High-Frequency Observations of Plankton and Particle Abundance from a Cabled Observatory Off Japan

In August 2014, a cabled observatory was deployed off Oshima Island, south of Tokyo, Japan, and operated until September 2018, yielding a 4-year dataset of oceanographic properties and plankton abundance at a fixed location. This study highlights the variability of key physical and biological parameters observed during 2 distinct periods of this time series. Data were collected using a suite of instruments such as thermistor chains, acoustic Doppler velocimeter (ADV), acoustic Doppler current profiler (ADCP), conductivity, temperature, depth (CTD), turbidity and fluorescence sensor, photosynthetically active radiation (PAR) sensor, wave height gauge, and CPICS (a plankton imaging system). Based on these observations, the kinetic energy dissipation rate was estimated over time and its correlation with plankton abundance, aggregate abundance, and plankton diversity was examined. The results demonstrated that temperature fluctuations followed a −5/3 power-law spectrum across the observed frequency range, consistent with turbulence theory. Plankton diversity, measured using the Shannon index, exhibited a power-law spectrum with a slope of −1 ( f −1 ) for harmonic components exceeding 4 h in duration, characterized by a daily peak corresponding to zooplankton diel vertical migration. Long-term measurements of turbulence remain scarce but are crucial for understanding seasonal to interannual variability in plankton and particle dynamics. These findings underscore the importance of ocean mixing in regulating size distribution and abundance, offering insights into the interplay between physical and biological processes in coastal ecosystems.