Chemistry 151 



Low pond water phosphate concentrations are caused more by retention of 

 most sediment phosphorus in unavailable organic or reductant-soluble 

 inorganic form and by strong sorption of the rest by sediments than by low 

 sediment total phosphorus levels. Either mineralization of sediment 

 organic phosphorus or transformation of reductant-soluble phosphorus to 

 an exchangeable form would result in a several-fold larger pool of 

 exchangeable inorganic phosphorus and in an increase in phosphate 

 concentration in the water column. Thus, the low productivity of the ponds 

 is related to the low temperatures which keep much of the phosphorus tied 

 up in the sediments. Higher temperatures would cause higher 

 decomposition rates and a consequent higher rate of supply of phosphorus. 

 This would not necessarily remove the phosphorus limitation on growth of 

 algae but would allow much higher rates of primary production. 



ORGANIC CARBON 



Dissolved Organic Carbon 



Most of the carbon in the pondwater is present as dissolved organic 

 carbon (DOC). The DOC is composed of a tremendous variety of 

 compounds but can be roughly divided into two parts; the larger of these is 

 high molecular weight compounds such as polymerized humic acids, and 

 the smaller is low molecular weight compounds such as simple 

 carbohydrates, amino acids, and small organic acids. 



One reason for the high concentrations of high molecular weight 

 compounds is that they are resistant to biological breakdown. Many of 

 them originate from the terrestrial system and enter the ponds during 

 runoff. Thus, they are the terminal products of decomposition in soils. Of 

 course, these resistant compounds are eventually broken down, perhaps by 

 an enzymatic hydrolysis or a photo-decomposition by UV, and the smaller 

 products are used by bacteria. 



In contrast, the low molecular weight compounds are readily used by 

 bacteria; this holds the concentrations at a very low level. These labile 

 compounds are derived mainly from algae and macrophytes by leakage 

 and decay although a portion also arises from breakdown of the long- 

 chain compounds. 



Total DOC 



The organic material passing through a Reeve Angel 984 H glass 

 fiber filter, which has a mean pass of 0.8 ^m (Sheldon 1972), is called 

 dissolved. Organic material retained by the filter is called particulate 

 although it is likely that some organic colloids are also retained by the 



