Remote control

Reduced Himalayan snow cover affects marine life in the Arabian Sea

By T V Jayan
Published: Tuesday 31 May 2005

-- the decreasing snow cover in Southwest Asia and the Himalaya is threatening marine life in central and western parts of the Arabian Sea, which lie thousands of kilometres (km) away. According to a study published in Science (April 22, Vol 308, No 5721), reduced winter and spring snow cover in these mountains is throwing up ideal conditions for widespread blooms of phytoplankton -- microscopic marine plants -- in the Arabian Sea. Marine creatures feed on phytoplankton but its uncontrollable growth deprives the region of oxygen putting other life forms under fatal stress. As it is, the Arabian Sea already hosts one of the world's largest minimum oxygen zones -- spread over 1.4 million square km -- located at a depth of 150-1,000 metres.

Chain of reactions When the snow cover is low, the land mass heats up more in summer, creating a larger temperature difference between the land and the sea. This gives rise to stronger winds from the sea to the land. In the Western Arabian Sea, these winds reach speeds of up to 36 knots (almost 67 km/hour) leading to upwelling of cooler nutrient-rich water. These winds are strong enough to churn up water from depths of about 150 metres or more. The phytoplankton that inhabit the surface layers of water, where there is enough light for photosynthesis, quickly absorb the nutrients and proliferate.

The surface layers of water have high oxygen content but the deep layers have little. The upwelling of water means the deep layers replace the oxygen-rich surface layer. While the phytoplankton bloom every summer and use up the oxygen, the marine life forms, especially fish, get less oxygen, eventually leading to their death.

Since 1997, scientists have observed a close link between the reduction in snow cover and the increasing number of dead fish in the sea. "Fishermen off the coasts of Somalia, Oman and Yemen have already been witnessing increased incidence of fish mortality," said India-born Joaquim Goes of the Bigelow Laboratory of Ocean Sciences in the us, who is the principal author of the study.

Goes and his co-authors Prasad Thoppil of University of Southern Mississippi and Helga do R Gomes and John Fasullo of University of Colorado conducted studies from 1997-2003 using data generated by satellites. During this period, they observed average summertime phytoplankton biomass along the coast going up to four and a half times and offshore to four times that in 1997.

Global warming But the problems do not stop there. Bringing deep water to the surface in this part of the globe could also dramatically increase global warming.

Some bacteria have found a way of dealing with the low oxygen levels at the bottom of the Arabian Sea. They extract oxygen from nitrate found in the water. The process, known as 'de-nitrification', produces nitrous oxide, a powerful greenhouse gas.

When de-nitrification occurs deep down, the nitrous oxide is trapped. But when the deep water rises to the surface -- a consequence of the decreasing snow cover -- the gas is released into the atmosphere. There, it acts as a greenhouse gas that is about 300 times more harmful to the environment than carbon dioxide. If the Himalaya continues to receive less winter snow, says Goes, "the Arabian Sea will become a chimney for nitrous oxide". That could induce climate change much worse than anticipated.

But if there is an intensification in the monsoon activity (that is, stronger monsoon winds blowing across the Indian subcontinent), why doesn't the landmass receive more rainfall?

According to Goes, who was formerly with the Goa-based National Institute of Oceanography, the rainfall depends on several other factors, including the temperature of the sea surface. If these temperatures were higher, one could expect more moisture being carried to the land, he says. The amount of rainfall also depends critically on the coincidence of monsoon westerlies with the trade winds. Several researchers, however, have attributed increased incidence of freak floods in Pakistan and Bangladesh to stronger monsoon showers.

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