PARTICULATE AND DISSOLVED ORGANIC CARBON IN OCEANS 207 



mainly carbohydrate, although protein stains sometimes show a slight positive 

 reaction. 2 Probably they are mainly polysaccharides. Chemical analyses 

 indicate the presence of algal cell-wall materials and refractory storage products. 



Thin and semitransparent flakes constitute another major fraction. These 

 flakes are irregular in shape, and the median size of the longest dimension is of 

 the order of 25 fJt. They are not obviously organic in nature on casual inspection, 

 but they stain heavily with both protein and carbohydrate stains. They are 

 indistinguishable visually from particles produced experimentally by adsorption 

 of organic matter on bubbles. Indeed, it was only after experimental 

 demonstration of this phenomenon 9 that they were recognized as being part of 

 the organic component and were included among particle counts in seawater 

 samples. 1 ° They are found in both surface and deep water, suggesting that there 

 might be other modes of origin in deep water, but if so, the mechanism remains 

 obscure. 



Finally, the collection on the filter includes large numbers of small particles 

 that are stained readily by histochemical stains but are not otherwise easily 

 characterized. They presumably include small detrital fragments and clay and 

 mineral particles with an organic coating. 



"DISSOLVED" ORGANIC MATTER 



Again this is an operational definition. Filter-passing matter is often called 

 dissolved organic matter, but it obviously includes colloids and small particles up 

 to the retention size of the filter. 



The method most commonly used for measurement of this fraction has been 

 wet oxidation. After preliminary acidification and degassing to remove inorganic 

 C0 2 , the sample is sealed and incubated with a chemical oxidant, most 

 commonly persulfate in recent years. This is followed by further degassing and 

 measurement of CO2 produced by oxidation of organic matter. The ease of this 

 method has led to fairly extensive application, although not without some 

 skepticism as to whether all the organic matter could be oxidized by this 

 method. 



Most of the measurements are of the order of 0.5 to 1 mg C/liter. The 

 highest values are found in the surface layer. In deep water, concentrations are 

 low and relatively uniform both vertically and horizontally. Extensive observa- 

 tions in the North Atlantic by Duursma 1 1 showed some slight evidence of a 

 mid-depth minimum in the vicinity of the oxygen minimum layer. However, no 

 evidence of this was found in the South Atlantic, 1 2 and, in general, none of the 

 slight variations that have been noted would seem to be very consistent or 

 significant. 



Skopintsev 1 3 developed a method for determining organic carbon by 

 evaporating a seawater sample to dryness and releasing C0 2 by dry combustion. 

 His results were considerably higher than the general level obtained by wet 



