SEASONAL AND INTERANNUAL VARIATIONS IN PHOTOSYNTHETIC CARBON ASSIMILATION AT STATION ALOHA

Autotrophic carbon assimilation measurements using a trace metal-free C-14 technique were performed at near monthly intervals between 1988 a nd 1992 in the North Pacific subtropical gyre (U.S. JGOFS-WOCE Sta. AL OHA; 22 degrees 45'N, 158 degrees 00'W). Integrated photosynthetic val ues ranged from 127 to 1055 mg C m(-2) day(-1) while the average carbo n assimilation number (P-B), defined as carbon assimilation rate per u nit chlorophyll a (chi a), varied between 1.6 and 12 g C (g chi a)(-1) h(-1) in the 0-45 m depth range. Consistently low P-B values (<5 g C (g chl a)(-1) h(-1), averaged in the upper 45 m of the water column) w ere 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 P-B was not a ssociated 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 nutr ient injections due to mixing events were minor or absent after Januar y 1991. Two non-exclusive hypotheses are presented to explain the rise of P-B in the absence of an enhancement of inorganic nutrient fluxes from below the euphotic zone: (i) high P-B values observed during 1991 -1992 are indicative of phytoplankton growth being balanced as a resul t of a decrease in the variability of nutrient injection due to a redu ction in the frequency of mixing events, and (ii) the rise of P-B duri ng 1991-1992 is caused by an ecosystem shift from nitrogen to phosphor us limitation. The stability of the water column during 1991-1992 may have increased the availability of reduced nitrogen relative to phosph orus due to the enhancement of nitrogen fixation. Because these hypoth eses do not require an increase in algal biomass or elemental fluxes a cross the base of the euphotic zone to explain an increase in autotrop hic carbon assimilation, they imply that nutrient dynamics within the euphotic zone of the North Pacific of the North Pacific subtropical gy re need to be understood in order to interpret changes in P-B and pred ict carbon fluxes. Copyright (C) 1996 Elsevier Science Ltd