The Arctic Ocean’s ice sheet covers in 2012 half the Area it used to cover in 1979. This is causing alarms among the environmental organisations and hope for those who want to gain access to new natural resources and new sea routes. Some people hope for a fishing bonanza, but they may be disappointed.
The waters around the Arctic account for a fifth of the world’s catch. The ice melt might lead to too much change to the eco systems and decrease productivity. But how can this be? As the ice melts, more light can reach the water, and that means more photosynthesis by marine algae supporting, directly or indirectly, the fish and mammals that live in the Arctic Ocean.
The most important reason why the ice melt might decrease productivity in the Ocean is that global warming may increase ocean stratification. Stratification is the tendency of seawater to separate into layers, because fresh water is lighter than salt and cold water heavier than warm. The more stratified water is, the less nutrients in it move around.
Most free-swimming sea creatures are pelagic. When they die, all these organisms sink to the bottom, where they become food for benthic creatures. Once they have been consumed their component molecules, including nutrients such as nitrates, phosphates and iron, are stuck in Davy Jones’s locker. For the surface to be productive, the locker must be opened and the nutrients lifted back up, so that they can feed the growth of phytoplankton.
One of the most important ways this happens is by upwellings of water from the bottom—great churning columns caused by the collision of cold and temperate waters. Two of the most important are in the Arctic: south of Greenland on the Atlantic side and south of the Bering Strait on the Pacific side. Nitrates are abundant at the surface in both places, which is why they are among the world’s richest fishing grounds. There are few upwellings in the tropics, which are thus nutrient-poor.
Stratification threatens this recycling system by suppressing the vertical movement of water. And global warming encourages stratification because it turns the ice into a layer of fresh water that sits on the surface.
Parts of the Arctic seem to be getting badly stratified. In winter, there is almost no density difference in the North Atlantic and the Barents Sea—as you would expect given the upwelling there. But in summer, the northern part of the Barents Sea is even more stratified than the tropical Atlantic and Pacific. And the Beaufort Sea’s stratification is high in both summer and winter.
A warming Arctic will not, in other words, be full of fish. It will simply be an ice-free version of the desert it already is
Source: The Economist
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