Increased marine production of N2O due to intensifying anoxia on the Indian continental shelf

Abstract

Eutrophication of surface waters and hypoxia in bottom waters has been increasing in many coastal areas, leading to very large depletions of marine life in the affected regions. These areas of high surface productivity and low bottom-water oxygen concentration are caused by increasing runoff of nutrients from land. Although the local ecological and socio-economic effects have received much attention, the potential contribution of increasing hypoxia to global-change phenomena is unknown. Here we report the intensification of one of the largest low-oxygen zones in the ocean, which develops naturally over the western Indian continental shelf during late summer and autumn. We also report the highest accumulations yet observed of hydrogen sulphide (H2S) and nitrous oxide (N2O) in open coastal waters. Increased N2O production is probably caused by the addition of anthropogenic nitrate and its subsequent denitrification, which is favoured by hypoxic conditions. We suggest that a global expansion of hypoxic zones may lead to an increase in marine production and emission of N2O, which, as a potent greenhouse gas, could contribute significantly to the accumulation of radiatively active trace gases in the atmosphere.

N2O cycling at the core of the oxygen minimum zone off northern Chile

“…Although these rates were estimated at only 2 stations, the estimates come from 2 of the most important upwelling areas off Chile during the summer, making them the most representative for the area to date. The values are similar to those reported by Omnes et al (1996) and Bianchi et al (1994) for the Mediterranean Sea (1.0 to 4.3 µM d -1 ), but are higher than those reported for other suboxic areas, such as the Baltic Sea (Rönner & Sörensson 1985, Brettar & Rheinheimer 1992, the Arabian Sea (0.09 to 0.11 µM d -1 ) (Naqvi et al 1993), the Indian Shelf (0.3 µM d -1 ) (Naqvi et al 2000), and the Peruvian OMZ (0.0032 µM d -1 ) (Elkins et al 1978), also measured through laboratory experiments.…”

“…Anoxia strongly reduced both N 2 O production and consumption in the experiments carried out off Iquique and Antofagasta. Denitrification rates, measured as N 2 Opd under anoxia (0.05 to 0.35 µM d -1 ), were in the same range as those previously reported by Rönner & Sörensson (1985) and Brettar & Rheinheimer (1992) for the Baltic Sea (0.015 to 0.15 µM d -1 ), Naqvi et al (1993) for the Arabian Sea (0.09 to 0.11 µM d -1 ), Naqvi et al (2000) in the Indian Shelf (0.3 BM d -1 ), and Bonin et al (2002) in an estuarine area (0.01 to 0.13 µM d -1 ). These drastic differences in NO 2 -reduction under anoxia led to higher NO 2 -accumulations off Antofagasta than off Iquique.…”

“…Our results show that below suboxic conditions, this process takes place -as deduced from global models (Nevison et al 2003) or through direct measurements on waters of the Indian Shelf (Naqvi et al 2000), the Baltic Sea (Brettar & Rheinheimer 1992), suboxic estuaries (De Bie et al 2002), and marine bacteria cultures (Goreau et al 1980, Bonin et al 1989, De Bie et al 2002where the N 2 O produced from NH 4 + oxidation increased within microenvironments having low O 2 and NH 4 + availability (Avrahami et al 2002). In addition to this, potential nitrification rates (0.091 to 0.62 µM d -1 ) have been measurable in the OMZ core off Iquique (V. Molina et al unpubl.…”

“…O 2 and organic carbon) contribute to N 2 O cycling is not yet clear, and few relevant studies have been carried out on OMZs (Law & Owens 1990, Naqvi & Noronha 1991, Naqvi et al 2000 or suboxic waters (Elkins et al 1978, Rönner & Sörensson 1985, Brettar & Rheinheimer 1992. High denitrification rates can lead to changes in the C and N exchanges between the ocean and the atmosphere, significantly influencing global climate change (Codispoti et al 2001).…”

Climate change: What's in the rising tide?

“…Nitrous oxide is 200-300 times more potent as a greenhouse gas than carbon dioxide. In 2000, Wajih Naqvi and his colleagues at the National Institute of Oceanography in Goa, India, reported an alarming accumulation of nitrous oxide in the Arabian Sea, along the western Indian continental shelf, following a monsoon washout of nitrate fertilizers 6 . The calculated emissions from the region in just 6 months accounted for as much as 5% of annual ocean emissions from a region that makes up only 0.05% of the world's oceans.…”