Answer: Currents are powerful physical forces in the seas. They move water and heat around the globe, and help determine the chemical make-up of the water column. Currents also are a major factor in ocean ecosystems. Two types of current motion, upwelling and downwelling, strongly influence the distribution and abundance of marine life.
Upwelling
Currents play a huge role in marine productivity, through a process called upwelling. Sea life is concentrated in the sunlit waters near the surface, but most organic matter is far below, in deep waters and on the sea floor. When currents upwell, or flow up to the surface from beneath, they sweep vital nutrients back to where they're needed most.
Nowhere is the link between ocean circulation and productivity more evident than around Antarctica. There, strong currents pump nitrogen and phosphate up from the deep sea to fuel vast blooms of algae and other plants. These plankton are eaten by swarms of shrimp-like crustaceans called krill. Because of upwelling nutrients, krill are abundant enough to feed the largest animals on earth, baleen whales, as well as myriad penguins, seals, and seabirds. In fact, despite the harsh conditions, the biomass of Antarctic krill is thought to be greater than that of any other animal on Earth.
Downwelling
The importance of upwelling to surface organisms is matched by the need of sea bottom life for downwelling, or the sinking of surface water. Surface water can be forced downward by the pressure of the “pile” of water that forms where currents converge or wind drives the sea against a coastline. But for bottom dwellers, the sinking of water caused by density changes is especially noteworthy. The global conveyer belt takes oxygen-rich surface water and flushes it through the deep sea. Without this renewal, the dissolved oxygen in bottom sediments and waters would quickly be used up by the decay of organic matter. Anaerobic bacteria would take over decomposition, leading to a build up of hydrogen sulfide. Few benthic animals would survive such toxic conditions.
In the most extreme cases, a lack of downwelling may lead to mass extinctions. Paleontologists have suggested that 250 million years ago, deep circulation slowed nearly to a stop, and the ocean began to stagnate. Low oxygen, sulfide and methane-rich waters filled the ocean deeps and then spread onto the continental shelves, wiping out 95% of all marine species in the greatest extinction event in Earth history.
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