WHITLEDGE: REGENERATION OF NITROGEN BY NEKTON 



trawls. The fish were transferred immediately to 

 a holding tank aboard the RV Atlantis II and 

 were maintained as long as a week with daily 

 feeding. Specimens were held at least 6 h before 

 experiments were initiated. Excretion measure- 

 ments were collected on several specimens after 

 the holding tank had been cleaned, rinsed with 

 ethyl alcohol, flushed, and filled with seawater. 

 The tank was covered with black polyethylene 

 sheeting to reduce light and to prevent contami- 

 nation by particulate matter. After the animals 

 were placed in the tank and the experiment was 

 initiated, water samples were collected every 10 

 min for periods of up to 3 h. Most of the experi- 

 ments were started in the late evening so the 

 temperature of the experimental tanks was 

 within 1°-2°C of ambient surface seawater. 

 There was no heating effect by sunlight so the 

 temperature ranged from 14.5° to 16°C for all 

 the experiments with <0.5° change in tempera- 

 ture during any of the experiments. "Fresh" 

 specimens were examined within 12 h of cap- 

 ture. Specimens that had been starved for 1 and 2 

 d were used to estimate nonfeeding excretion 

 rates. After all water samples had been col- 

 lected, the specimens were blotted on towels and 

 weighed. The animals were subsequently dried 

 in a circulating oven at 70°C until a constant dry 

 weight was obtained. The whole dried fish were 

 ground into a powder for determination of per- 

 centage body nitrogen and carbon. 



Excretion samples were analyzed for ammo- 

 nium, urea, nitrate, silicate, phosphate, dis- 

 solved organic nitrogen, and dissolved organic 

 phosphorus. The samples were freshly run and 

 were filtered through a 0.45 jum glass-fiber filter 

 to remove particulate matter. The chemical 

 methods used were similar to those described by 

 Freiderich and Whitledge ( 1972) except for urea 

 and dissolved organic nitrogen and phosphorus 

 which were determined by the methods of 

 DeManche et al. (1973) and Armstrong et al. 

 (1966). 



RESULTS 



Excretion Measurements 



Eight excretion experiments were performed 

 on a total of five species of demersal fish. In addi- 

 tion, excretion measurements were taken from 

 two blue sharks, Prionace glauca, and several 

 octopi, Octopus vulgaris. The typical tank con- 

 centrations of nitrogen compounds measured 



during the experiments are shown in Figure 2. 

 The rate of ammonium excretion was approxi- 

 mately twice as large as the rate for urea. The 

 rate of excretion for ammonium was more nearly 

 linear than that for urea, although the nonlin- 

 earity for urea is probably within the precision 

 limits of the method. The sum of ammonium and 

 urea represents the identified nitrogen excretion 

 in the experiments. The difference between this 

 sum of ammonium and urea and total nitrogen 

 excretion (as measured after ultraviolet irradia- 

 tion) is probably composed of organic nitrogen 

 compounds such as dissolved amino acids, tri- 

 methylamine oxide (Grollman 1929; Wood 1958), 

 or creatine (Whitledge and Dugdale 1972). All 

 experiments conducted showed a nearly linear 

 increase in ammonium concentrations over the 

 short duration of the experimental periods. Like- 

 wise the increases in urea and total excreted 

 nitrogen were nearly linear but were more var- 

 iable than ammonium. 



A summary of all nitrogen excretion experi- 

 ments is shown in Table 1. Well-fed demersal 

 species such as Diplodus senegalensis excreted 

 from 1.03 to 1.44 nS NH 4 -N/mg dry weight per 



6.0 



0) 



- 4.0 



D 



I 



< 

 rr 



o 



o 

 o 



2.0 - 



20 



40 60 



TIME (min) 



80 



FIGURE 2. —Tank concentrations of ammonium, urea, and total 

 nitrogen as measured by ultraviolet irradiation during an 

 excretion experiment. 



329 



