ESTIMATING PHYTOPLANKTON PRODUCTION FROM AMMONIUM 

 AND CHLOROPHYLL CONCENTRATIONS IN NUTRIENT-POOR 

 WATER OF THE EASTERN TROPICAL PACIFIC OCEAN"° 



William H. Thomas" and Robert W. Owen, Jr.* 



ABSTRACT 



Previous work has shown that nitrogen is the limiting nutrient in poor (nitrate-free) water in the 

 eastern tropical Pacific Ocean and has suggested that ammonium is the principal nitrogen source for 

 phytoplankton in this water. Enrichment and uptake experiments with various concentrations of 

 ammonium have provided values for the half-saturation constant, Kg, and the maximum growth rate, 

 /i^ax' which can be used to calculate actual growth rates with the hyperbolic model relating growth 

 rate to limiting nutrient concentration. At two stations, growth rates calculated from ammonium con- 

 centration agreed well with those calculated from chlorophyll and 14c production, and the hyperbolic 

 equation could be combined with that using production and chlorophyll to calculate production alone. 

 In this paper calculated production rates are compared with those observed from 14c uptake mea- 

 surements for a number of EASTROPAC cruises. The regression between calculated production and 

 observed production is highly significant and the slope is close to 1.0, indicating reasonable agreement, 

 particularly when all of the errors in the calculation, especially in Ks, are considered. The results 

 suggest rather close control of phytoplankton production by the limiting nutrient, ammonium, in these 

 near-surface, nutrient-poor waters. 



This paper describes how concentrations of a 

 limiting nutrient in sea water and some mea- 

 sure of the standing crop of phytoplankton can 

 be used to estimate phytoplankton production. 

 Estimated production is compared with observed 

 i'*C production, and the two sets of values are 

 shown to agree reasonably well when all the 

 errors in the estimation are considered. 



The EASTROPAC Expedition series has de- 

 lineated particularly well areas that are rich in 

 nutrients and that are nutrient-poor in the 

 eastern tropical Pacific Ocean. Rich areas in- 



' Contribution from the Scripps Institution of Ocean- 

 ography. 



" This work was part of the research of the STOR 

 (Scripps Tuna Oceanography Research) Program. It 

 is also a result of the EASTROPAC Expedition, a co- 

 operative study of the biological, chemical, and physical 

 oceanography of the eastern tropical Pacific Ocean. The 

 work was supported by National Science Foundation 

 Grant No. GB-8618 to the senior author and by contracts 

 #14-17-0007-963 and #14-17-0007-989 between the Bu- 

 reau of Commercial Fisheries (now the National Marine 

 Fisheries Service) and the Institute of Marine Resources. 



' Institute of Marine Resources^ Scripps Institution of 

 Oceanography, University of California, San Diego, La 

 Jolla, Calif. 92037. 



* National Marine Fisheries Service Fishery-Ocean- 

 ography Center, La Jolla, Calif. 92087. 



Manuscript received September 1970. 



FISHERY BULLETIN: VOL 69, NO. I, 1971. 



elude the Peru Current, the Costa Rica Dome, 

 and an area of equatorial upwelling extending 

 across the EASTROPAC area (from the Amer- 

 ican coast to long 119° W). Poor areas lie to 

 the north and south of the equatorial upwelling 

 zone and to the west of the Peru Current and 

 Costa Rica Dome. Rich and poor near-surface 

 waters were mapped in previous papers (Thom- 

 as, 1969, 1970b) and will be shown in detail 

 in the EASTROPAC Atlas (Thomas, unpub- 

 lished data) . Nutrient values for rich and poor 

 water are also given in Table 1 of Thomas 

 (1970a). 



Corresponding areal and seasonal changes in 

 the phytoplankton production in this region have 

 been observed and attributed in part to mecha- 

 nisms of nutrient supply (Owen and Zeitzschel, 

 1970). No accounting has been possible, how- 

 ever, for the variations observed within the 

 nutrient-poor surface layer of the region. 



Near-surface water in poor areas is especially 

 low in nitrate-nitrogen; this nutrient is gener- 

 ally not detectable (<0.1 /ng-at./liter). Ammo- 

 nium-N is present in concentrations ranging up 

 to 1 /ng-at./liter and organic nitrogen can reach 



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