The effect of Kuroshio Current on nitrate dynamics in the southern East China Sea revealed by nitrate isotopic composition

Abstract

In spring 2014 (May–June), in the southern East China Sea (ECS) and east of Taiwan, nutrient and isotope samples were collected and analyzed. Also, dissolved oxygen (DO), chlorophyll‐a, and physical parameters were determined. The Kuroshio subsurface water intruded into the ECS and separated into two branches on the continental shelf: the nearshore Kuroshio branch current (NKBC) and the offshore Kuroshio branch current (OKBC). Nitrate concentration in the Kuroshio subsurface water slightly decreased east of Taiwan by assimilation and was supplied by upwelling currents northeast of Taiwan. The variations of DO, nitrate, and nitrogen isotopes in the bottom water showed that continuous nitrification occurred in the NKBC after intrusion into the ECS. This process might contribute to the hypoxia zone near the coast of Zhejiang Province, China. Our results also indicated that internal nitrogen cycles appeared in NKBC since the isotope fractionation ratio of oxygen and nitrogen (18ɛ:15ɛ) in NO3− was 1.40. The NO3− flux and budget were calculated in the NKBC via numerical simulations. It demonstrated that at least ∼0.52 kmol NO3−·s−1 was produced by nitrification from DH9 to DH5 transect, and ∼0.11 kmol NO3−·s−1 was consumed in the DH4 transect. Moreover, according to the Rayleigh model, primary production in most of southern ECS was supported by the intrusion of the Kuroshio subsurface water, causing 5‰ isotope fractionation. In some of nearshore stations which located in the northern investigated area, the assimilated nitrate was contributed from both the NKBC and coastal currents originated from the Changjiang diluted water.

Intrusion Pattern of the Offshore Kuroshio Branch Current and Its Effects on Nutrient Contributions in the East China Sea

“…Moreover, the high‐salinity water column was not observed in the west of Taiwan. It indicated that the KBC originated from the northeast of Taiwan rather than from the Taiwan Strait, which is consistent with most studies (Kondo, ; Liang et al, ; Wang et al, ; Yang et al, ). Therefore, the DH11 transect will not be further discussed since this article focus on the Kuroshio.…”

“…The δ 15 N and δ 18 O values were expressed relative to the atmospheric nitrogen content and the Vienna Standard Mean Ocean Water (V‐SMOW) according to the following equation: where R = 15 N/ 14 N or 18 O/ 16 O. The results were calibrated using the isotopic standards USGS‐34, USGS‐32, and IAEA‐N3 and combinations of these standards with various ratios (see Table in Wang et al, ). The analytical precisions of the δ 15 N and δ 18 O values were generally better than ±0.2‰ and ±0.5‰, respectively.…”

“…To this end, the isotopic composition of nitrate ( ) in the KBC and affected areas of the ECS were measured. The results showed that nutrients carried by the KBC and produced by internal regeneration supported primary production throughout most of the ECS and that the KBC was a significant source of nutrients for biological activity along the ECS continental shelf (Wang et al, ). However, the seasonal variations in the Kuroshio Current and its intrusion patterns are still insufficient, especially since the route of the Kuroshio is affected by the global climate (Hsin et al, ; Hwang & Kao, ; Shen et al, ).…”

Key Dynamical Factors Driving the Kuroshio Subsurface Water to Reach the Zhejiang Coastal Area

“…For example, several published data showed that the intrusion of the KSSW played a dominant role in the phosphate input to the ECS (Chen, ; Chen & Wang, ; Fang, ; Liu et al, ; Yang et al, ). In addition to phosphate, the dissolved oxygen, nitrate, and nitrogen isotopes were transported by the bottom water, and the process of continuous nitrification occurred in the Kuroshio subsurface branch current after intruding into the ECS, which might contribute to the hypoxia zone near the coast of the Zhejiang Province (Liu & Kang, ; Wang et al, ).…”

Carbon Chemistry in the Mainstream of Kuroshio Current in Eastern Taiwan and Its Transport of Carbon into the East China Sea Shelf

“…Numerical studies have already proven that the nearshore Kuroshio branch current could even reach as far as the nearshore area within the 50m-isobath near 30.5 • N and has affected the coastal ecosystem profoundly [31]. What is more, the intrusion of Kuroshio and its related variations are considered an important inducement for algal blooms and hypoxia in nearshore region of the ECS [31,32]. Enormous efforts have been made to study the effects of the Kuroshio Current on potential CO 2 sequestration in the ECS, on the basis of multidisciplinary projects conducted in the sea surrounding Taiwan [16,[33][34][35].…”