McCarthy and kamykowski: urea and other nitrogenous nutrients 



of the shark infestation, and it is evident that 

 the urea values were elevated significantly dur- 

 ing Period B. It is unlikely that the significant 

 difl^erences in the medians and variabilities of 

 the nitrate, nitrite, and ammonium values be- 

 tween periods were related to the presence of the 

 sharks per se. The trends for each nutrient 

 were markedly diflferent: For nitrate the Period 

 B median concentration was less than that in 

 either Period A or Period C, for nitrite the me- 

 dian was unchanged, and for ammonium the me- 

 dian in Period B was greater than that in Period 

 C but not diflferent from that in Period A. Kamy- 

 kowski (see footnote 4) presents data which 

 suggests that a mechanism involving internal 

 waves acting together with intermittent wind- 

 related events (e.g., upwelling) can explain the 

 pattern of increases in surface nitrate and cor- 

 responding decreases in surface temperature 

 found in the study. Strasburg (1958) has shown 

 that blue sharks in the central Pacific Ocean were 

 found most frequently where the temperature 

 was between 7° and 15°C; however, the temper- 

 ature differences with respect to either time or 

 depth which were encountered during the pres- 

 ent study were not sufficient to provide an ex- 

 planation for the shark infestation. 



The nitrite regression equations contrasting 

 the two stations sampled on the same day reveal 

 that the dynamics of the nitrite production and 

 utilization is uniform over the scale of a few 

 kilometers. If the greater nitrite concentration 

 below the thermocline represents a greater rate 

 of production, our finding is consistent with the 

 ideas put forth by Vaccaro and Ryther (1960) 

 which suggest that nitrite liberation by phyto- 

 plankton in the presence of excess nitrate will be 

 intensified at reduced light levels. The lack of 

 correlation between the chlorophyll a and nitrite 

 concentrations, however, implicates a complex 

 interaction between components of the nitrite 

 system. 



The lack of correlation between ammonium 

 and urea concentrations for Period A and Period 

 C follows the pattern seen earlier in data from 

 the English Channel (Newell, 1967), the Peru 

 Current (McCarthy, 1970), and the central Pa- 

 cific (McCarthy, 1971), but the strong positive 

 correlation in Period B deserves further con- 



sideration. The median ammonium values for 

 Periods A and B were not diff"erent, and although 

 the rates must have been similarly balanced, 

 the sources as well as rates of both supply and 

 utilization could have been considerably diflfer- 

 ent for the two periods. If the shark infestation 

 was responsible for the elevated urea concentra- 

 tions during this period, was it perhaps not also 

 responsible for enough of the ammonium present 

 to have produced the observed relationship? It 

 would seem unlikely that the sharks were directly 

 responsible since the selachii (modern sharks) 

 are reported to excrete 80-90% of their nitrog- 

 enous end products as urea, 2-10% as ammoni- 

 um, and the balance as amino acids and an 

 unidentified fraction (Scheer, 1963). During Pe- 

 riod B a squid (Loligo opalescens) spawn was 

 noted off the coast of La Jolla. These usually 

 last approximately 1 week, and during this time 

 the adult squid reportedly do not feed (A. 0. 

 Flechsig, personal communication). Sharks 

 collected during this period were found to have 

 been feeding on squid (N. Marshall and G. Sul- 

 livan, personal communications), but it seems 

 unlikely that the sharks were attracted to this 

 area by the squid since such spawns are fre- 

 quently noted without the presence of sharks. 

 The squid spawns do, however, regularly attract 

 birds (Larus spp.), and we also noted greater 

 abundances of birds during this time. Nitrogen 

 is excreted principally as ammonium by cepha- 

 lopods (Barnes, 1963) and as uric acid by birds 

 (Needham, 1931). It has been reported that 

 uric acid is unstable in seawater and rapidly de- 

 grades to urea (Williams, 1970)' so the birds 

 might have been an additional source of urea. 

 It is therefore possible that Period B was also 

 unique with respect to the presence of ammonio- 

 telic organisms which were either directly or in- 

 directly supporting or supported by the shark 

 population and which were in sufficient abun- 

 dances to have resulted in the positive correlation 

 between ammonium and urea concentrations. 



' Williams, P. M. 1970. The stability of organic ni- 

 trogen compounds in seawater. University of California, 

 Institute of Marine Resources. Research on the marine 

 food chain progress report for U.S. Atomic Energy Com- 

 mission Contract At (11-1) GEN 10, P.A. 20. Part 1, 

 p. 8b. [Unpubl. manuscr.] 



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