Oxygen minimum zones

The balance between relative rates of aerobic respiration and water movement were considered in Section 4.3.4. We saw that a subsurfece concentration minimum, the oxygen minimum zone (OMZ), is a common characteristic of vertical profiles of dissolved oxygen and is produced by in situ respiration. Waters with O2 concentrations less than 2.0 ppm are termed hypoxic The term anoxic is applied to conditions when O2 is absent. (Some oceanographers use the term suboxic to refer to conditions where O2 concentrations fall below 0.2 ppm but are still detectable.) As illustrated by Figure 4.21b, this water column is hypoxic in the OMZ. The dissolved oxygen concentrations are presented as % saturations in Figure 4.21c. With the exception of the mixed layer, the water column is undersaturated with respect to dissolved oxygen with the most intense undersaturations present in mid-depths. Surface supersaturations are the result of O2 input from photosynthesis and bubble injection. 

Oxygen minimum zones (gray) where intermediate and deep water have concentrations <1 ml O2/L. Source-. From Levin, L. A. (2002). American Scientist 90, 436-444. 

Denitrification in continental shelves and OMZs (oxygen minimum zones). Source From Seitzinger, S et al. (2006). Ecological Applications 16, 2064-2090. (See companion website for color version.)... 

Oxygen minimum zone (OMZ) Subsurface undersaturations in dissolved oxygen gas found within... 

Secondary nitrite maximum Subsurface concentration maximum in nitrite found in the oxygen minimum zone. Mainly caused by denitrification. 

Douglas, E.L., Friede, W.A. and Pickwell, G.V. (1976). Fishes in oxygen minimum zones blood oxygenation characteristics. Science, New York 191,957-959. 

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). 

Castro-Gonzalez, M., and Fraias, L. (2004). N2O cycling at the core of the oxygen minimum zone off northern Chile. Mar. Ecol. Progr. Set. 280, 1—11. 

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. 

Ward, B. B., Glover, H. E., and Lipschultz, P. (1989a). Chemoautotrophic Activity And Nitrification In The Oxygen Minimum Zone Off Peru. Deep-Sea Research Part A-Oceanographic Research Papers 36, 1031-1051. 

Montoya,. P., and VoB, M. (2006). Nitrogen cycling in anoxic waters Isotope signatures of nitrogen transformations in the Arabian Sea oxygen minimum zone. In Past and Present Water Column Anoxia (Neretin, L., ed.). NATO Science Series, Springer, Dordrecht, pp. 259—281. 

Below 100 m, NOa increases to a maximum of approximately 42 pM at 800 m, the core of the oxygen minimum zone at greater depths the NOa concentrations decrease to about 36 pM near the seafloor (Fig. 16.7C). It is important to note that... 

Hamersley, M. R., Lavik, G., Woebken, D., Rattray, J. E., Lam, P., Hopmans, E. C., et al. (2007). Anaerobic ammonium oxidation contributes significandy to nitrogen loss from the Peruvian oxygen minimum zone. Limnol. Oceanogr. 52, 923—933. 

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... 

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