| Summary: | Measurements were taken during four seasonal cruises of the R/V Onnuri in spring (May 15–21) 2017, summer (August 16–23) 2016, autumn (November 6–15) 2015, and winter (February 8–14) 2017. Depth profiles of temperature and salinity were measured using a calibrated conductivity–temperature–depth/pressure (CTD) recorder (SBE 911; Sea-Bird Electronics Inc., Bellevue, WA, USA). Seawater samples were collected using a rosette sampler with 10 L Niskin bottles mounted onto the CTD assembly at eight depths (0, 10, 20, 30, 50, 75, 100, and 150 m) for each station in both transects F (33 °N) and H (32 °N). We could not conduct winter survey in H transect. Dissolved Inorganic Carbon (DIC), total alkalinity (TA), and nutrients (phosphate and silicate) were analyzed under laboratory conditions. Bottom seawater samples were collected 5 m above the bottom at stations where the depth was less than 100 m. The DIC was measured by acid extraction with infrared CO 2 detection (AS-C3, Apollo SciTech Inc., USA), and the TA was determined by open-cell Gran acidimetric titration (AS-ALK2, Apollo SciTech Inc., USA). The uncertainties of measurements were evaluated daily using certified reference materials for seawater provided by the Scripps Institution of Oceanography (San Diego, USA). The DIC and TA measurements had precisions of approximately ±1.5 and ±2 μmol kg – 1 , respectively. The seawater CaCO 3 saturation state for aragonite (Ωarag) was calculated with CO2SYS program version 2.1 using salinity, temperature, depth, phosphate, silicate, TA, and DIC data. The carbonic acid dissociation constants of Mehrbach et al. (1973), as refit by Dickson and Millero (1987), were used to determine Ωarag. The uncertainty in the estimation of Ωarag was approximately ±0.02.
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