THE OCEANOGRAPHY SOCIETY PACIFIC BASIN MEETING, July 1994, p.60

Temporal variations in photosynthetic carbon assimilation efficiencies at Station ALOHA (22 45'N; 158 00'W)

R.M. Letelier, J. Dore, C.D. Winn and D.M. Karl

Autotrophic carbon assimilation measurements using the trace metal-free ¹⁴C technique were performed at nearly monthly intervals between 1988 and 1993 in the Central North Pacific (CNP) gyre (US-JGOFS Station ALOHA; 22 45'N, 158 00'W). Integrated photosynthetic values ranged from 230 to 1055 mg C m^-2 d^-1 while the average carbon assimilation efficiency (pB) , estimated as carbon assimilation per unit chlorophyll a (chl a) per time ranged between 1.6 and 12 mg C (mg chl a)^-1 h^-1 in the 0-45 m depth stratum. Consistently low pB values (< 5 mg C (mg chl a)^-1 h^-1, averaged over the upper 45 m of the water column) were observed during the first two years of this study. These low values increased to > 5 mg C (mg chl a)^-1 h^-1 and varied as a function of the mixed-layer depth between 1991 and 1992. This increase coincided with the shoaling of the mixed-layer and a decrease in its depth fluctuation between cruises. It also coincided with a decrease in adenosine-5'-triphosphate (ATP) concentrations and dark carbon assimilation measured in the upper 45 m of the water column. These observations suggest that the variability in nutrient input, rather than the absolute input of nutrients, plays an important role in the control of the light saturated pB of phytoplankton in the upper euphotic zone of the CNP.