Water humus and oxygen demand

The question arises, as to how the decomposition of water humus in the ocean proceeds. The method of estimation of oxygen decrease in water, incubated in the dark at specific temperature, is commonly applied as a qualitative index of OM decomposition. The biochemical oxygen demand (BOD) method was suggested to estimate the degree of purification of waste waters 

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). 

Experimental values of BOD, obtained by Rakestraw (1947) are considerably below those found by Seiwell (1937) in the North Atlantic. The annual BOD, estimated from the surface layer, from 2000 and 4600 m depth (at the temperature in situ) amounted to 4.3 to 0.6 and 0.3 ml O2 1 , respectively. They were calculated using the results of the BOD estimation after ten days, assuming the annual rate of BOD to be constant. In reality. 

The vertical profiles of BOD in the North Atlantic waters studied by Aisatullin and Leonov (1972) show, however, a relationship between the profiles of the OM input to the water and its oxidation. The ability to decompose either particulate or dissolved OM in the ocean water seems to be potentially similar. According to Menzel and Goering (1966), the decrease of particulate organic C is for surface water from 16 to 57% of the initial for a period of 90 days at 20° C. At 200 m and 1000 m depth the changes are 

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