Marine hypoxia/anoxia as a source of CH<sub>4</sub> and N<sub>2</sub>O

Abstract

Abstract. We review here the available information on methane (CH 4 ) and nitrous oxide (N 2 O) from major marine, mostly coastal, oxygen (O 2 )-deficient zones formed both naturally and as a result of human activities (mainly eutrophication). Concentrations of both gases in subsurface waters are affected by ambient O 2 levels to varying degrees. Organic matter supply to seafloor appears to be the primary factor controlling CH 4 production in sediments and its supply to (and concentration in) overlying waters, with bottom-water O 2 -deficiency exerting only a modulating effect. High (micromolar level) CH 4 accumulation occurs in anoxic (sulphidic) waters of silled basins, such as the Black Sea and Cariaco Basin, and over the highly productive Namibian shelf. In other regions experiencing various degrees of O 2 -deficiency (hypoxia to anoxia), CH 4 concentrations vary from a few to hundreds of nanomolar levels. Since coastal O 2 -deficient zones are generally very productive and are sometimes located close to river mouths and submarine hydrocarbon seeps, it is difficult to differentiate any O 2 -deficiency-induced enhancement from in situ production of CH 4 in the water column and its inputs through freshwater runoff or seepage from sediments. While the role of bottom-water O 2 -deficiency in CH 4 formation appears to be secondary, even when CH 4 accumulates in O 2 -deficient subsurface waters, methanotrophic activity severely restricts its diffusive efflux to the atmosphere. As a result, an intensification or expansion of coastal O 2 -deficient zones will probably Correspondence to: S. W. A. Naqvi (naqvi@nio.org) not drastically change the present status where emission from the ocean as a whole forms an insignificant term in the atmospheric CH 4 budget. The situation is different for N 2 O, the production of which is greatly enhanced in low-O 2 waters, and although it is lost through denitrification in most suboxic and anoxic environments, the peripheries of such environments offer most suitable conditions for its production, with the exception of enclosed anoxic basins. Most O 2 -deficient systems serve as strong net sources of N 2 O to the atmosphere. This is especially true for coastal upwelling regions with shallow O 2 -deficient zones where a dramatic increase in N 2 O production often occurs in rapidly denitrifying waters. Nitrous oxide emissions from these zones are globally significant, and so their ongoing intensification and expansion is likely to lead to a significant increase in N 2 O emission from the ocean. However, a meaningful quantitative prediction of this increase is not possible at present because of continuing uncertainties concerning the formative pathways to N 2 O as well as insufficient data from key coastal regions.

Denitrification in the water column of the central Baltic Sea

“…However, if it is transported upwards, and not reduced during its passage through the denitrification zone, and reaches the oxic waters above, it may contribute to the general N 2 O supersaturation of the central Baltic Sea (Rö nner, 1983;Brettar and Rheinheimer, 1991). However, it appears that denitrification is able to consume most of the produced N 2 O during stable anoxic periods and that large production of mainly occurs during transition from oxic to anoxic conditions (Naqvi et al, 2010). Surprisingly Hietanen et al (2012) only found complete denitrification to N 2 and no accumulation of N 2 O.…”

Methane production induced by dimethylsulfide in surface water of an upwelling ecosystem

“…A typical consequence of the local wind forcing is the fertilization of surface water column that increases primary production during the austral spring-summer (with ranged from 1.2 to 10 g C m À2 d À1 , Farías et al, 2009) and also leads to strong outgassing of N 2 O and CH 4 (Cornejo et al, 2007;Naqvi et al, 2010).…”

“…Among costal ecosystems, eastern boundary coastal upwelling areas could be major sources or hotspots of marine CH 4 (Monteiro et al, 2006;Kock et al, 2008;Naqvi et al, 2010). There, a combination of physical and biological processes must co-occur to produce high air-sea CH 4 efflux.…”

Distributions and sea-to-air fluxes of nitrous oxide in the South China Sea and the West Philippines Sea

“…In the water column with low oxygen concentrations and in sediments or inside suspended particles, nitrification and denitrification can occur simultaneously and are often closely coupled (Seitzinger, 1988;Ward et al, 1989;Capone, 1991;Middelburg et al, 1996;Barnes and Owens, 1998;Naqvi et al, 1998;Robinson et al, 1998;Usui et al, 2001;Nevison et al, 2003;Codispoti et al, 2005;Bange, 2008). The yield of N 2 O from both processes is highly dependent on the concentration of dissolved oxygen (Goreau et al, 1980;de Bie et al, 2002;Naqvi et al, 2010).…”

“…Denitrification occurs under low oxygen (o6 μmol L À 1 ) to anoxic conditions (Nevison et al, 2003). Denitrification produces and consumes N 2 O in oxygen minimum zones and the net result is that oxygen minimum zones have some of the highest N 2 O concentrations in the ocean (LaMontagne et al, 2003;Naqvi et al, 2010;Zamora et al, 2012;Arévalo-Martínez et al, 2015). In the water column with low oxygen concentrations and in sediments or inside suspended particles, nitrification and denitrification can occur simultaneously and are often closely coupled (Seitzinger, 1988;Ward et al, 1989;Capone, 1991;Middelburg et al, 1996;Barnes and Owens, 1998;Naqvi et al, 1998;Robinson et al, 1998;Usui et al, 2001;Nevison et al, 2003;Codispoti et al, 2005;Bange, 2008).…”