Seasonal dynamics of seawater CO2 system at a coastal site near the southern tip of Izu Peninsula, Japan

To elucidate how variables of the seawater CO2 system in surface waters have been changing seasonally and the mechanisms responsible for these changes in the area off the south coast of central Honshu Island, Japan, we made time-series measurements of total dissolved inorganic carbon (DIC), total alkalinity (TA), inorganic macronutrient and chlorophyll a concentrations as well as temperature and salinity at a near-shore station off Shimoda city near the southern tip of the Izu Peninsula over 19 months from November 2011 to June 2013. The oceanographic setting in our study site is characterized by seasonal stratification, low salinity in summer, and well-mixed water column in winter. A large bloom of phytoplankton developed in spring, thereafter nutrients were depleted within the mixed layer in summer. DIC exhibited large seasonal variations and ranged from 1967 µmol kg−1 in summer to 2073 µmol kg−1 in winter when normalized to a salinity of 35, because of biological production in the spring and summer and vertical mixing in autumn and winter, as shown by multiple regression analysis in which variations of normalized DIC (nDIC) are estimated from variations in temperature (13.3–25.5 °C) and chlorophyll concentrations (0.27–3.9 μg l−1). We observed a decrease of TA due to dilution by precipitation and river runoff in summer and a trend of salinity-normalized TA increase over time. The saturation state of the calcium carbonate mineral aragonite Ωarg varied seasonally from 2.40 in winter to 3.58 in summer. The Ωarg in the present study was calculated using DIC, TA, temperature and salinity, and we applied sensitivity analysis for these parameters. Ωarg was predominantly controlled by the large variations of DIC. The combination of these results from the station off Shimoda and from an offshore station at 34° N, 137° E to the west allow us to predict Ωarg with high precision for temporal and spatial variations. It suggests that in winter, there is a meridional gradient of Ωarg in the region off the southern coast of central Honshu Island, with considerably lower Ωarg in the coastal zone.