Primary productivity and nitrogen uptake in the subsurface chlorophyll maximum on the Eastern Agulhas Bank

Abstract

Measurements of primary productivity and nitrogen uptake using 15N-labelled N0 3, NH 4 and urea were performed on the eastern Agulhas Bank during January 1992. Primary productivity normalized to biomass ( P B) was exceptionally large for stations characterized by shallow mixing depths. In addition, P B was not well correlated with incident light. Exclusion of those stations with large PB ratios yielded a light utilization index of 1.43 ± 0.48. Primary productivity was largely regeneration based as indicated by photic zone integrals of the f-ratio ( x = 0.17). Ammonium uptake and regeneration were in approximate balance over the photic zone. However, there was a tendency for vertical displacement of these fluxes at some stations. On average, half the total new production occurred in the Chl max layer. New production was N0 3 limited in the upper photic zone (100−25% light depths) though was independent of N0 3 in the thermocline. Photic zone integrals of new production showed a moderately good relationship ( r 2 = 0.74) with surface N0 3 concentration. Possible mechanisms behind the formation and persistence of the subsurface Chl max are considered. In situ growth is recognized as important; by virtue of the significantly larger f-ratios at the Chl max ( x = 0.20) compared to the upper mixed layer ( x = 0.12), though not to the exclusion of other mechanisms. Herbivory and concomitant NH 4 release and inhibition of N0 3 uptake is proposed as a possible control over the vertical positioning of the Chl max. The application of these results to the estimation of regional primary productivity and new production are discussed. Simple models based on the light utilization index and incident light are not promising though the data set is too small to offer any firm conclusions in this regard. The potential exists for estimating new production directly from near-surface N03 concentration but will need to be substantiated in further cruises.