Macroscale‐wide nutrient inversions in the subsurface layer of the Japan Sea during summer

Abstract

The nutrient concentrations at depths of 0–200 m in the southern area of the Japan Sea were investigated at 97 stations during six cruises between June and October in 2013 and 2014. The nutrient concentrations at the surface were depleted to less than 0.1 µM, except for silicates, which remained at 0.8–5 µM, and increased below the nutricline, at depths of 20–125 m. The vertical profiles of nitrate, silicate, and/or phosphate concentrations between 131°30′E–139°40′E and 35°50′N–40°40′N showed a peak in the subsurface layer at 40, 71, and 6 stations, respectively, and nutrient inversions occurred at macroscale widths. The subsurface nutrient maximum occurred at depths of 20–150 m in waters at temperatures of 15–16°C and potential densities of 25.3–25.5 σθ, on average. The depths of the subsurface nutrient maximum were generally associated with a salinity maximum originating in the bottom water of the shallow Tsushima Strait. The nutrient inversions were disturbed by phytoplankton consumption, as indicated by the presence of the subsurface chlorophyll maximum at the same depth or below the salinity maximum at stations without nutrient inversions. Therefore, it was inferred that remineralization of nutrients near the bottom, from the East China Sea to the Tsushima Strait, and horizontal advection by the Tsushima Warm Current below the euphotic layer induced macroscale subsurface nutrient inversions in the southern Japan Sea.

Long‐term decrease in phosphate concentrations in the surface layer of the southern Japan Sea

“…We propose that horizontal advective transport of low phosphate concentration water from the ECS is the most plausible mechanism for the declining trend in the phosphate concentration. Horizontal advection from the ECS controls the nutrient conditions in the JS [ Kim et al ., ; Kodama et al ., ; Takikawa et al ., ]. The surface water in the Tsushima Warm Current was replaced by water originating in the ECS, including the Kuroshio and Taiwan Warm Current, during one summer, and this water remained during winter based on the ejection period (Figure ).…”

“…The data were divided into 11 layers based on the sampling depth (0–20, 21–50, 51–100, 101–150, 151–200, 201–300, 301–400, 401–500, 501–1000, 1001–2000, and >2001 m depths) considering the vertical profiles of nutrient concentrations. That is, during summer, the nutrient concentration is almost depleted in the 0–20 m depth layer, and the nutricline and nutrient maximum is observed in 20–50 m depth layer [ Kodama et al ., ]. In addition, nutrient concentration had a steep slope from the surface to 500 m depth and it was stable or decreased below the 500 m depth (Figure ).…”

“…However, Kim and Kim [] used a box model and nutrient concentration data sets to conclude that the increase in excess nitrate concentrations is not predominantly due to atmospheric deposition in the JS. Additionally, nutrient conditions in the East China Sea (ECS) strongly affect the surface layer of the southern JS because of horizontal advection of the Tsushima Warm Current from the Tsushima Strait to the Tsugaru and Soya straits [ Kodama et al ., ; Takikawa et al ., ]. Kodama et al .…”

“…Kodama et al . [] indicated that the Tsushima Strait water is advected to the offshore region of the Tsugaru Strait, based on the distribution of the subsurface nutrient inversion structure within one stratified season (May–October) by the Tsushima Warm Current. Moreover, Kim and Kim [] pointed out the importance of horizontal advection in long‐term nutrient variations in the JS, but the effects of horizontal advection were not demonstrated in the previous studies mentioned above.…”

Spatial variations in zooplankton community structure along the Japanese coastline in the Japan Sea: influence of the coastal current

“…CC BY 4.0 License. Hirose et al, 2009;Onitsuka et al, 2010;Kodama et al, 2016;Kodama et al, 2015), and is the spawn and nursery field of important fisheries resources (Goto, 1998(Goto, , 2002Ohshimo et al, 2017;Kanaji et al, 2009) Zooplankton are transported by the TWC: the distributions of the giant jellyfish, Nemopilema nomurai, which originates from the Chinese coastal area of the East China Sea (Kitajima et al, 2015;Uye, 2008), and the giant salps, Thetys vagina (Iguchi and Kidokoro, 2006), depend on the TWC in the Japan Sea. However, studies of the relationship between the TWC and small 5 zooplankton such as copepods are very limited, especially concerning their spatial distribution.…”