Oceans Oxygen Minimum Zones

Anderson, J.J., Okubo, A., Robbins, A. S., and Richards, F. A. (1982). A model for nitrite and nitrate distributions in oceanic oxygen minimum zones. 

Five depth profiles of NO and the corresponding NO production rates have been measured in the ETNP (Ward and Zafiriou, 1988) NO concentrations were in the range from 0 up to 65 pmol At four stations located in the open ocean, maximum NO concentrations were observed at the upper boundary of the oxygen minimum zone (OMZ, O2<10 pmol L ), whereas one coastal station showed an increase of NO from Opmol at the surface to about 20 pmol at the bottom in about 250m. Maximum NO production rates were found at the upper boundary of the OMZ at the open ocean stations. However, Ward and Zafiriou (1988) could not unambiguously identify the NO formation process because NO production rates and nitrification rates (i.e., NH oxidation rates) were not correlated. NO accumulation appeared when O2 concentrations were lower than 100 pmol L , whereas in the core of the OMZ with O2 concentrations close to 0 pmol denitrification seemed to cause a rapid turnover of NO. Highest ever-reported concentrations of dissolved NO were found off Peru ranging from 0 up to 400 pmol (Zafiriou, personal communication in Ward and Zafiriou (1988)). 

N2O profiles from oceanic regions with suboxic zones such as the Arabian Sea and the eastern tropical North Pacific Ocean, which are sites of intense denitrification activities, generally show a two-peak structure (Fig. 2.3) N2O maxima are found at the upper and lower boundaries of the oxygen minimum zone (OMZ), whereas in the core of the suboxic zone, N2O concentrations are considerably depleted (Bange et ah, 2001b Cohen and Gordon, 1978). In anoxic water masses such as found in the central Baltic Sea, the Cariaco Basin, and Saanich Inlet, N2O concentrations are close to the detection limit or not detectable (Brettar and Rheinheimer, 1991 Cohen, 1978 Hashimoto et ah, 1983 Ronner, 1983 Walter et ah, 2006b). 

Nitrifying bacteria are traditionally considered to be obligate aerobes they require molecular oxygen for reactions in the N oxidation pathways and for respiration. They are reputed to be microaerophiles, however, who thrive best under relatively low oxygen conditions. Microaerophily may be important in interface environments such as the sediment water interface and in the oxygen minimum zones of the ocean. The role of oxygen in sedimentary nitrification and coupled nitrification/ denitrification is discussed above in the section on nitrification in sediments. 

In the near future we anticipate further progress in ocean acidification as a result of increased atmospheric CO2 concentrations (Caldeira and Wickett, 2003) with sea surface pH potentially reaching as low as 7.8, a decrease of 0.5 pH units since the middle of the 20th century. More extensive periods of stratification and a spreading of oxygen-minimum zones in the world s oceans are also expected. Each of these processes is likely to impact on the oceanic N-cycle and the role cyanobacteria play within these systems. Specifically, these climate induced changes are likely to have significant effects on the composition of marine cyanobacterial communities and hence on the N dynamics they carry out. 

Both N03 and particles in and near the major oceanic denitrifying regions have a that is high relative to typical open ocean values (Brandes et al., 1998 Chne and Richards, 1972, 1975 Liu and Kaplan, 1989 Sutka et al., 2004 Voss et al., 2001). Within the oxygen minimum zones (OMZs) proper, the of... 

The denitrification is still insufficiently quantitatively understood in aquatic ecosystems, especially by comparison to terrestrial ecosystems. There are different views on whether the oceans are a source or sink of nitrous oxide. Various data indicate that the ocean is, on average, supersaturated with respect to N2O, and that N2O supersaturations are positively correlated with N03 and negatively correlated with O2. A number of studies have suggested that denitrification is a major sink for fixed nitrogen in the oceans. Recent studies suggests that denitrification losses in the oceans are on the order of 0.13 x 10 tons/yr (9.2 x 10 mol N per year) that exceed known oceanic N inputs. More than half of this denitrification (0.067 x 10 tons/yr (4.8 x lO mol N per year) takes place in sediments, with the remainder in pelagic oxygen minimum zones (see Box 3 for more details). 

Wakeham S.G. (1987) Steroid geochemistry in the oxygen minimum zone of the eastern tropical North Pacific Ocean. Geochim. Cosmochim.Acta 51, 3051—69. 

Smith, S.L., Morrison, J.M. and Codispoti, L.A. (2002) The role of the oxygen minimum zone in biogeo-chemical cycles, in Report of the Indian Ocean Synthesis Group on the Arabian Sea Process Study, JGOFS Report No. 35 (eds L. Watts, P. Burkill and S. Smith), Scientific Committee on Oceanic Research, International Council of Scientific Unions, Bergen, pp. 57—64. 

Dickens GR, Owens RM (1994) Late Miocene-early Pliocene manganese redirection in the central Indian Ocean Expansion of the intermediate water oxygen minimum zone. Paleoceanogr 9 169-181 Dodd JC, Burton CC, Bums RG, Jeffries P (1987) Phosphatase aetivity assoeiated with the roots and the rhizosphere of plants irrfeeted with vasoular-arbusoular myeorrhizal fungi. New Phytologist 107 163-172... 

Figure 3 Vertical profiles of Os concenfrafion and Os/ Os rafio in sea wafer from fhe Indian Ocean and fhe easfern fropical North Pacific. The Pacific data (Woodhouse et al. 1999 EPSL 173 223) include analyses of both filtered and unfiltered samples no systematic difference between the two is apparent. Note that the low Os concentrations in the Pacific profile coincide with the core of a very strong oxygen minimum zone. Indian Ocean data (Levassuer et al. 1998 Science 282 272) indicate that Os behaves conservatively.

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