the largest and live for one to five days (Fig. 12). But what phytoplankton lack in size, they make up in numbers. Two types of cyanobacteria (blue-green algae), Synechococcus and Prochlorococcus, may be the two most abundant organisms on Earth and are found in concentrations of up to 500,000 cells per milliliter of sea water (footnote 2).
Figure 12. Marine phytoplankton
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Source: © United States Geological Survey.
Since average temperatures are much less variable in the ocean than on land, temperature is less of a limiting factor for primary production in the oceans. However, light is key. As sunlight penetrates down through the ocean, it is absorbed or scattered by water and by particles floating in the water. The compensation depth, where net energy produced from photosynthesis equals the energy that producers use for respiration, occurs at a depth where light is reduced to about 1 percent of its strength at the surface. In clear water this may be as deep as 110 meters, but in turbid coastal water it may be less than 20 meters. Activities that make water more murky, such as dredging or water pollution, thus are likely to reduce biological productivity in the areas where they occur.
Mixing also affects how much light phytoplankton receive. If winds stir the ocean so that the mixed layer extends far below the compensation zone, phytoplankton will be pushed down levels where there is not enough light for photosynthesis, so their net production will be lower than if they drift constantly in well-lit water. In areas of the ocean where wind speeds vary widely from season to season, primary productivity may rise and fall accordingly.
In addition to light, phytoplankton need nutrients for photosynthesis. Carbon is available in the form of CO2 from the atmosphere. Other important macronutrients including nitrogen (N), phosphorus (P), and silicon (Si), and micronutrients such as iron (Fe), are dissolved in seawater. On average, phytoplankton use nutrients in a ratio of 106 C: 16N: 1P: .001 Fe, so if one of these elements runs out in the mixed layer, productivity goes down. Ocean mixing and coastal upwelling bring new supplies of nutrients up from deeper waters.
When optimal light, temperature, and nutrient conditions occur, and limited numbers of grazers (larger organisms that feed on phytoplankton) are present, plankton population explosions called blooms occur. Grazers reproduce more slowly than phytoplankton, so it takes time for them to catch up to their food supply. Major blooms can color large stretches of ocean water red, brown, or yellow-green depending on what species is present (hence the popular term "red tide," although tides do not cause blooms). Many of these events are not harmful in themselves, but they deplete oxygen in the water when the organisms die and decompose. Some types of phytoplankton algae produce neurotoxins, so blooms of these varieties are dangerous to swimmers and consumers of fish or shellfish from the affected area.
Blooms can be triggered by runoff that carries fertilizer or chemicals into ocean waters or by storms that mix ocean waters and bring nutrients to the surface. They often occur in spring, when rising water temperatures and longer daylight hours stimulate phytoplankton to increase their activity levels after slow or dormant periods during winter. Most plankton blooms are beneficial to ocean life because they increase the availability of organic material, much like the flowering that takes place on land in spring.
A massive spring bloom occurs each year in the North Atlantic from March through June, raising ocean productivity levels from North Carolina to Canada. This bloom extends all the way north to the southern edge of Arctic sea ice and moves northward through the spring and summer as ice melts. Figure 13 shows a portion of the bloom.
Figure 13. North Atlantic spring bloom, March 28, 2003 (Cape Cod to Newfoundland shown)
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Source: © National Aeronautics and Space Administration. Earth Observatory.
Phytoplankton are eaten by animal microorganisms such as zooplankton, which range from single-celled creatures like foraminifera (small amoebae with calcium carbonate shells) to larger organisms such as krill and jellyfish. Copepods, the most abundant type of small crustaceans, are one to two millimeters long and serve as important food sources for fish, whales, and seabirds. Phytoplankton blooms attract zooplankton, which in turn are eaten by larger predators. "Phytoplankton are the plants of the ocean and the base of the food web," says MIT biologist Penny Chisholm. "If they weren't there, there would be nothing else living in the oceans."
Climate cycles can have major impacts on biological productivity in the oceans. When Pacific coastal upwelling off South America slows or stops during an El Niño event, plankton growth falls, reducing food supplies for anchovies and sardines that prey on the plankton. These fish die off or move to colder waters, which in turn reduces food for large predators like tuna, sea lions, and seabirds. Major El Niño events in the 20th century devastated the South American fishing industry and killed thousands of predator species as far north as the Bering Straits.
Other climate cycles have similar impacts. Generally, warm phases of the Pacific Decadal Oscillation decrease productivity off of the western United States and cold phases increase it. Decadal fluctuations in salmon, several ground fish, albacore, seabirds, and marine mammals in the North Pacific have been associated with the PDO. The mechanisms underlying these associations are speculative, but probably represent a combined effect of atmospheric conditions such as wind strength; upper ocean physical conditions, such as current strength, depth of wind mixing, and nutrient availability; and biological responses to these conditions across many trophic levels. Similarly, successful recruitment of several species of fish, including sardines and cod, has been associated with different phases of the NAO in different regions of the North Atlantic.