Diel and daily changes of water mass structure and biogeochemical processes in early spring in the western subtropical North Pacific

Abstract To assess the impact on biological processes of short-term changes of physical and chemical fields in the surface ocean, we made conductivity–temperature–depth observations and sampled water for biogeochemical parameters every 3 h for 72 h from 18:00 on 21 February to 18:00 on 24 February 2024 at subtropical observation station KEO (32.5° N, 144.5° E). The mixed layer was shallower than 100 m until 06:00 on 24 February, but it then deepened to 150 m within 3 h. Temporal changes of physical and biogeochemical parameters indicated that the water could be classified into five types. Type I: There was a significant diel variation of nitrate isotope ratios at depths of 0–50 m from 18:00 on 21 February to 09:00 on 23 February. The indication was that phytoplankton had taken up nitrate during the day, and nutrients were supplied from water beneath the euphotic zone at night. Type II: This water was found at depths of 50–150 m from 18:00 on 21 February to 09:00 on 23 February. Nitrate concentrations and isotope ratios varied with the mixing of two water masses within the euphotic zone and the twilight zone (depth of ~ 150 m). Type III: The mixed layer deepened to 150 m after 09:00 on 24 February. The temperature of the water was lower than that of Type 1 water, and the effect of phytoplankton nitrate uptake was observed in water below the euphotic zone. The community composition of the phytoplankton was different in this water. Type IV: This water was observed at depths of 50–100 m during 12:00–18:00 on 23 February. This water was upwelled three-dimensionally within a short time from the twilight zone to the lower part of the euphotic zone, and it contained high nutrient concentrations. Type V: This water remained at depths greater than 150 m throughout the observation period. There were few temporal variations of observed parameters. Our observations revealed high primary production rates in this oligotrophic subtropical region, and water masses with different physical and biogeochemical characteristics were directly adjacent to each other.