ASLO/TOS Ocean Research Conference, Honolulu, HI, February 2004, p. 33.

Estimation of photosynthetic activity, as determined by Fast Repetition Rate Fluorometry (FRRF), in the North Pacific Subtropical Gyre (NPSG)

G. Corno¹, R. M. Letelier¹, M. R. Abbott¹ and D. M. Karl²

¹Oregon State University, Corvallis, USA

²University of Hawaii, Honolulu, USA

Time-series FRRf measurements were made in the NPSG (Station ALOHA, 22 45 N, 158 00 W), over a complete year. The efficiency of Photosystem II (PSII), derived from normalized variable fluorescence (FV/FM), was high through the water column at all time (FV/FM=O.50-0.60). A constant maximum (FV/FM=0.65-0.85) occurred below the Deep Chlorophyll Maximum Layer, between 120 and 150 m. We defined this depth range as the High Efficiency Area (HEA). The effective absorption cross section of PSII showed similar patterns as FV/FM. Variability on short (seconds to minutes) and long (weeks) time scales in both parameters decreased with depth, reaching a minimum in the HEA. Fluorescence-derived photosynthetic parameters confirmed the presence of surface, high light-adapted populations and deep, low- light adapted assemblages. Our results suggest high photosynthetic efficiency in this oligotrophic region, despite the supposed-chronic nutrient limitation affecting surface populations. Surface variabltty may, however, reflect more flexible physiological adaptations due to irregular nutrient supply. These findings highlight the potential use of optical measurements to further understanding photo-autotrophs dynamics in pelagic environments, across a high spatial and temporal resolution.