Dissolved Fe in the East China Sea Under the Influences of Land Sources and the Boundary Current With Implications for Global Marginal Seas

Abstract

The East China Sea (ECS) lies on a wide shelf and embraces one of the world's largest rivers, Changjiang (Yangtze River), on the west and strong boundary current, Kuroshio, on the east, which can act as potentially huge sources of dissolved iron (dFe) to the overlying water. However, their magnitude, distribution, and transformation in the ECS are not yet well characterized. Here, we reported the results from the first large scale investigation of dFe concentrations in the water column from the ECS collected with an X‐Vane assemblage in August 2013. Samples from the Changjiang and other coastal rivers were also measured to constrain terrestrial fluxes. In surface waters, dFe concentrations show a strong gradient with patchy nature from the coast adjacent to the river mouth to the mainstream of the Kuroshio Current. The behavior of dFe in the ECS is closely linked to nutrient cycling and development of the sub‐surface chlorophyll maximum. Our dFe budget reveals that mass transport over the East China Sea Shelf (ECSS) is dominated by the input from the Taiwan Strait Warm Water, atmospheric deposition, and exchange with the open boundary current further offshore. The incursion of the Kuroshio Current across shelf break is a source of natural fertilization. The export of dFe from the ECSS nourishes other marginal seas (e.g., Japan Sea/East Sea) and the Kuroshio further downstream. Comparison with the literature data provides evidence that marginal seas play a critical role in bridging terrestrial and oceanic spheres in terms of Fe biogeochemistry.

Enhancement of Summer Nitrogen Fixation by the Kuroshio Intrusion in the East China Sea and Southern Yellow Sea

“…Our results clearly revealed relatively depleted NO 3 − but replete DRP associated with the intrusion path of nearshore Kuroshio Branch Current from the northeast of Taiwan to the Zhejiang coastal waters (Figure 2f), being highly consistent with active N 2 fixation therein (Figure 4). The dFe concentration measured simultaneously in the ECS Kuroshio mainstream (0.8 nmol L −1 ; Zhang et al., 2022) was much higher than those in the upstream Kuroshio of the Luzon Strait (0.25 nmol L −1 ; Wen et al., 2022) and east of Taiwan (<0.4 nmol L −1 ; Sato et al., 2021), because of large inputs of trace elements from rivers, atmospheric deposition, Taiwan Strait Water, and Kuroshio intrusion (Guo et al., 2014; Zhang et al., 2022). Therefore, high temperature, deficient N, extremely low NO 3 − /DRP, and abundant dFe were conducive to N 2 fixation in oceanic and mesohaline waters of the ECS influenced by the Kuroshio.…”

“…The ECSYS receives a large amount of freshwater and associated nutrients (particularly N) from the Changjiang and induces the CDW to extend northeastward (Chen, 2009; Zhang et al., 2020). Therefore, the ECSYS inner shelf and Changjiang Estuary are characterized by abundant N and high N:P ratio, whereas outer shelf is N depleted (Chen, 2009; Zhang et al., 2022). The data presented here were collected from two cruises aboard the R/V Dongfanghong 2 # during July and August 2013, which occupied a high spatial resolution across the ECSYS (Figure 1).…”

“…Depth‐integrated densities (DIDs) of their trichomes or heterocysts were calculated using trapezoidal integration over the sampling depths. Partial data on salinity, temperature, turbidity, nutrients, Chl‐ a , and filamentous cyanobacteria were derived from previous studies (Jiang et al., 2019; Zhang et al., 2022).…”

“…In addition, severe N deficiency (N:P ratio usually <6) according to Redfield N:P ratio (16; Redfield, 1958) and relatively abundant dissolved Fe (dFe; 0.47–10.01 nmol L −1 ) on the surface are observed in the offshore ECS (Zhang et al., 2022), because of riverine/atmospheric inputs as well as intrusion of nearshore Kuroshio Branch Current (Guo et al., 2014; Zhang et al., 2022). This intrusion across the shelf break transports a substantial amount of dFe to the broad shelf, representing natural Fe fertilization in the ECS (Zhang et al., 2022). The dFe concentrations in the ECS shelf and the adjacent Kuroshio mainstream were found to be higher than in the upstream Kuroshio of the Luzon Strait (0.25 nmol L −1 ; Wen et al., 2022) and east of Taiwan (<0.4 nmol L −1 ; Sato et al., 2021).…”

“…Previous studies have demonstrated that abundant diazotrophic cyanobacteria including Trichodesmium is transported by the Kuroshio (Cheung et al., 2019; Jiang et al., 2019; Shiozaki et al., 2018), which is widely distributed and even blooms (>1,000 trichomes L −1 ) in the ECSYS, particularly in warm seasons (Jiang, Li, et al., 2018; Marumo & Asaoka, 1974; Shiozaki et al., 2010). In addition, severe N deficiency (N:P ratio usually <6) according to Redfield N:P ratio (16; Redfield, 1958) and relatively abundant dissolved Fe (dFe; 0.47–10.01 nmol L −1 ) on the surface are observed in the offshore ECS (Zhang et al., 2022), because of riverine/atmospheric inputs as well as intrusion of nearshore Kuroshio Branch Current (Guo et al., 2014; Zhang et al., 2022). This intrusion across the shelf break transports a substantial amount of dFe to the broad shelf, representing natural Fe fertilization in the ECS (Zhang et al., 2022).…”