Oxygen minimum layer

Anoxic or almost anoxic conditions are found in stagnant basins such as the Black Sea or in basins with sill depths in the oxygen minimum layer like the Santa Barbara or San Pedro basins off California (16). Under such conditions anoxic conditions may develop in the water under the sill depth or in the sediment just below the sediment-water interface. 

Naqvi, S. W. A., and Shailaja, M. S. (1993). Activity of the respiratory electron transport system and respiration rates within the oxygen minimum layer of the Arabian Sea. Deep Sea Res. I. 40, 687-696. 

In the same water samples, stored at 8—9° and 4—5°C, the results proved to be more constant after 100 days the oxygen content remained at the same level. In samples taken from the two deeper layers, the content remained practically unaltered after the first 50 days. The high oxygen consumption in the surface layers was attributable to planktonic material. BOD increased slightly in the water from the oxygen minimum layer when after 42 days the bottles were incubated at 25°C. 

During 305 days the oxygen decrease at the in situ temperature in those successive layers constituted 0.61, 0.08 and 0.14 ml O2 1" for the waters from surface, the oxygen minimum layer and the deep layers, respectively. Thus, OM is biochemically more resistant in the oxygen minim vim layer than in the deeper layers. It is noteworthy that BOD in the oxygen minimum layer did not exceed that in the deeper layer. Plunkett and Rakestraw (1955) indicated that in this layer of the Pacific Ocean there was evidence neither for a decrease of dissolved C nor for an accumulation of particulate OM (Rakestraw, 1958). The same results were obtained for the Atlantic Ocean (Menzel and Ryther, 1968). 

Skopintsev, B.A., 1961. Study of oxygen minimum layer in the North Atlantic Ocean, autumn, 1959. Okeanol. Issled., No. 13 108—114 (in Russian). 

In practiced field work, selection of the sampUng sites and depths often depends not only on the demands of the chemists, but also on those of hydrographers, biologists and/or scientists from other disciplines. So, the chemists often have to share water samples with other groups. Obviously, sampling at standard depths is unsuitable if specific problems are to be studied, such as near bottom gradients, the chemistry of the oxygen minimum layer or the conditions in discontinuity layers. 

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. 

Figure 2 The depth helow the sediment-water interface at which pore-water dissolved O2 concentrations fall to zero on three continental margins one in the northeast Atlanic (circles Lohse et al., 1998) one in the northwest Atlantic (squares Martin and Sayles, suh-mitted) and one in the northeast Pacific (triangles Reimers et al, 1992). In aU cases, the thickness of the oxic layer is less than that in pelagic sediments. It is thicker in Atlantic margin sediments than in the northeast Pacific because of the intense oxygen minimum in the northeast Pacific.

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

The existence of a maximum thickness beyond which the performance deteriorates is due to the concerted impact of oxygen and proton transport limitations. Considered separately, each of these limitations would only serve to define a minimum thickness below which performance worsens due to an insufficient electroactive surface. The thickness of the effective layer, in which current density is predominantly generated, is given by the reaction penetration depth ... 

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