Apparent oxygen utilization

The key to imderstanding these remineralization reactions is the parameter Apparent Oxygen Utilization (AOU), defined as... 

Annual percent saturation of dissolved oxygen in (a) surface waters (Om) and (b) 50 m water depth. Source-. From Garcia, FI. E., et al. (2006). World Ocean Atlas 2005, Volume 3 Dissolved Oxygen, Apparent Oxygen Utilization, and Oxygen Saturation. NOAA Atlas NESDIS 63, U.S. Government Printing Office, p. 342. 

AOU, apparent oxygen utilization The difference in O2 concentration between that in a deepwater sample and its NAEC. It is a measure of the amount of O2 consumed via the respiration of organic matter since a deep-water mass was last at the sea surface. 

The first quantitative estimates of denitrification in the eastern tropical North Pacific were made by Codispoti and Richards (1976). Codispoti and Richards used apparent oxygen utilization, AOU, and phosphate data to stoichiometricaUy extrapolate back to the nitrate concentration present when a given water mass was previously at the surface. In this way they were able to develop a relationship between pNOj and sigma-f for waters of the ETNP-ODZ, from which they calculated nitrate deficit as outlined in Eq. (6.4). 

Doval M. D. and Hansell D. A. (2000) Organic carbon and apparent oxygen utilization in the western South Pacific and the central Indian Oceans. Mar. Chem. 68, 249-264. 

Figure 15.7. Stoichiometric correlations among nitrate, phosphate, oxygen, sulfide, and carbon. The correlations can be explained by the stoichiometry of reactions such as equation 3 concentrations are in micromolar, (a) Correlation between nitrate nitrogen and phosphate phosphoms corrected for salt error in waters of the western Atlantic, (b) Correlation between nitrate nitrogen and apparent oxygen utilization in same samples. The points falling off the line are for data from samples above 1000 m (Redfield, 1934, p. 177). (c) Correlation between nitrate nitrogen and carbonate carbon in waters of the western Atlantic, (d) Relation of sulfide sulfur and total carbonate carbon in waters of the Black Sea. Numbers indicate depth of samples. Slope of line corresponds to AS /AC = 0.36. (From data of Skopintsev et al., 1958, as quoted in Redfield et al., 1966.) (e) Correlation of the concentration of nitrogen to phosphate in the Atlantic Ocean (GEOSECS data). The slope through all the data yields an N/P ratio close to 16.

The amount of O2 deficit due to organic matter respiration in a water sample can be estimated by knowing the temperature, salinity, and O2 concentrations. The difference between the O2 calculated to be at equilibrium, [0 ], and the measured O2 value is called the apparent oxygen utilization or AOU ... 

Cross sections of apparent oxygen utilization (AOU) in the Atlantic, Indian and Pacific Oceans. Modified from figures supplied by Robert Key (Key et a/., 2004). 

Emerson, S., T. W. Watanabe, T. Ono and S. Mecking (2004) Temporal trends in Apparent Oxygen Utilization in the upper pycnocline of the North Pacific 1980-2000. J. Oceanogr. 60,139-47. 

It follows that deep seawater contains nutrients from two sources. First, it may contain nutrients that were present with the water when it sank from the surface. These are called "preformed nutrients". Second, it may contain nutrients derived by the in situ remineralization of organic particles. These are called oxidative nutrients. The oxidative nutrients can be estimated from the RKR equation. From this model, we might expect the four dissolved chemical species (O2, CO2, NO3, PO4) to vary in seawater according to the proportions predicted. The key to understanding these remineralization reactions is the parameter Apparent Oxygen Utilization (AOU), defined as ... 

Ogura, N., 1970a. The relation between dissolved organic carbon and apparent oxygen utilization in the Western North Pacific. Deep-Sea Res., 17 221—231. 

Apparent oxygen utilization rates from the central Arctic that are so high, they need to be balanced by transport of high production water from over the continental shelves. 

Figure 12 Apparent oxygen utilization plotted against measured A C for WOCE Pacific Ocean samples taken at depths greater than 4000 m and north of 40°S. The slope of the line (-0.831+0.015) can be used to estimate an approximate oxygen utilization rate of 0.1 pmol kg y if steady state and no mixing with other water masses is assumed.

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