Measurement of the Vertical Profile of Phytoplankton by Using Fast Repetition Rate Fluorometry: Baseline Observation at the Okinawa Trough and Izu-Ogasawara Waters, Japan

In deep-sea mining, leakage of lifting water might have an adverse effect on phytoplankton. Therefore, understanding the fluctuation of phytoplankton is important for environmental impact assessment in deep-sea mining. In this study, the maximum fluorescence intensity (F
_m
) and maximum quantum yield (F
_v
/F
_m
) of phytoplankton in the Okinawa trough and Izu-Ogasawara waters were determined using fast repetition rate fluorometry (FRRF). The peak F
_m
was noted at a depth of 80-100 m, and the maximum F
_v
/F
_m
was recorded at a depth of 100-130 m. This result suggests the existence of subsurface chlorophyll maximum and distribution of phytoplankton groups with high photosynthetic capacity in the lower layer. Therefore, understanding the dynamics of phytoplankton in not only the surface layer but also the subsurface layer of the open ocean is important. Our observation results could be used as important baseline data. Furthermore, during monitoring surveys, immediately observing the fluctuation in phytoplankton and detecting the influence of lodged lifting water at an early stage are necessary. FRRF can rapidly measure the photosynthetic parameters of phytoplankton in-situ; this could render it suitable for use in monitoring surveys. Thus, observations using FRR fluorometer might be useful for environmental impact assessment in deep-sea mining.