Oxygen levels down in oceans by 2%, marine life under threat

MUMBAI: The Indian Ocean is losing oxygen more slowly than the Pacific and Atlantic but the potential impact on marine ecosystems is large, according to a new report from the International Union for Conservation of Nature.
The report, which was released this past weekend, found that oxygen levels in the world’s oceans are declining due to global warming and nutrient pollution—from fertiliser runoff—posing a threat to marine life and fisheries.
Indian Ocean has seen a smaller and more varied decline than other oceans. With a quarter of the global human population living around the Indian Ocean though, “its environment, biodiversity and living resources are most vulnerable to human-induced changes, especially deoxygenation,” the report said.
“The magnitude of change here is smaller than in the Pacific and Atlantic because the physical characteristics are different, especially with the influence of the monsoon,” says S W A Naqvi, a scientist with the Council of Scientific and Industrial Research who contributed to the IUCN report. However, “these two basins (the Bay of Bengal and Arabian Sea) also contain large volumes of water which are naturally oxygen depleted; this means that even if change is small, there is going to be a large impact.”
Globally, the level of oxygen has reduced by 2% between 1960 and 2010, with the largest losses in the Tropical and North Pacific Ocean, the Southern and Arctic Ocean and the South Atlantic Ocean. Over 700 low oxygen zones have been identified in the world’s oceans, up from 45 in 1970s.
Models predict a further decline in oxygen in the order of 1% to 7% by 2100. These numbers are likely underestimations, the report warned.
In the northern Indian Ocean—for which most data is available—the decline is not uniform. There has been substantial oxygen loss in the upper waters of the western Arabian Sea, more mixed trends in the central portion, and a comparatively slight decline in the Bay of Bengal. Both the Arabian Sea and the Bay of Bengal have large oxygen minimum zones (OMZ)—or functionally dead zones—that lie in the deep seas.
Decreasing oxygen and rising organic load in Red Sea and Persian Gulf could lead to an expansion in the Arabian Sea OMZ, the report said.
In addition, coastal waters off western India experiences a seasonal “hypoxia” or dead zone, when the summer monsoon brings in oxygen-poor, nutrient-rich waters resulting in a layer of low or no oxygen water. This phenomenon causes occasional mass fish kills seen along the coast, including in Goa this August.
The oxygen concentrations appear to have reduced in this seasonal upwelling, says Naqvi, with implications for Indian fisheries. Pollution and sewage discharge have also led to low oxygen in India’s estuaries and ports, including Mumbai, Kochi, and Surat, the report notes.