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

Abstract

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 (σt) 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.

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

“…Monthly water column data yields the annual primary production of 273.0 g C m −2 yr −1 in the UB, about 20 % higher than the production inferred from the ocean color data, primarily due to the existence of the SCM layer (Kwak et al, 2013). The maintenance of the high primary production in the UB has been attributed to windinduced coastal upwelling (Park and Kim, 2010), surface circulation (Yoo and Park, 2009), and mesoscale eddies (Hyun et al, 2009).…”

“…According to Kwak et al (2013), diatoms are generally the most dominant phytoplankton throughout the year in the UB, and especially, the subsurface chlorophyll maximum layer was found at 20-30 m, and it corresponded to the depth of maximum contribution of diatoms (> 90 %) in spring and summer 2010. Hence, any increase in the CF is thought to be mainly due to the spring bloom rather than due to any changes in the phytoplankton species.…”

“…Reports have been given of the relationship between NO 3 and temperature in the East Sea in August based on longterm NFRDI data (Son et al, 2006), and also in other regions (Omand et al, 2012;Palacious et al, 2013). Nutrient and temperature data obtained in the UB on May in 2010 and on March and April in 2011 (Kwak et al, 2013) are used to test any possible temperature-NO 3 relationship in spring (Fig. 2b).…”

“…The SCM layer is mainly observed between 30-40 m depth, and may last for six months in the UB from May to October (Rho et al, 2012). Previous studies suggest that the primary production at the SCM contributes significantly to the total annual primary production in the UB (Shim et al, 1992;Kwak et al, 2013). Development, duration, and temporal variability of the SCM, however, are poorly known.…”

Annual New Production of Phytoplankton Estimated from MODIS-Derived Nitrate Concentration in the East/Japan Sea

“…Generally, new production is defined as primary production based on nitrate concentration as a nitrogen source coming from deep waters to the euphotic zones, which could increase phytoplankton population size in marine ecosystems [31,32]. Currently, only a few field-measured new productions have been obtained in the EJS [2,[33][34][35][36]. However, there has been no information on the temporal and spatial variation of the new production that is currently available in the EJS.…”