Monthly measured primary and new productivities in the Ulleung Basin as a biological "hot spot" in the East/Japan Sea

Abstract. The Ulleung Basin (UB), located in the southwestern part of the East/Japan Sea (EJS), is considered having an unusually high productivity for a deep basin. Recently changes have been reported in physical, chemical, and biological properties. Here we measured the primary and new productivities in the UB using a ¹³C-¹⁵N dual isotope tracer technique. Measurements took place every month for the first time throughout a year for a better estimate of the annual primary production in the EJS. Temporal variations of temperature, salinity, and density (σ_t) in the study area were highly seasonal as expected for an ocean in the temperate zone. Nutrient distributions reflected these seasonal fluctuations in the vertical structure of the water column. Diatoms were in general the most dominant phytoplankton ranging from 15.5 to 82.2% with an average of 42.0% (S.D. = ±9.9%). Based on those average daily productivities from our monthly measurements, the annual primary, new, and regenerated production in the UB were 273.0 g C m⁻² yr⁻¹, 62.6 g N m⁻² yr⁻¹, and 48.7 g N m⁻² yr⁻¹, respectively. Our estimated high f ratio (0.59) in the UB, indicated that the predominant nitrogen source for primary production was nitrate. This is comparable with the nitrogen source in a productive coastal-upwelling region. New carbon production by phytoplankton is estimated as 145.6 g C m⁻² yr⁻¹ (S.D. = ±40.8 g C m⁻² yr⁻¹) which indicates that a large portion (53.9%) of the total annual primary production might potentially be exported from the diatom-dominated euphotic zone to a deeper zone in the UB. Further intense integrated field observations will be necessary to improve our understanding of the current marine ecosystem in the UB as an important biological production area in the EJS.