100 R. T. Prentki et al. 



Isoetes lacustris in a mountain lake in the Black Forest (Tessenow and 

 Baynes 1975). In some of these situations, the oxygenated sediments may 

 result in part from the extremely low rates of sediment respiration. 



NITROGEN * 



Concentrations in Water 



Although phosphorus is usually thought to be the most important 

 limiting nutrient in freshwater, the nitrogen compounds were also present 

 in very low concentrations in the Barrow ponds and could be limiting algal 

 production. Also, the concentration of a particular nitrogen compound 

 may interact with the specific transport systems of algae to allow one 

 species to have a competitive advantage over another. For this reason, an 

 intensive study of the nitrogen concentrations was made during 1970 and 

 1971. In addition, the uptake and cycling of nitrogen was studied in a 

 series of experiments using '''N as a tracer. 



In 1970 and 1971, twice-weekly measurements were made of 

 inorganic nitrogen (NO2, NO3, NH3) in Ponds B, C, D, and E and weekly 

 measurements were made in Ikroavik Lake. Several other ponds and 

 subponds were also studied. In addition, in 1971 particulate nitrogen (FN) 

 and dissolved organic nitrogen (DON) were measured six times in B, C, 

 andD. 



The nitrate and nitrite analyses were carried out in 1970 and 1972 

 with the copper-cadmium column technique (Strickland and Parsons 

 1965). In 1971, a sodium phenate reduction modified for high levels of 

 dissolved organic substances was used (Barsdate and Alexander 1971). 

 Ammonia analyses followed Soldrzano (1969) with a sodium hypochlorite 

 and phenol treatment and measurement of the blue indophenol. 

 Particulate nitrogen was measured on a Coleman Automated Nitrogen 

 Analyzer and dissolved organic nitrogen after oxidation with a strong UV 

 lamp. 



Ammonia is generally the predominant inorganic nitrogen form in 

 the ponds; concentrations reached 85 Mg N liter"' on occasion and never 

 fell below 9.5 ^l% N. Monthly averages for 1970 and 1971 are given in 

 Table 4-8 while the actual data for Pond B, 1971, are given in Figure 4-12. 

 Nitrate levels were often below the limit of detection, although during the 

 summer of 1971 the concentrations remained higher than in 1970. 

 Seasonally, nitrate levels tend to be maximal early in the season and to 

 decline as the season progresses; ammonia shows two maxima, one early 

 in the spring as soon as the ice melts (preceding the nitrate maximum) and 



*V. Alexander 



