New production in the central equatorial Pacific

Abstract

Uptake rates of nitrogen (''NO3 and "NH,') and primary productivity were measured along a transect across the equatorial Pacific region at ca 13.5' W from 15" S to 15' N, in April 1988. Latitudinally, higher rates of total and new production (i.e. the production resulting from NO3-uptake) were observed near the equator coinciding with higher concentrations of chl a and NO, Despite significant variation in NO3-concentrat~on, variability of phytoplankton biomass and production was low along the transect. Vertically, NO3-uptake rates tended to increase with depth in the euphotic zone following the increase in NO3-concentration. Between 6 ' s and 4" N, where NO3-was abundant, the average euphotic zone integrated f-ratio (NO3-uptake X 6.6/primary productivity) was 0.39. North and south of this region, where NO3-was low or undetectable, a lower mean f-ratio was found (0.22 and 0.09, respectively). Despite NO3-abundance at the equator, regenerated N was the major source of inorganic N used by phytoplankton. Low utilization of available NO< was likely the consequence of grazing effects. A high grazing pressure in the equatorial Pacific region would reduce the absolute N consumption by reducing phytoplankton bion~ass, and also would increase the availability of regenerated N (i.e. NH,+). Since NH,' is generally the preferred form of N used by phytoplankton, its availability may be an important regulator of new production in this nitrate-rich environment.