A newly observed physical cause of the onset of the subsurface spring phytoplankton bloom in the southwestern East Sea/Sea of Japan

Abstract

Abstract. An ocean buoy, UBIM (Ulleung Basin Integrated Mooring), deployed during the spring transition from February to 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 Sea/Sea of Japan. 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. Low-frequency modulation of the mixed layer depth ranging from 10 m to 53 m during the entire mooring period is mainly determined by shoaling and deepening of isothermal depths depending on the placement of UBIM on the cold or warm side of the frontal jet. The occurrence of the spring bloom at 30 m is concomitant with the appearance of colder East Sea Intermediate Water at buoy UBIM, which results in subsurface cooling and shoaling of isotherms to the shallower depth levels during the bloom period than those that occurred during the pre-bloom period. Isolines of temperature-based NO 3 are also shown to be uplifted during the bloom period. It is suggested that the springtime spreading of the East Sea Intermediate Water is one of the important factors that triggers the subsurface spring bloom below the mixed layer.

Summertime episodic chlorophyll <i>a</i> blooms near the east coast of the Korean Peninsula

“…Conversely, offshore advection of high CF coastal plume water of northern origin, similarly to E09 and other northern water advection events, may also be significant, as is also the case for the E10 bloom. Based on previous research conducted in other coastal systems, E10 is similar to results on temporal and spatial variations of CDOM, CF and primary productivity by cross-shore (onshore and/or offshore) advection of high SST and high CF plume water associated with local circulations (Brzezinski and Washburn, 2011;Warrick et al, 2007). Thus, cross-shore advection of low SSS and high CF water associated with ambient circulation plays an equally significant role in shaping and triggering bloom events in the coastal area.…”

Biogeochemistry and ecosystems of continental margins in the western North Pacific Ocean and their interactions and responses to external forcing – an overview and synthesis

“…In contrast to previous studies on the primary production and chlorophyll a distribution based on weekly or monthly composites of satellite ocean color data, Son et al (2014) reveals highly resolved temporal variation of biochemical properties of the upper layer of the Ulleung Basin in the southwestern JES. Time series measurement of biochemical, optical and physical parameters by an ocean buoy, UBIM (Ulleung Basin Integrated Mooring), during the spring transition highlights an important role of advection of subsurface water mass (i.e., the East Sea Intermediate Water) in triggering the subsurface spring bloom below the surface mixed layer in the Ulleung Basin.…”

Distinct characteristics of the intermediate water observed off the east coast of Korea during two contrasting years

“…Kim and Chung [] found a layer of salinity minimum beneath the TWW in the southwestern EJS (or Ulleung Basin; UB, Figure a), and first named this cold, fresh, and oxygen‐rich water “East Sea Intermediate Water (ESIW).” Since then, the ESIW has been a key tracer in understanding the intermediate depth circulation particularly in the southwestern EJS [ Cho and Kim , ; Kim and Kim , ; Yoshikawa et al ., ; Yoon and Kawamura , ; Park et al ., ]. Southward intrusion of the ESIW is also shown to trigger the subsurface spring bloom in the southwestern EJS [ Son et al ., ].…”

“…Since then, the ESIW has been a key tracer in understanding the intermediate depth circulation particularly in the southwestern EJS [Cho and Kim, 1994;Yoshikawa et al, 1999;Yoon and Kawamura, 2002;Park et al, 2004]. Southward intrusion of the ESIW is also shown to trigger the subsurface spring bloom in the southwestern EJS [Son et al, 2014].…”