NEW PRODUCTION IN THE CENTRAL EQUATORIAL PACIFIC

Uptake rates of nitrogen ((NO3-)-N-15 and (NH4+)-N-15 and primary productivity were measured along a transect across the equatorial Pacific region at ca 135-degrees-W from 15-degrees-S to 15-degrees-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 NO3-. Despite significant variation in NO3- concentration, 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-degrees-S and 4-degrees-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 NO3- 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 biomass, and also would increase the availability of regenerated N (i.e. NH4+). Since NH4+ 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.