FISHERY BULLETIN: VOL. 70, NO. 4 



The strong positive correlation between ph^eo- 

 pigment' chlorophyll ratios and ammonium con- 

 centrations in Period B suggests that ammonio- 

 telic herbivores were important, and perhaps the 

 presence or abundances of these organisms were 

 also related to the shark presence. 



Another possible explanation for the positive 

 correlation between ammonium and urea con- 

 centrations is that bacterial hydrolysis of urea 

 during Period B was responsible for increased 

 ammonium production. ZoBell and Feltham 

 (1935) isolated marine bacteria capable of hy- 

 drolyzing urea from the same area used for the 

 present study, but indirect evidence (McCarthy, 

 in press) indicates that bacterial hydrolysis is 

 probably of much less importance than phyto- 

 plankton uptake in the fate of urea in near sur- 

 face waters off the coast of Southern California. 



Fish, zooplankton, and phytoplankton have 

 long been known to occur in schools, patches, and 

 layers in near surface waters, and their spatial 

 array probably contributed to the irregular dis- 

 tributions of both ammonium and urea seen in 

 this study. On a scale of a few meters the input 

 due to zooplankton and the utilization due to phy- 

 toplankton may approach an equilibrium, but the 

 immediate effect of a large fish or fish school 

 passing through a particular volume would be 

 elevated ammonium and urea concentrations 

 which would decrease with time at rates depen- 

 dent on both diffusion and biological utilization. 

 Since the blue shark has no urinary sinus (Dr. 

 T. Enns, personal communication), both the 

 branchial and the renal systems would be ex- 

 pected to release urea continuously. One can 

 calculate both branchial (Boylan, 1967) and 

 renal (Forster, 1967) excretioix rates for Squor- 

 Ins, but it is questionable whether these rates 

 would be representative of Prionace glauca. The 

 major problem in attempting to estimate the 

 shark urea production during the present study 

 is, however, that of reliably estimating the shark 

 biomass. 



An increased rate of ammonium or urea pro- 

 duction in a parcel of water will not necessarily 

 be reflected in an increased seawater concentra- 

 tion of the metabolite. To the extent that rates 

 of utilization are dependent on substrate concen- 

 trations, e.g., nitrogen uptake by phytoplankton, 



then an immediate increase in the rate of utili- 

 zation might be expected in response to an in- 

 creased rate of production. There are no sewage 

 outfalls in the vicinity of La Jolla Bay, but the 

 supply of ammonium and/or urea to waters 

 proximate to such discharges may result in 

 significant enrichment with these nutrients 

 (Eppley et al, in press). 



Because of the spatial variability in both sup- 

 ply and utilization and the low concentrations 

 usually encountered, it is difficult to interpret, or 

 even detect, short-term changes in ammonium 

 and urea concentrations in near-surface sea- 

 water. Surface waters into which fish and 

 crustaceans migrate daily in some degree of 

 synchrony to feed is one setting in which such 

 changes might be expected, and have been ob- 

 served. Beers and Kelly (1965) presented data 

 which suggested a correlation between variations 

 in ammonium concentrations in the upper 500 m 

 of the Sargasso Sea and the diurnal migration 

 of zooplankton, and Emmet (1969) referred to 

 unpublished work which showed that a diurnal 

 maximum in urea concentration occurs in open 

 ocean surface waters early in the morning. 



If, as Lorenzen (1967) has suggested, a 

 phaeo-pigment/chlorophyll ratio is indicative of 

 herbivore grazing pressure, the significant cor- 

 relation between this ratio and ammonium con- 

 centrations reported in Period B of this study 

 might imply that (1) both fecal material and 

 ammonium excreted by herbiverous zooplankton 

 have residence times which are similar, or (2) 

 ammonium is released with the degradation of 

 herbivore fecal pellets. The lack of correlation 

 in Periods A and C might imply that herbivores 

 were not significant sources of ammonium dur- 

 ing these periods. The lack of correlation be- 

 tween phaeo-pigment/chlorophyll ratios and 

 urea concentrations might imply that (1) her- 

 bivore excretion is not an important source of 

 urea, (2) the residence times of phaeo-pigments 

 and urea are sufficiently different to mask any 

 association, (3) urea excreted by nonherbivores 

 obscured a relationship between phaeo-pigment/ 

 chlorophyll ratios and urea concentrations, or 

 (4) urea is not released with the degradation of 

 herbivore fecal pellets. Herbivores zooplankton 

 have been shown to release substantial quantities 



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