Chemistry 173 



added lights, the nitrogen fixation was increased, which implies that there 

 is a link between photosynthesis and fixation. When high amounts of 

 phosphorus were added each day, the fixation rate increased in direct 

 proportion to the amount added. We interpret this as indicating that the 

 algae are usually phosphorus limited. When excess P is added, the rate of 

 supply of N becomes limiting and algae, such as blue-green algae, that can 

 fix N gain a competitive advantage. 



When P was added to an artificial pond containing no sediment, 

 productivity did not increase until extra N was added. This illustrates the 

 importance of the sediments in supplying nitrogen to the water. 



Phosphorus 



The concentration of dissolved reactive phosphorus (DRP) in the 

 pond water was always low, generally between 1 and 2 ^ug P liter ~\ 

 However, there was a brief period in mid- or late June when the 

 concentrations reached 4 or 5 Mg P liter "\ The P came from runoff and 

 from leaching of the standing dead vegetation; when the algal 

 photosynthesis increased and the rate of supply decreased in late June the 

 concentration of DRP dropped below 1 ng P liter ~\ 



Dissolved unreactive phosphorus (DUP) is re-introduced into the 

 ponds each spring through litter decomposition, sediment leaching, and 

 runoff. When the snowpack is deep, the runoff is large and the DUP 

 remaining in the ponds is low. The concentration in the ponds averaged 12 

 Mg DUP liter"'; this was 56% of the total phosphorus. About 70% of the 

 DUP was refractory while the rest could be easily broken down and made 

 available to algae. 



Another important phosphorus fraction is the particulate (PP). This 

 was fairly constant in concentration and averaged 10 Mg PP liter '. Large 

 zooplankton were not included so this includes only algal, bacterial, and 

 detrital phosphorus. 



Trough ponds are strikingly different from polygon ponds in that all 

 forms of phosphorus are more abundant. Concentrations were 3.7 to 

 7.6 Mg DRP liter"', 18.6 to 60.6 ^ig DUP liter"', and 20.3 to 31.5 ng PP 

 liter"'; these are 2 to 7 times the concentrations in the low-centered 

 polygon ponds. 



The concentrations of phosphorus in the interstitial water are much 



^higher than those in the water column above but most of the difference is 



due to high DUP. Thus, the DRP was mostly 1 to 5 Mg liter"', while the 



DUP was 24 to 138 Mg P liter "'. There were no observed differences due to 



the presence of roots or reduced sediments. 



The sediment phosphorus was investigated by chemical 

 fractiiDnation and ^^P. An iron-rich sediment holds nearly all of the 

 inorganic P in the ponds. It is held by sorption of phosphate on or 

 occlusion within hydrous iron oxide rather than by the formation of iron. 



