Chemistry 171 



Oxygen 



The oxygen in the pond water was near saturation in the deep areas of 

 the pond but decreased to one-third of saturation in the shallow Carex bed. 

 This was the result of high sediment respiration rates and poor circulation 

 of the shallow water. Overall, the water was undersaturated due to 

 sediment respiration. 



Eh and pH 



The Carex beds also influenced the sediment pH and Eh. In the center 

 of the ponds, only the top 1 to 2 cm is oxidized. But beneath the plant beds, 

 the oxidized zone extends to 8 or 10 cm. This results from the downward 

 transfer of O2 within the roots. The E7 at 14 cm was about 50 mV in the 

 pond center and 125 mV beneath the Carex roots. Thus, the sediments are 

 reducing but not strongly reducing. The pH fell to a low of 5.8 at the top of 

 the anaerobic layers of the sediment but then rose slightly to 6.4 in all the 

 deeper layers. 



Nitrogen 



The various compounds of nitrogen are present in the pond water in 

 low concentrations. Nitrite was always insignificant as levels were below 

 0.5 Mg N liter " ' . Nitrate concentrations were high early in the year (up to 

 60 Mg N liter ~ ' with an average of 7 to 40 ng N) but always fell late in the 

 summer (1 to 9 ^g N liter '). Ammonia has two peaks; one occurs early in 

 the spring and precedes the nitrate maximum and the other occurs in late 

 July and early August. Concentrations were always at moderate levels 

 (average monthly values from 20 to 50 Mg N liter '). There was also an 

 ammonia peak in the meltwater (100 to 200 ng n liter '). However, most 

 of the nitrogen is present in organic form. The dissolved organic nitrogen 

 (DON) was most abundant (600 to 1000 ng N liter " ') while the particulate 

 organic nitrogen (PON) was about the same concentration as the total 

 inorganic forms (40 to 100 Mg N liter ~ '). These concentrations of nitrogen 

 are much higher than those in arctic lakes; in part, the regeneration of 

 ammonia from the sediments keeps the concentrations at these moderate 

 levels. 



Concentrations were high in the sediments from the center of the 

 pond (5580 ng DON liter"', 3000 ^ig NH3-N liter ', 11 Mg NO3-N 

 liter '). The rooted sedges and grasses take up ammonia so the amount in 

 the sediment near the plants was low (50-70 ^g NH3-N liter '). 



There is nitrogen fixation in the sediments but the rate is very low. 

 The average value of 0.31 mg N m '^ day ' is equal to 28 mg N m \ yr"' 



