| Citation: | Hui Ding, Qinsheng Wei, Ming Xin, Yuhang Zhao, Bin Zhao, Mingyu Wang, Fei Teng, Xuehai Liu, Baodong Wang. An inner shelf penetrating front and its potential biogeochemical effects in the East China Sea during October[J]. Acta Oceanologica Sinica, 2024, 43(11): 1-11. doi: 10.1007/s13131-024-2432-6
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As the boundaries or interfaces between different water masses, the oceanic front is a highly dynamic zone with sharp gradients in various environmental variables (Belkin et al., 2009). The front usually occurs on multiple scales. Spatially, it may range from a few meters up to thousands of kilometers; temporally, the front may be short-lived (days) quasi-stationary, seasonally persistent or prominent (Belkin et al., 2009; Wang et al., 2015, 2021). Oceanic fronts are extremely involved in physical-biogeochemical interactions, playing an important role in regulating hydrodynamics, material transport and associated biogeochemical-ecological processes (Huang et al., 2010; Wang et al., 2013; Woodson and Litvin, 2015; Liu et al., 2018; Wei et al., 2016, 2020, 2022; Belkin, 2021; Li et al., 2021b; Lv et al., 2022). In recent decades, with enhanced observations and the widespread use of satellite-derived remote sensing data, oceanic fronts have attracted increasing attention.
The East China Sea (ECS) (Fig. 1), which is mainly surrounded by the Chinese mainland and Taiwan Island, Ry
In this study, mainly based on in-situ observations in the ECS during autumn 2021, we attempt to identify an inner shelf PF and explore its potential biogeochemical-ecological effects. The results may deepen our understanding of PF in the ECS and provide insight into the roles of PF in regulating regional oceanographic processes.
The investigation was conducted onboard the R/V Xiangyanghong No. 18 during October 13−26, 2021. The cruise generally covered the region from the outside of Zhoushan Islands to the northern part of the Taiwan Strait, including 43 stations (Fig. 1). In terms of observational sections, the latitude of 30°N (generally from 122.5°E to 127.25°E) was chosen as Section S1; moreover, four sections (namely, from S2 to S5), which are basically perpendicular to the coastline, were designed parallel from north to south off Zhejiang (Fig. 1).
The hydrological parameters of temperature and salinity were determined using SBE-911. At each station, samples of nutrients and Chl a were collected according to regular water-sampling layers of surface (i.e., 2 m layer), 20 m, 30 m and bottom (i.e., ~2 m above the seabed). Nutrient samples were filtered by a 0.45 μm cellulose acetate membrane on board and preserved using polypropylene bottles in an about −20°C environment. They were analyzed in a land laboratory using an automatic nutrient analyzer (QuAAtro 39, SEAL Analytical GmbH, Germany). Specifically, nitrite nitrate, ammonia, phosphate and silicate were determined by diazo azo spectrophotometry, copper cadmium reduction, sodium hypobromite oxidation, phosphomolybdate blue spectrophotometry, and silomolybdate blue spectrophotomhetry, respectively; their detection limits were 0.02 μmol/L, 0.02 μmol/L, 0.04 μmol/L, 0.03 μmol/L and 0.25 μmol/L, respectively. The concentration of dissolved inorganic nitrogen (DIN) is the sum of the nitrite, nitrate and ammonia concentrations. Water samples of Chl a were filtered through Whatman GF/F filters on board, and the filters were folded and wrapped in aluminum foil and stored in liquid nitrogen until analysis. During laboratory measurement, the Chl a sample was first extracted with 90% acetone under low temperature and dark conditions for 24 hours. Then, its content was measured using a Trilogy fluorescence meter (Turner Designs, USA).
Moreover, the wind speeds during the investigation period were used in this study, which are the scatterometer vectors from the SeaWinds instrument onboard the QuikScat satellite. The data were averaged into mean-state vectors and subsetted into a one-half degree resolution for better visualization, as adopted by Yuan et al. (2005) and He et al. (2010). Jet Propulsion Laboratory Soil Moisture Active Passive (SMAP) Level 3 Sea Surface Salinity (SSS) 8-day running mean (
The horizontal distributions of temperature and salinity in the ECS during October 2021 are shown in Fig. 2. Overall, the temperature decreased from surface to bottom (Figs 2a-e); in contrast, the salinity exhibited an increasing trend downward (Figs 2f-j). Moreover, the salinity varied from low near the shore to high offshore, and a salinity front generally formed between the nearshore low-salinity water and the offshore saline water. Notably, a low-salinity water mass (salinity < 32) located at the southeast of Hangzhou Bay, extended offshore near Section S2 in surface and 10 m layers (Figs 2f and g), which was forming a seaward tongue-shaped salinity front. Meanwhile, surface high-salinity water (>33) occurred in the southwest of the study area (Fig. 2f). In the layers below 10 m (Figs 2g-j), this high-salinity water intruded northeastward on a large scale from the southeastern ECS and even occupied the entire area outside the inner shelf. In addition, this saline water was also characterized by a low temperature at bottom (Fig. 2e).
Vertically, the water column was generally stratified at each section (Fig. 3), with a thermocline and halocline occurring at subsurface, below which was a saline and low-temperature water mass. Remarkably, the seaward extension of low-salinity water was observed in the upper layers of the inner ECS shelf, especially along Sections S1 and S2 (Figs 3f and g). In addition, the cold and saline bottom water, which was mainly concentrated in the steep area along each section, tended to be uplifted onshore (Fig. 3). Influenced by the offshore extension of nearshore low-salinity water and onshore intrusion of saline bottom water, a dominant salinity front formed with its upper part inclining seaward, especially along Sections S1 and S2 (Figs 3f and g).
Figs 4a-l show the horizontal distributions of nutrients in the ECS. Overall, nutrient concentrations generally increased from surface to bottom. They were relatively high off the Zhejiang coast in surface (Figs 4a, e and i) and 20 m (Figs 4b, f and j) layers, forming a dominant nutrient front. In 30 m layer (Figs 4c, g and k), high nutrient concentrations were observed in the northwestern region of the study area. Notably, a low-nutrient region extended from the northeast of Taiwan to the northwest in the upper layers (i.e., surface, 20 m and 30 m layers), which approximately corresponded to the northwestward saline water (Figs 4f-j). At bottom (Figs 4a, e and i), nutrient concentrations were high in the middle shelf and the southeastern ECS. The vertical distributions of nutrients along each section were similar (Figs 5a-o). In general, nutrient concentrations were relatively low in the upper layers and increased downward, especially in the offshore region of each section; high-nutrient areas formed in the bottom steep area along each section. A dominant nutricline existed at each section, and its location was roughly uplifted shoreward. Notably, upper-layer nutrients were greatly depleted in the nearshore region of Sections S1 and S2, while they were maintained at a high level in the coastal area of Sections S3, S4 and S5. Moreover, nutrient concentrations presented a downward decreasing trend in the nearshore region of Sections S4 and S5 (Figs 5d and e, i and j, n and o). This phenomenon well corresponded to the vertical distributions of salinity, which exhibited an increasing trend from surface to bottom (Figs 3i and j). These results indicate that the nutrients in this region mainly originated from the transport of coastal low-salinity water.
Figures 4m-p show the horizontal distributions of Chl a in the ECS. Overall, Chl a content decreased from surface to bottom. A high-Chl a region was observed at surface in the northwestern part of the study area, generally extending seaward along the Section S2 and forming a seaward tongue-shaped Chl a front (Fig. 4m). This high-Chl a region was in accordance with the low-salinity area southeast of Hangzhou Bay (Fig. 2f). Chl a content in the southeastern part of the study area was relatively low, roughly corresponding to the offshore saline water-influenced region (Fig. 2f) with low nutrients (Figs 4a, e and i). As shown by the vertical distributions of Chl a (Figs 5p-t), high Chl a content mainly formed in the upper layers in the nearshore region of Sections S1 and S2, presenting a tongue-shaped distribution from the coast to offshore (Figs 5p and q). In contrast, Chl a content was low in the coastal area of Sections S3, S4 and S5 (Figs 5r-t). In the offshore region of each section, Chl a content was maintained at a relatively low level and gradually decreased downward.
During the period of investigation, the northwesterly wind began to prevail in the study area, as indicated by Fig. 6a. Subsequently, the low-salinity coastal water in the inner ECS shelf tended to spread southward along the coast of Zhejiang, as indicated by Figs 2f and g. It was mainly composed of the southward CDW (Mao et al., 1963; Beardsley et al., 1985) and the local ZFCC (Zeng et al., 2012). The offshore saline water, which extended northwestward from the southeastern ECS, denoted the influence of the Kuroshio (Hsueh, 2000; Yang et al., 2012; Qi et al., 2014). In particular, the upper-layer saline water with high temperature (Figs 2a-d, f-i) and bottom saline water with low temperature (Figs 2e and j) imply ECS shelf water and onshore intrusion of the KSSW (Yang et al., 2012; Xu et al., 2018; Cui et al., 2021), respectively. The ECS shelf water is mainly composed of the Kuroshio Surface Water (KSW)—originated saline water and the northward Taiwan Warm Current Water (Qi et al., 2014; Lian et al., 2016; Li et al., 2021a). Based on the temperature-salinity scatter plot (Fig. 6b), water masses could be further identified in the ECS.
The above-mentioned hydrographic conditions largely regulated the nutrient distributions. Generally, influenced by the southward extension of low-salinity coastal water (mainly from the ZFCC transport) (Figs 2f and g), a relatively high-nutrient zone formed in the surface (Figs 4a, e and i) and 20 m (Figs 4b, f and j) layers in the southern nearshore region of the study area. Due to the oligotrophic ECS shelf water, the offshore surface region was characterized by low nutrients (Figs 4a, e and i). Partially caused by the northwestern intrusion of the KSSW, nutrients were maintained at a high level in the offshore bottom water (Figs 4d, h and l), especially for phosphate (Fig. 4h). Previous studies have revealed that the onshore branch of the KSSW can deliver a large amount of nutrients into the ECS shelf (Chen, 1996; Yang et al., 2013; Xu et al., 2020; Wang et al., 2018, 2023). Our results show that the highest nutrients were mainly concentrated in the bottom layer of the middle ECS shelf. Actually, the core of this high-nutrient region was roughly in accordance with the well-known summertime hypoxic zone in the ECS (Chen et al., 2007; Wei et al., 2017). We believe that in addition to originating from KSSW transport, the release from local mineralization of organic matter in the hypoxic zone might also contribute to the nutrient levels in this region, as also revealed by Zhu et al. (2017) and Wei et al. (2017, 2021b). Moreover, stratification (Fig. 3) could greatly hinder the vertical transport of nutrients and thus play a role in maintaining high nutrient levels in the bottom layer of the middle ECS shelf.
Previously, the PF was found to be one of the most important physical processes in the inner ECS shelf (Yuan et al., 2005, 2010; He et al., 2010; Wu, 2015; Ye et al., 2022; Zhou and Wu, 2023). As indicated in Figs 2f and g, the tongue-shaped low salinity water, which mainly extended offshore in the upper layers (i.e., surface and 10 m layers) in the northwestern region of the study area, clearly suggests the existence of a cross-shelf PF in the ECS. Actually, the seaward extension of low-salinity water along Sections S1 and S2 (Figs 3f and g) could further confirm the occurrence of PF. In addition, the distribution of satellite-derived mean SSS during the cruise also indicates the occurrence of PF in the ECS shelf (Fig. 6c). Overall, this cross-shelf PF generally covered a large distance from the coast, resulting in an offshore intrusion into the middle shelf. The salinity of this penetrated water was mainly between 29 and 32 (Figs 2f and g), and the horizontal penetration scale of PF could reach ~200 km from the coast, which was comparable to the satellite-derived results of He et al. (2010). The analyses on the variability in PF revealed that it was mainly observed during summer and winter in the inner ECS shelf (He et al., 2010). Here, our results confirmed that the PF could occur in autumn, as also illustrated by Zhou and Wu. (2023). The PF location was approximately southeast of Hangzhou Bay (Figs 2f and g), generally penetrating offshore near the Zhoushan Islands. The formation of PF is largely due to complicated hydrodynamics (He et al., 2010; Yuan et al., 2010; Wu, 2015; Ye et al., 2022; Zhou and Wu, 2023), among which the accumulation of southward-transported coastal water under favorable wind conditions is one of the most important factors. The northwesterly wind (Fig. 6a) during the cruise was conducive to the southeastward extension of the coastal diluted waters (Figs 2f and g). Moreover, these inshore waters tended to be transported eastward when encountering the northward saline ECS shelf water off Zhejiang. The relatively sharp salinity gradients between the PF-dominated region and the saline seawater to the southeast partially implied their interactions (Figs 2f and g). In addition, the curved headlands near the Zhoushan Islands could also favor the offshore penetration of coastal water plumes (Zhou and Wu, 2023).
In the southern nearshore region off Zhejiang, forced by the shoreward Kuroshio water, the inshore diluted water was mainly trapped along the coast and thus a coastal salinity front formed (Figs 2f-j). This coastal front was connected to the nearshore part of the PF near 29°N in the surface and 10 m layers (Figs 2f and g). Notably, the cross-shelf PF mainly occurred in the upper layers in the north, while the coastal salinity front could be maintained from surface to bottom in the south and extend to the northern region along the shorelines (Figs 2f-j). As a result, a spatial separation existed between the upper-layer PF and bottom salinity front in the northwestern region of the study area. In contrast, the surface and bottom salinity fronts were well matched with each other in the southern coastal region. Moreover, our observations show that the saline KSSW could reach the shore much closer in the northern PF-dominated region than in the south, as indicated by 23℃ isotherm and 34 isohaline in the vertical distributions of temperature and salinity along Sections from S1 to S5 (Fig. 3). Influenced by the seaward PF in upper layers and the more shoreward intrusion of the KSSW at bottom, a stronger thermocline and halocline could be achieved in the northwestern region of the study area (i.e., the nearshore of Sections S1 and S2), as implied by the vertical distributions of temperature and salinity along Sections S1 and S2 (Figs 3a and b, f and g). However, in the southwestern region of study area (i.e., the southern coastal area off Zhejiang), stratification was greatly weakened, and the water column tended to be vertically mixed, especially in the nearshore region of Sections S4 and S5 (Figs 3d, e, i and j).
Fronts are among the most important physical processes and may play an important role in the transport and exchange of substances (Li et al., 2003; Dong et al., 2011; Ren et al., 2015; Wei et al., 2020). Undoubtedly, the unique PF in the ECS could drive the delivery of seawater from the inner shelf to the middle shelf, thus enhancing material exchange and influencing regional biogeochemical-ecological processes. In fact, the seaward extension of tongue-shaped low-salinity water (Figs 2f and g) should have a significant impact on the cross-shelf transport of nutrients under the influence of PF. However, in this PF-dominated area, nutrient concentrations were very low (Figs 4a, e and i), showing the decoupling of nutrient distributions and PF. This might be largely attributed to the biological consumption of phytoplankton, as indicated by the tongue-shaped high-Chl a zone (Fig. 4m). The relationship between Chl a and salinity in the surface layer (Fig. 7a) could further suggest phytoplankton blooms within the low-salinity PF-dominated region. Hence, the seaward-transported nutrients along with the cross-shelf PF were mainly depleted to support primary production, as implied by Fig. 7b. These results were responsible for the inconsistency of the PF (Fig. 2f) and nutrient distributions (Figs 4a, e and i). Actually, the significant stratification in the PF-dominated area (Figs 3a and b, f and g) was also conducive to the accumulation of phytoplankton in the upper layers (Fig. 4m and Figs 5p and q). Moreover, the settlement of suspended particulate matter (SPM) over a relatively long distance and period within the seaward PF-dominated region would considerably reduce the seawater turbidity, potentially improving the light transmission property. This was previously confirmed by Wei et al. (2021a), who demonstrated that the seaward surface plume front far from the coast favors the settlement of SPM and reduction of turbidity in the Changjiang River Estuary and adjacent areas. As a result, suitable conditions could be achieved for phytoplankton growth in the PF-dominated area, and the in-situ observed high-Chl a zone in a seaward tongue shape was a further significant indicator or signal of PF in the inner ECS shelf. Exactly, we believe that the nutrients depleted by phytoplankton within the PF-dominated region (Fig. 7b) were mainly from the southward CDW and ZFCC, as suggested by low-salinity water in Figs 2f and g. Here, quantitatively estimating the contribution of PF to nutrient transport is challenging because of the complex processes, which suggests the need of model constructing for physical-biogeochemical-ecological interactions in the future. In addition, the shoreward transport of nutrients along with the KSSW tended to be upwelled in the northern nearshore region, as indicated by Figs 5a, b, f, g, k and l. The high-nutrient area, which occurred in 30 m layer in the northwestern region of the study area (Figs 4c, g and k), further confirmed the upward transport of nutrients underneath the PF. This process might also play a role in supporting primary production when rapid nutrient consumption occurred in the PF-dominated upper layers (Figs 5a and b, f, g, k and l).
In contrast, the surface-to-bottom consistent coastal front in the southern region off Zhejiang (Figs 2f-j) might act as a barrier for the seaward transport of substances, as previously indicated by Liu et al. (2015) and Qiao et al. (2020). Thereby, the low-salinity water rich in nutrients was mainly trapped along the coast in this area (Figs 2f and g and Figs 4a, e and i). Moreover, the dynamic environment under much weaker stratification (Figs 3d and e) on the nearshore side of this southern coastal front was not conducive to the settlement of SPM. As a result, light transmission could greatly be reduced, potentially leading to poor light conditions for phytoplankton growth in upper layers. Actually, the vertical convection induced by the unstable dynamic environment in this area was also disadvantageous for the accumulation of phytoplankton. Therefore, a low Chl a content was observed near the southern coastal region off Zhejiang (Fig. 4m and Figs 5r-t). The weak phytoplankton production also meant low consumption of nutrients, which was in turn responsible for high nutrient concentrations in the southern coastal area off Zhejiang (Figs 4a, e and i). Consequently, the nutrients from the transport of the southward ZFCC could be maintained in the southern nearshore region. The aforementioned analyses demonstrate that the PF combined with the coastal front may play an important role in shaping/regulating hydrodynamics, nutrient distributions and the Chl a regime over the inner ECS shelf.
Using field observations in the ECS during October 2021, this study investigated a cross-shelf PF in the inner ECS shelf, and the potential biogeochemical-ecological effects of PF were explored by multidisciplinary analyses.
(1) A pronounced inner shelf PF was identified, and the salinity within this seaward-penetrated PF was mainly between 29 and 32. This front was approximately southeast of Hangzhou Bay, generally penetrating offshore near the Zhoushan Islands. It covered a large distance from the coast, with a horizontal penetration scale of ~200 km.
(2) The cross-shelf PF mainly occurred in the upper layers, and a spatial separation existed between the upper-layer PF and bottom salinity front in the northern coastal region of the study area. In contrast, the surface and bottom salinity fronts were well matched with each other in the southern coastal region off Zhejiang. Moreover, a stronger thermocline and halocline were maintained in the PF-dominated region.
(3) Suitable conditions could be achieved for phytoplankton growth in the PF-dominated area, and the in-situ observed high-Chl a zone in a seaward tongue shape was a significant indicator or signal of PF. The phytoplankton consumption was responsible for the decoupling of nutrient distributions and PF. The southern coastal front off Zhejiang might restrict the seaward transport of nutrients, and the dynamic environment under weak stratification in this region was disadvantageous for the growth and accumulation of phytoplankton; thus a low Chl a content was observed near the southern coastal region. The PF combined with the coastal front plays an important role in shaping/regulating nutrient distributions and the Chl a regime over the inner ECS shelf.
Acknowledgements: Data and samples of this research were collected onboard the R/V Xiangyanghong No. 18 implementing the open research cruise NORC2021-02 supported by the NSFC Shiptime Sharing Project (project number: 42049902). We are grateful to the survey team and crew for their help and cooperation during the field investigation.|
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Figures(7)
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Beijing Renhe Information Technology Co. Ltd
Hui Ding, Qinsheng Wei, Ming Xin, Yuhang Zhao, Bin Zhao, Mingyu Wang, Fei Teng, Xuehai Liu, Baodong Wang. An inner shelf penetrating front and its potential biogeochemical effects in the East China Sea during October[J]. Acta Oceanologica Sinica, 2024, 43(11): 1-11. doi: 10.1007/s13131-024-2432-6
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