WHITLEDGE: REGENERATION OF NITROGEN BY NEKTON 



the fish totals 2.96 mg-at/m 2 per day (Table 4), 

 which is about 26.9% of the ammonium used by 

 phytoplankton and 14. l%of total inorganic nitro- 

 gen utilized. 



Results of zooplankton regeneration experi- 

 ments obtained at the same time showed varia- 

 tions related to size of the organisms and depth of 

 water. Smaller zooplankton were most abundant 

 and had largest excretion rates inshore while the 

 largest zooplankton biomass was located just off- 

 shore of the shelf break where the larger zoo- 

 plankton with smaller excretion rates were 

 found (Smith and Whitledge 1977). The mean 

 ammonium regeneration rate calculated from 

 zooplankton that were separated into four size 

 classes of 102, 223, 505, and 1,000 M m was 4.7 mg- 

 at/m 2 per day over the shelf, which is about 42.7% 

 of the ammonium used in primary production 

 and 22.4% of total inorganic nitrogen uptake. 



The release of ammonium from the sediments 

 off northwest Africa was estimated by placing 

 bell jars on the bottom in the shallow inshore re- 

 gion (25 m) where divers could collect initial and 

 final samples using plastic bottles (Rowe et al. 

 1977). The mean ammonium release rate from 

 the two locations was 5.64 mg-at N/m 2 per day. 

 This value represents 0.23 Mg-at/1 per day if mix- 

 ing occurred over the entire water column in the 

 nearshore region. The ammonium content of 

 pore water in the upper few centimeters and at 

 the seawater-sediment interface was quite large 

 in the two nearshore stations compared with 

 samples collected at 50 and 200 m. Likewise the 

 gradients of ammonium production at the sedi- 

 ment-water interface was shown to decrease 

 from MOO /xg-at/1 per cm at 25 m to <40 /ug-at/1 



Table 4.— Nitrogen budget for northwest Africa upwelling 



ecosystem. 



per cm offshore of the shelf break. So using the 

 concentration of ammonium in pore water and 

 sediment-water interface gradients as indica- 

 tors of ammonium flux from the sediments, the 

 sediments were estimated to be releasing about 

 5.6 and 1.9 mg-at/m 2 per day at water depths of 

 50 and 200 m. These sediment-release values 

 would provide 78.9% of ammonium used in pri- 

 mary production and 24.2% of the total inorganic 

 nitrogen uptake over the inner shelf. A smaller 

 portion of productivity evidently sinks to the 

 sediments hence smaller benthic release rates 

 are observed. 



The input to the water column from the sedi- 

 ments nearshore at depths of 30 m or less are 

 probably very significant in creating and main- 

 taining a high concentration of ammonium 

 found in the shallow waters (Fig. 5) that are often 

 discolored due to a large air-derived suspended 

 load. The ammonium-release rates from the sedi- 

 ment are larger than nearshore pelagic regen- 

 eration rates and the large aeolian sediment load 

 (Sarnthein and Walger 1974; Rowe et al. 1977; 

 Milliman 1977) was presumably large enough to 

 inhibit phytoplankton nutrient uptake as a result 

 of light attenuation and to discourage large bio- 

 masses of zooplankton (Codispoti and Friederich 

 1978). It is therefore probable that the primary 

 productivity not eaten by the small-sized zoo- 

 plankton falls to the bottom, so an appreciable 

 quantity of ammonium is placed in the water col- 

 umn by zooplankton excretion and particulate 

 organic matter decomposition on the bottom. 



41 



STATIONS 

 44 42 63 



4364 



10- 



'Phytoplankton ammonium uptake = 7.5 mg-at/m 2 per day. 



Phytoplankton nitrate uptake = 16.0 mg-at/m 2 per day 

 2 Phytoplankton ammonium uptake = 16.2 mg-at/m 2 per day. 



Phytoplankton nitrate uptake = 6.2 mg-at/nr per day. 

 Source for footnotes 1 and 2: Dugdale and Maclsaac, unpubl. results 



25 20 15 IO 5 



DISTANCE OFFSHORE (km) 



Figure 5.— Distribution of ammonium (pg-at/l) observed in a 

 transect of stations across to shelf at lat. 21°40'N. 



333 



