Seasonal and interannual variations in photosynthetic carbon assimilation at Station

Autotrophic carbon assimilation measurements using a trace metal-free 14C technique were performed at near monthly intervals between 1988 and 1992 in the North Pacific subtropical gyre (U.S. JGOFS-WOCE Sta. ALOHA; 22°45′N, 158°00′W). Integrated photosynthetic values ranged from 127 to 1055 mg C m−2 day−1 while the average carbon assimilation number (PB), defined as carbon assimilation rate per unit chlorophyll a (chi a), varied between 1.6 and 12 g C (g chl a)−1 h-1 in the 0–45 m depth range. Consistently low PB values (< 5 g C (g chl a)−1 h−1, averaged in the upper 45 m of the water column) were observed during the first 2 years of this study but increased to > 5 g C (g chl a)−1 h−1 during 1991–1992. This rise in PB was not associated with an increase in chi a. Furthermore, it occurred during a period of increased water column stability. Reduction in ATP and (NO3− + NO2−) concentrations in the upper euphotic zone suggests that nutrient injections due to mixing events were minor or absent after January 1991. n-exclusive hypotheses are presented to explain the rise of PB in the absence of an enhancement of inorganic nutrient fluxes from below the euphotic zone: (i) high PB values observed during 1991–1992 are indicative of phytoplankton growth being balanced as a result of a decrease in the variability of nutrient injection due to a reduction in the frequency of mixing events, and (ii) the rise of PB during 1991–1992 is caused by an ecosystem shift from nitrogen to phosphorus limitation. The stability of the water column during 1991–1992 may have increased the availability of reduced nitrogen relative to phosphorus due to the enhancement of nitrogen fixation. Because these hypotheses do not require an increase in algal biomass or elemental fluxes across the base of the euphotic zone to explain an increase in autotrophic carbon assimilation, they imply that nutrient dynamics within the euphotic zone of the North Pacific subtropical gyre need to be understood in order to interpret changes in PB and predict carbon fluxes.