c 

 o 



\- 

 ni 



o , 



(J g 



'tr -^ 



CT — 



o ^ 



25 



20 



15 



E 



o 

 o — - 



^ ^ 10 



cc 



10-3 10-2 10-1 100 101 



Particle size ( A/m) 



102 



103 



Figure 4-17. Size distribution of organic carbon particles in 

 sea water (adapted from Sharp 1973) . 



of particle size. Although all living cells are contained in size fractions 

 greater than approximately 0.5 yM they account for <5% of the total mass of 

 carbon in the water column. In fact, it might be argued that any marine 

 organism larger than a 10 MM phytoplankton cell is insignificant in the 

 organic matter cycle in the oceans. 



Three size groups of organic matter are present in sea water: particulate, 

 intermediate, and dissolved. Particulate matter is any microbial organism or 

 nonliving material that is large enough to be injested by a filter-feeding 

 organism or to support bacterial attachment. Dissolved matter can be 

 assimilated through cell membranes. The intermediate class of material is too 

 small to be caught and digested but too large to assimilate through the cell 

 membrane. In the following discussion, "biologically active" organic matter 

 represents the larger and small-sized groups. Intermediate sized particles 

 are ill-defined and are probably converted and transformed primarily by 

 physical and chemical processes. 



The original source of organic matter is plant photosynthesis. The primary 

 source in nearshore areas and estuaries is terrestrial vegetation (introduced 

 by freshwater runoff), emergent wetland plants, benthic diatoms, and attached 

 algae. In the open ocean, phytoplankton is the most important source of 

 organic matter. Concentrations of organic material are much more variable in 

 coastal waters, due to local inputs, than in the open ocean. 



4-51 



10-80 



