Low benthic mineralization and nutrient fluxes in the continental shelf sediment of the northern East China Sea
Introduction
Although the continental shelf area comprises a small fraction (~8%) of the world's oceans' surface (Walsh, 1988; Gattuso et al., 1998; Rippeth et al., 2005), the primary production in the continental shelf accounts for about 15–30% of annual global primary production (Longhurst et al., 1995; Fasham, 2003). Because of the shallow water depth (< 200 m) of continental shelf, the large fraction of organic carbon (OC) produced by primary production in the euphotic depth or laterally transported from the eutrophic coastal water can be deposited into sediment layer (Walsh, 1991; Denis and Grenz, 2003). The OC settled into sediment is degraded via complex microbial respiration pathways as soon as it arrives at the seafloor, and the byproducts of the OC degradation are fed back into the water column or sunk into the sediment layer. A sequential cycle of “OC deposition–remineralization–release from or sink into sediment” can control the biogeochemical cycles of materials in ocean, and thus the details of each process need to be quantified (Allison et al., 2007; Fennel, 2010).
Sedimentary OC is composed of labile and refractory fractions according to their origin (Burdige, 2007). As the OC settled into sediment, the labile fraction of OC is quickly degraded via complex microbial respiration pathways using different electron acceptors such as oxygen, nitrate, manganese oxide, iron oxide, and sulfate (Canfield et al., 1993), while the refractory OC can be buried into the deep sediment layer (Berner, 1980; Canfield, 1989). Because the buried OC is isolated from the biosphere to the geosphere, its flux into the sediment layer can provide information about the carbon sink to the ocean. The OC burial flux depends on bottom water O2, sedimentation rate (SR), primary production, water depth (Jahnke, 1996; Hedges et al., 1997; Burdige, 2007), and its continental shelf, which has high primary production and terrestrial OC load from the land, thereby impacting its benthic carbon cycles. Therefore, it is particularly important to quantify the OC burial flux associated with the fate of primary production and terrestrial OC, which can significantly contribute to global carbon cycles (de Hass et al., 2002; Burdige, 2007; Zhang et al., 2009). At the same time, the released nutrients from OM oxidation at the sediment–water interface can supply a considerable fraction (5–22%) of nitrogen and phosphate requirements for primary production in the continental shelf area (Lourey et al., 2001; Denis and Grenz, 2003). High benthic nutrients flux (BNF) is one of the important processes that control the nutrient cycles in the continental shelf and the pelagic primary productivity; therefore, high BNF also has implications for primary productivities and carbon cycles (Pratihary et al., 2014).
The East China Sea (ECS) is the largest marginal sea in the Northwest Pacific Ocean. The ECS is characterized by high levels of nutrient concentration and primary production resulting from the runoff of the Changjiang River and the year-round upwelling of Kuroshio in Northeast Taiwan (Liu et al., 2000; Gong et al., 2003; Zhang et al., 2007). Moreover, the ECS receives large amounts of sediments (4.14 × 108 ton y−1; Wang et al., 2008) containing 2–5 × 106 ton y−1 of particulate organic carbon from the Changjiang River (Wu et al., 2013). The combination of high primary production and particulate organic carbon discharge can promote vertical fluxes of sinking particles, thus having profound impacts on the carbon cycle in the ECS. However, few studies have examined the biogeochemical OC cycle in the sediments (Deng et al., 2006; Song et al., 2016).
In this study, we present a data set that includes the oxygen uptake rate of sediment, SR, and benthic nutrient fluxes (BNFs) via the sediment water interface in the northern part of the ECS. This data set allows us (1) to show the spatial distribution of OC oxidation rates and BNFs, (2) to estimate burial OC flux and its efficiency in the sediment layer, (3) to quantify the contribution of BNFs to support primary production in the water column via benthic–pelagic coupling, and (4) to assess the importance of sediments on carbon and nutrient cycles in the continental shelf sediment of the ECS.
Section snippets
Study area
The ECS is located in the Northwest Pacific Ocean and hugged by the China mainland on the west. The western boundary current, Kuroshio, flows along the deep Okinawa Trough in the east; ca 60% of the surface area of the ECS is covered by a wide shelf of approximately 7.7 × 105 km2 with an average water depth of 72 m (Fig. 1) (Deng et al., 2006; Chen, 2009). Via the two largest rivers in the Asian continent, Yangtze (Changjian) and Yellow (Huanghe) rivers, massive amounts of terrestrial materials
Oceanographic features
Except for H1, the water depth at the stations ranged from ~80 to 90 m (Fig. 2; Table 1). The surface mixed layer (< 20 m) was covered with a seasonal thermocline. Below the thermocline, the temperature and salinity were homogeneously mixed vertically (Fig. 2). The surface water temperatures ranged from 18.93 to 19.73 °C, and those were not distinguished spatially (Table 1). The surface salinity at H1, which is nearest to the Yangtze, was the lowest. The bottom water temperature and salinity
Benthic O2 uptake with different approaches
The in situ results on benthic O2 uptake can suggest more robust results compared with onboard incubation because it can represent the natural conditions at the in situ condition and measure a larger area than incubation cores (Glud, 2008). The core incubation in the laboratory may possess a potential artifact because of the poor representation of macrofauna irrigation of small core size compared with a larger in situ benthic chamber (Devol and Christensen, 1993; Forja and Gómez-Parra, 1998;
Conclusions
Based on the results of TOU, vertical profiles of OC content, and SR measured in the Northern part of ECS, the partitioned OC flux in the sediment was established. The benthic–pelagic coupling effects was also assessed using the BNFs and primary production. These results lead us to the following comprehensive conclusions.
- 1)
The ex situ TOU yielded at F8 was about two times higher than in situ results applying the whole incubation time, but the same in the initial time (<≈3 h) gave comparable
Declaration of Competing Interest
None.
Acknowledgments
We would like to thank the captains and crews of R/V Onnuri. Financial support was provided by the Korea Institute of Ocean Science Technology (KIOST; PE99812 & PE99813) and the Ministry of Ocean and Fisheries (MOF; PG51650).
Declaration of Competing Interest
The authors declare that they have no know competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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