REGENERATION OF NITROGEN BY THE 



NEKTON AND ITS SIGNIFICANCE IN THE NORTHWEST 



AFRICA UPWELLING ECOSYSTEM 



Terry E. Whitledge 1 



ABSTRACT 



Nitrogen and phosphorus excretion rates were measured for octopus and six species of fish in the 

 northwest Africa upwelling region near lat. 21°40'N. The nekton excretion rates ranged from 0.44 to 

 4.61 /ig NHVN/mgdry weight per day and the whole body C:N (by atoms) of the specimens was 4. 85. 

 The calculated nitrogen turnover time in the well-fed specimens was about 65 days. The estimated 

 rates of ammonium regeneration over the shelf (<200m) for all the nekton wasabout3mg-at/m 2 per 

 day which was 27% of the phytoplankton ammonium uptake requirements. On the slope (>200m) the 

 nekton regenerated 1.8 mg-at/m 2 per day which was 11% of phytoplankton ammonium uptake. The 

 ammonium production by bacterioplankton. zooplankton, nekton, and sediments accounted for 

 226% of the ammonium utilized in the nearshore shelf region and 83% in the offshore region. 



Ammonium is an important source of nitrogen 

 for phytoplankton growing in the sea. Estimates 

 of nutrient uptake using 15 N tracer experiments 

 have indicated that ammonium regeneration 

 may be responsible for 44 to 83% of nitrogen 

 utilized by phytoplankton in the North Pacific 

 gyre (Eppley et al. 1973) and up to 50% in the 

 Peru upwelling system (Dugdale and Goering 

 1970). The source of ammonium in the marine 

 environment may be recycled through any of 

 several animal groups. Ammonium regenera- 

 tion in Long Island Sound was found to be pre- 

 dominantly from zooplankton and benthos (Har- 

 ris 1959), while in Georgia coastal waters, 

 phosphate regeneration (and presumably am- 

 monium regeneration) was produced by zoo- 

 plankton that are large enough to be sampled 

 adequately by nets (Pomeroy et al. 1963). 



The regeneration of nitrogen by zooplankton 

 has been examined in several coastal upwelling 

 ecosystems. The red crab, Pleuroncodes plan i pes, 

 copepodites, and adult Acartia regenerated 

 about 16% of total phytoplankton nitrogen up- 

 take (Whitledge in press) in the Baja California 

 upwelling system while zooplankton in the Peru 

 upwelling ecosystem regenerated about 15% of 

 total nitrogen uptake (Whitledge 1978). In the 

 northwest Africa upwelling region off Cape 



■Oceanographic Sciences Division, Department of Energy 

 and Environment, Brookhaven National Laboratory, Upton, 



33.7 -2>2>fiT 



NY 11973. 



Manuscript accepted October 1981. 

 FISHERY BULLETIN: VOL. 80. NO. 2. 1982. 



Blanc (Fig. 1), the zooplankton were shown to re- 

 cycle 33% of nitrogen productivity over the shelf 

 (Smith and Whitledge 1977). 



The focus in most regeneration studies has 

 been zooplankton because fish and benthic orga- 

 nisms have relatively smaller biomasses in many 

 oceanic areas. However, the biomass of the 

 anchoveta in the Peru upwelling ecosystem was 

 estimated to be 15 times greater than the zoo- 

 plankton biomass (Dugdale and Goering 1970), 

 and the fish regenerated 22% of the phytoplank- 

 ton total nitrogen uptake and 59% of the ammo- 

 nium uptake (Whitledge 1978). Since the fish in 

 the Peru upwelling system produce a significant 

 quantity of recycled nitrogen, another major 

 fishing area, the northwest Africa upwelling 

 system, was studied to examine the relative im- 

 portance of nutrients regenerated by fish in com- 

 parison with that by zooplankton (Smith and 

 Whitledge 1977), benthic processes (Rowe et al. 

 1977), and bacterioplankton (Watson 1978). In 

 addition, the biology of many species of fish has 

 not been investigated with respect to changes of 

 nitrogen excretion rates over time and under 

 various conditions so an attempt was made to in- 

 crease our understanding of this elimination 

 process. 



METHODS 



Near-bottom fish specimens of Diplodus sene- 

 galensis, Pegellus couperi, Cantharus cantharus, 

 and Pomadasys incisus were captured in bottom 



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