The best fishing areas are right offshore, in the coastal oceans, and also along the equator, especially in the eastern part of the ocean basins. Why should this be so? The answer, as mentioned in the last section, is upwelling and mixing of subsurface waters into the sunlit zone. These deeper waters have nutrients, which stimulate the growth of microscopic photosynthetic algae ("phytoplankton"). Phytoplankton, in turn, serve as food for the zooplankton. Both phytoplankton and zooplankton feed fish.
Diagram illustrating the principle of coastal upwelling
. Currents, created by winds, are diverted by the Coriolis force
, and this results in water being carried away from shore. Deep, cold water rises to replace these waters, resulting in coastal upwelling.
Most of the upwelling in the ocean occurs in two settings: coastal upwelling along the shores bathed by eastern boundary currents (that is, along the eastern portions of the great central gyres) and equatorial upwelling along the equator. Normally, the upwelling water derives from depths between 100 and 300 m, depending on the strength of the upwelling motion. Upwelling is driven by winds, and both the coastal upwelling along the eastern boundary currents and equatorial upwelling rely on the trade winds (See the satellite image depicting the productivity zones; note how they relate to coastal and equatorial upwelling zones.)
Upwelling and mixing also occurs within the storm systems of the west-wind belt. The mixing by winter storms brings nutrients to the surface; this results in plankton blooms in early summer when the Sun is high over the horizon and delivers the light necessary for photosynthesis. Also, the Sun warms the surface waters and provides a measure of stability, so that phytoplankton is kept in sunlit surface waters, rather than being mixed downward into the dark regions. Regions characterized by a succession of strong mixing alternating with warming and much sunlight yields the highest productivity. By bringing cold waters to low latitudes, the flows associated with upwelling have a considerable effect on the heat budget of the planet. Basically, upwelling in the tropics, along the eastern boundary currents and in the eastern equatorial high productivity regions, cools the tropics.
Diagram illustrating the principle of equatorial upwelling. Winds along the equator (dotted line) create currents, which are then diverted north and south by the Coriolis force. The cold, deep waters from below rise to the surface to replace these diverted waters, causing upwelling.
Composite image of satellite photos since 1978 depicting phytoplankton concentration along North and South America. The key for the image is in units of milligrams of phytoplankton pigment per cubicmeter
of seawater. Note how high populations
of plankton correspond to areas where coastal upwelling is strong, like the Peruvian coast and the Pacific Northwest. (From: NASA
Upwelling and Climate Change
In any one region, upwelling is intermittent, it can be strong in some years and weak in others. The success of fishermen is greatly affected by this, since a weakening of an upwelling system can bring economic disaster. Upwelling brings nutrient-rich deep waters to the surface, where algae can thrive in the sunlight, feeding the fish. Without nutrients, no algae, and without algae, no fish (as happens during El Niño conditions).
How will global warming affect upwelling and coastal productivity? The strong dependency of upwelling processes on the strength of trade winds contains one hint. Remember that trade winds are zonal winds, which feed off the latitudinal temperature gradient. As this gradient weakens (high latitudes being warmed more than low, in all computer simulations), zonal flow will weaken. Monsoons, as we have seen, would be favored over trades. Thus, the upwelling that derives its energy from trade winds will weaken. Indeed, off California, upwelling of cold water has become less common since 1975, and the productivity of the California Current has diminished accordingly.
A similar decrease in productivity may be expected elsewhere in the eastern boundary currents, and in the eastern equatorial high production regions. Upwelling that depends on monsoon activity (as in the Arabian Sea) should be much less effected, or even benefit from the change.
A comparison of phytoplankton concentration during upwelling periods off the coast of Peru: (A) 1983, a severe El Niño and (B) 1985, a non-El Niño period. Note how much smaller the bloom (circled) is during El Niño conditions, when nutrient upwelling ceases. The color key needed to read the phytoplankton concentrations is the same as that in the above satellite image. (From: NASA