Why Are Upwelling Zones So Rich? Nutrients Brought Back to the Light

Photosynthesis requires light and nutrients together, and the open ocean rarely supplies both in the same place. Light penetrates only the top hundred metres or so, while nutrients, principally nitrate, phosphate and iron, sink continuously as dead organisms and waste particles fall out of the surface layer, accumulating in deep water where there is no light to use them. Warm surface water is also less dense than the cold water beneath, which produces a stable stratification resisting mixing, so in much of the tropical ocean the surface stays permanently nutrient-starved despite abundant light. That is why satellite images of ocean productivity show enormous blue expanses that are biologically close to empty, and why the phrase ocean desert is accurate rather than rhetorical. Anything that breaks the stratification and lifts deep water into the light therefore transforms productivity.

Coastal upwelling depends on a piece of physics that is not intuitive. Wind blowing along a coast drags surface water with it, and because the earth rotates, moving water is deflected: to the right in the northern hemisphere and to the left in the southern. The net transport of the wind-driven surface layer therefore ends up at right angles to the wind rather than along it, a result derived by Vagn Ekman and named after him. Where that transport is directed offshore, surface water moves away from the coast and deep water rises to replace it, typically from a few hundred metres down, bringing nutrients with it. The geography that produces this is specific: a coast oriented so that prevailing winds blow along it in the correct direction, which is why the great upwelling systems sit on the eastern edges of ocean basins where trade winds run parallel to the shore. Equatorial upwelling works by a related mechanism, with the deflection acting in opposite directions either side of the equator and pulling surface water apart.

Upwelling supports food webs that are unusually short and unusually productive:

• •Phytoplankton bloom in response to the nutrients, with diatoms dominating, which are large and easily eaten rather than being passed through several trophic steps
• •Small schooling fish, principally anchovy and sardine, feed directly on that plankton, which means very little energy is lost between primary production and a commercially valuable fish
• •The Peruvian anchoveta fishery has at times been the largest single-species fishery in the world by tonnage
• •Seabirds and marine mammals concentrate in enormous numbers, and the guano deposited by those birds on the Peruvian coast was itself a major export commodity in the nineteenth century
• •Oxygen minimum zones form beneath productive upwelling, because the sinking organic material is decomposed by bacteria that consume oxygen, creating layers hostile to most fish
• •The systems are variable rather than steady, and the Peruvian system in particular collapses during El Nino events when the upwelling weakens or brings warm nutrient-poor water instead

Concentrating so much productivity into narrow strips makes these systems economically vital and ecologically exposed. The Peruvian anchoveta fishery has collapsed and recovered repeatedly, with the crash from 1972, driven by heavy fishing coinciding with a severe El Nino, standing as a textbook case of a fishery pushed past its limit by treating a good year as normal. Management has improved substantially, with closures triggered by stock assessments, though the fishery still supplies fishmeal for aquaculture and livestock rather than feeding people directly, which is a persistent criticism. Climate change adds two conflicting pressures: stronger land and sea temperature contrasts may intensify the winds that drive upwelling in some systems, while increased stratification may reduce the nutrient content of the water that rises. Oxygen minimum zones are expanding, which compresses fish habitat vertically. And because the systems are already variable, distinguishing a trend from natural fluctuation requires long records that in most cases do not exist.

Most of the open ocean has light at the top and nutrients only in the dark depths, which is why it is biologically empty. Wind blowing along certain coasts pushes surface water offshore because rotation deflects it at right angles, and deep nutrient-rich water rises to replace it. The resulting blooms feed anchovy and sardine directly, producing extremely short food chains and enormous fisheries in a few narrow strips. Those systems collapse during El Nino and are compressed by expanding low-oxygen zones.