Time-series measurements of biochemical and physical properties in the southwestern East/Japan Sea during the spring transition in 2010

Abstract

Abstract. An ocean buoy, UBIM, deployed during the spring transition from February and May, 2010 reveals for the first time highly-resolved temporal variation of biochemical properties of the upper layer of the Ulleung Basin in the southwestern East/Japan Sea. Meteorological data shows the typical spring transition occurred during the mooring period, weakening of wind speed, increase in shortwave radiation, and change in total heat flux from net cooling to net heating. Power spectrum of chlorophyll fluorescence (CF) peaks at semidiurnal tidal, near-inertial, diurnal, and subtidal frequencies. The diurnal variation of CF is characterized by high CF during the daytime and low CF at night. Dissolved oxygen and CF are correlated with high (low) dissolved oxygen accompanied by high (low) CF, indicating the dissolved oxygen is mainly determined by biological activities. The time series measurement captured the onset of subsurface spring bloom at 30 m, and collocated temperature and current data gives an insight into a mechanism that triggers the onset of the spring bloom not documented so far. The entire mooring period can be divided into pre-bloom period from the beginning of the mooring to early April, and bloom period afterwards. Mean CF values during the pre-bloom and bloom periods are 0.9 μg L−1 and 1.9 μg L−1, respectively. Mean mixed layer depth (MLD) shoaled from 22 m during the pre-bloom period to 15 m during the bloom period. Despite of the increase in shortwave radiation, average PAR values at 20 m show lower value during the bloom period as compared to that during the pre-bloom period. Low-frequency modulation of MLD ranging from 10 m to 53 m during the entire mooring period is mainly determined by shoaling and deepening of isothermal (isopycnal) depths. Temperature structure in the upper 110 m is characterized by alternating uplifting and lowering of isotherms, which is caused by the placement of the mooring site on the cold (cyclonic) or warm side of the frontal jet, the East Korean Warm Current. The frontal variability is thought to be due to the low-frequency path variatio of the East Korean Warm Current. The occurrence of the spring bloom at 30 m is concomitant with the appearance of colder East Sea Intermediate Water (ESIW) at buoy UBIM that results in the subsurface cooling and the shoaling of isotherms to the shallower depth levels than those occurred during the pre-bloom period. It is suggested that the springtime spreading of the ESIW is one of the important factors that triggers the onset of subsurface spring bloom below the mixed layer. The time lag between the peaks of CF and the occurrence of the shallowest isothermal depths is about several days, which appears to be the timescale for the growth of phytoplankton.