PARTICULATE AND DISSOLVED ORGANIC 

 CARBON IN THE OCEANS 



GORDON A. RILEY 



Institute of Oceanography, Dalhousie University, Halifax, Nova Scotia 



ABSTRACT 



Total nonliving organic carbon in the oceans averages 1 mg C/liter, more or less, and totals 

 10 1 2 metric tons or more in all the world's oceans. This is estimated to be of the order of 

 100 times the carbon content of living organisms in the sea. The preponderance of nonliving 

 material is indicative of gradual accumulation and a long residence time, suggesting that 

 much of the organic matter is virtually unusable biologically. However, recent evidence 

 suggests that perhaps 1% or a little more consists of free amino acids and other biologically 

 labile substances that are recycled more rapidly. These substances can be used for growth of 

 small heterotrophic algae and bacteria that then serve as a food supply for filter-feeding 

 animals. Bacterial action and certain physical processes tend to aggregate small particles and 

 dissolved substances into larger particles that provide a substrate for heterotrophs and also 

 make the particles more available to the animal population. The latter excretes soluble 

 organics that help to maintain the food web. These complicated relations between the ocean 

 creatures and the nonliving organic components are the subject of this paper. 



Nonliving organic matter includes particles and masses of material of micro- 

 scopic and even macroscopic size, small filter-passing particles and colloids, and 

 material in true solution. The amount, in terms of organic carbon content, is of 

 the order of 1 mg C/liter. Of this, one-tenth or less consists of particles large 

 enough to be retained by filters with a pore size of 0.45 to 1.2 ju, such as are 

 commonly used for separation. More than half the total will go through a Diaflo 

 membrane that passes substances with a molecular weight of 50,000 or less. 



Carbon measurements commonly involve destruction of the organic matter 

 by wet or dry oxidation or by strong ultraviolet (UV) radiation, followed by 

 measurement of the C0 2 evolved. There have been technical difficulties in 

 development of the methods, and results obtained by different methods are not 

 precisely comparable. However, these difficulties are minor compared with the 

 problem of biochemical characterization of the material, which is chemically 



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