ASSESSING PRIMARY PRODUCTION VARIABILITY IN THE NORTH PACIFIC SUBTROPICAL GYRE: A COMPARISON OF FAST REPETITION RATE FLUOROMETRY AND 14C MEASUREMENTS1

In situ ¹⁴C uptake (dawn to dusk) and fast repetition rate fluorometry (FRRF) measurements at nearly monthly intervals were compared at Station ALOHA (22°45′N, 158°00′W) between August 2002 and September 2003 in order to determine the feasibility of using FRRF profiling as a means for estimating primary production (PP). The FRRF and ¹⁴C rates were significantly correlated (r²=0.906, P value <0.05, n=70) with slopes of 2.00 and 1.90 for chl a and light normalized data, respectively. However, the relationship between ¹⁴C‐ and FRRF‐derived carbon fixation varied vertically and temporally. The FRRF: ¹⁴C ratio was >1.5 in near‐surface water (5–25 m depth) and approached 1.0 deeper in the euphotic zone. Vertical variations probably reflected the effect of different physiological processes (i.e. Mehler reaction, dark respiration, and excretion) on overall photoautotrophic respiration. In particular, the decrease in Mehler reaction rates with increasing water depth may have accounted for the decrease in difference between ¹⁴C and FRRF measurements with depth. The influence of in situ light field variability in controlling the absorption cross‐section of photosystem II (PSII) (σ_PSII′) may also have been responsible for some of this difference. When compared with total community respiration (R), the derived light‐driven photoautotrophic respiration (reported here as the difference between FRRF and ¹⁴C measurements) represented approximately 50% of R integrated over the euphotic zone. Our results show that FRRF and ¹⁴C measurements were well correlated in oligotrophic waters but the exact relationship between the two processes varies both temporally and vertically, such that a unique relationship between these two techniques could not be derived from first‐order principles.