FISHERY BULLETIN: VOL. 70, NO. 4 



McCarthy's were collected from 9 profiles (5 

 samples each) in the euphotic zone of the Peru 

 Current. One of the obvious features of both 

 sets of data is the high degree of variability be- 

 tween samples, even those only a few meters 

 vertically distant from one another. There was 

 no consistent pattern in the profile data although 

 higher values tended to occur at intermediate 

 depths in the euphotic zone. It has been shown 

 that urea accounts for a significant fraction of 

 phytoplankton nitrogen uptake off the coast of 

 southern California (McCarthy, in press), and 

 it has been suggested that animal excretion is 

 the major source of urea in the euphotic zone 

 of that area (McCarthy and Whitledge, 1972). 



Ammonium concentrations are usually low in 

 coastal waters, but this ion can, at times, be the 

 most abundant form of nitrogen available for 

 phytoplankton utilization. It is the major ni- 

 trogenous excretory product of most marine an- 

 imals (Parry, 1960; Baldwin, 1964) and as such 

 is recycled rapidly in surface waters. The im- 

 portance of ammonium in phjrtoplankton nutri- 

 tion has been demonstrated in the eastern trop- 

 ical Pacific by Thomas (1966) and Thomas and 

 Owen (1971). 



Nitrite in neritic waters often shows marked 

 differences with depth, and the concentration 

 is usually somewhat less than that of ammonium 

 ( Vaccaro, 1965) . Nitrite can be formed by bac- 

 teria through either the oxidation of ammonium 

 or the reduction of nitrate. Phytoplankton can 

 utilize nitrite as a source of nitrogen and have 

 been shown to excrete extracellular nitrite when 

 growing on high levels of nitrate (Vaccaro and 

 Ryther, 1960; Carlucci, Hartwig, and Bowes, 

 1970). 



In temperate coastal areas the distribution of 

 nitrate in seawater usually shows a predictable 

 seasonal pattern which is well documented (see 

 Vaccaro, 1965). During the winter, vertical 

 mixing and low rates of plant assimilation keep 

 the nitrate concentration plentiful in near sur- 

 face waters and rather uniform in vertical dis- 

 tribution. With the onset of spring, density 

 stratification substantially reduces the vertical 

 transfer of nitrate, and it is removed from the 

 wind mixed surface waters via phytoplankton 

 assimilation. Bacteria are probably responsible 



for regenerating nitrate through the oxidation 

 of ammonium and nitrite (Harvey, 1966). 



SAMPLE COLLECTION AND 

 ANALYSIS 



Samples were collected with PVC Van Dorn 

 bottles off the coast of La Jolla, Calif., twice 

 weekly from 7 February to 17 April 1970, at 

 three stations in 50 m of water. Station II (Fig- 

 ure 1) was approximately 1 km directly seaward 

 of Scripps Institution of Oceanography pier on 

 the southern edge of Scripps Canyon, Station I 

 was approximately 1.5 km SW of Station II on 

 the southern edge of La Jolla Canyon, and Sta- 

 tion III was approximately 1.5 km NW of Sta- 

 tion II over the more gently sloping bottom north 

 of both canyons. Water for the analysis of 

 chemical and biological parameters was collected 

 from the surface, 10, 20, 30, and 40 m depths at 

 all three stations on each sampling day. Nu- 

 trient analyses were run on samples collected 

 at Station II and alternately on those collected 

 at Stations I and III (with a few exceptions). 

 A Secchi disk reading and bathythermograph 

 cast were always taken at Station II. 



Immediately after sample collection, aliquots 

 were drawn for oxygen determinations. They 

 were "pickled" by the addition of the manganous 

 sulphate and alkaline iodine solutions and were 

 returned to the laboratory for the completion of 

 the analysis. Within 2 hr after sample collec- 

 tion, aliquots were analyzed in duplicate for 

 ammonium and urea while others were analyzed 

 for chlorophyll a and phaeophytin «. Other ali- 

 quots were: 1) frozen and later analyzed (with- 

 in 2 weeks) for nitrate, nitrite, silicate, and 

 phosphate; 2) preserved for microscopic deter- 

 mination of phytoplankton species and numbers; 

 and 3) stored (approximately 3 weeks) for sa- 

 linity determinations. 



The determinations of dissolved oxygen by a 

 modified Winkler technique (Carritt and Car- 

 ]:)enter, 1966), chlorophyll a and phaeophytin a 

 by fluorometry (Holm-Hansen et al., 1965), ni- 

 trate by the cadmium-copper reduction and sub- 

 sequent determination of nitrite (Wood, Arm- 

 strong, and Richards, 1967), nitrite by diazoti- 

 zation (Bendschneider and Robinson, 1952), 



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