In situ observation of phytoplankton productivity by an underwater profiling buoy system: use of fast repetition rate fluorometry

To gain a better understanding of variability in phytoplankton productivity in the oceans, we developed an underwater profiling buoy system that uses a fast repetition rate (FRR) fluorometer. We used the profiling buoy system to observe phytoplankton productivity in Sagami Bay, Japan, from 15 April to 10 May 2005. During the observation period, chlorophyll a (chl a) concentration increased gradually, peaking at >5 mg chl a m(-3) at a depth of 10 m, indicating the occurrence of phytoplankton blooms. Gross oxygen production (P-O2), as determined by FRR fluorometry, increased with the development of phytoplankton blooms and peaked at 49.4 mmol O-2 m(-2) h(-1) on 2 May 2005, when both the chl a concentration and surface irradiance were high. To validate FRR-based P-O2, the light-utilization efficiency of a water column psi(O2) was calculated and compared with previous values determined for this location by the oxygen evolution method. The psi(O2) derived from FRR fluorometry showed good agreement with those by the oxygen evolution method, suggesting that FRR fluorometry was able to roughly estimate in situ daily P-O2. Our profiling buoy system has potential benefits for the measurement of oceanic phytoplankton productivity at a high resolution.