McCarthy and kamykowski: urea and other nitrogenous nutrients 



RESULTS 



Representative plots of the nutrient data are 

 shown in Figures 2 and 3. The nitrate concen- 

 trations consistently increased with depth. When 

 a thermocline was present, the median nitrate 

 concentration above was 0.49 {xg at. N/liter while 

 that below was 12.81 /x at. N/liter. The median 

 nitrite concentration above the thermocline was 

 0.04 fjig at. N/liter while that below was 0.36 fjig 

 at. N/liter. The median ammonium concentra- 

 tion above the thermocline was 0.09 [xg at. N/ 

 liter while that below was 0.34 ixg at. N/liter. 

 Both the medians and the variabilities for ni- 

 trate, nitrite, and ammonium concentrations 

 above the thermocline were significantly differ- 

 ent from those below. The median urea con- 

 centration above the thermocline was 0.16 (xg 

 at. N/liter while that below was 0.12 fxg at. 

 N/liter, and neither the medians nor the var- 

 iabilities were significantly different. 



Urea data for stations made on the same day 

 are plotted in Figure 4. We first noticed that 

 the urea values appeared to vary markedly from 

 one month to another, and since elasmobranches 

 excrete nitrogen primarily as urea (Baldwin, 

 1964), this pattern suggested that the quantity 

 of urea in the water may have been affected by 

 a heavy infestation of 1-2 m long blue sharks 

 (Prionace glauca). The infestation was first 

 noted on 28 February, and initially one could 

 see a few tens of dorsal fins extending above 

 the surface at any time in the vicinity of any 

 of the stations. After a few days the sharks 



became much more wary of the skiff and al- 

 though they remained in great numbers, they 

 were usually deeper. Shark sightings continued 

 to be numerous until 21 March and thereafter 

 they were rare. The periods before (7-24 Febru- 

 ary) , during (28 February-21 March) , and after 

 (24 March-17 April) the infestation, hereafter 

 referred to as Periods A, B, and C, are indicated 

 in Figure 4. These periods define an initial stage 

 of low urea values, a secondary stage of higher 

 and more variable urea values, and a final stage 

 which appears similar to the first. Medians and 

 ranges for nitrate, nitrite, ammonium, and urea 

 for the three periods are given in Table 1 and 

 were used for the statistical analyses reported 

 in Tables 2, 3, and 4. The highest concentration 

 of urea detected at any time during the sampling 

 program, 1.28 /xg at. N/liter, was in a surface 

 sample collected approximately 1 m from a 21/2-"^ 

 blue shark which was near Station I. 



Comparisons between Periods A, B, and C 

 Table 4) show that the periods differed with re- 

 spect to nitrate, nitrite, and ammonium as well 

 as urea. The median nitrate concentration in 

 Period B was significantly lower than those in 

 either Period A or Period C. The median nitrite 

 concentrations were similar for all three periods. 

 The median ammonium concentration for Period 

 C was significantly lower than that for Period A. 

 The median urea concentration for Period B was 

 significantly higher than those for either Period 

 A or Period C. Both the variability and the range 

 of urea concentrations were considerably higher 

 in Period B than in either Period A or Period C. 



Table 1. — Medians, ranges, and number of samples for nitrate, nitrite, ammonium, and urea within each period. 

 The medians and ranges for each form of nitrogen are in units of /ig at. N/liter. 



1265 



