Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul Concentrations of porewater heavy metals, their benthic fluxes and the potential ecological risks in Daya Bay, South China Ling Zhang a,b , Zhixin Ni c , Yunchao Wu a,b , Chunyu Zhao a,d , Songlin Liu a,b , Xiaoping Huang a,b,d, ⁎ a CAS Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China b Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China c Ministry of Natural Resources South China Sea Bureau, Guangzhou 510300, China d University of Chinese Academy of Sciences, Beijing 100049, China ARTICLE INFO Keywords: Heavy metal Porewater Flux Environmental impact Daya Bay ABSTRACT Heavy metal (Cr, Mn, Fe, Ni, Cu, Zn, Cd, Pb) concentrations in surface sediment porewater and their benthic fluxes were investigated in Daya Bay, South China, to study their accumulation and transfer at the sediment- water interface, as well as the impact of human activities on heavy metals. Heavy metals in porewater displayed different patterns in three partitions (top, center and inlet), which was mainly attributed to the difference in the biogeochemical conditions, hydrodynamic force inner the bay and the human activities along the bay. Ecological risk assessment results showed that heavy metals in porewater dramatically exceeded the background values. The average release of heavy metals from sediment were (6.1 ± 3.3) × 10 4 –(2.7 ± 1.6) × 10 8 ga -1 in the bay, so they had potential risks to the water environment, and sediment should be paid more attention to as the endogenesis of contamination. 1. Introduction Estuarine and coastal bays, which are regions of active land-ocean interaction, respond sensitively to natural processes and anthropogenic activities. In recent decades, economic development and population increase have resulted in a great quantity of heavy metals to estuarine and coastal bays through surface runoff, atmospheric deposition, was- tewater discharge etc. (Huang et al., 2014; Li et al., 2018; Feng et al., 2019). Heavy metals can't be degraded easily in bay systems, and can migrate in different environments and finally, most of them deposited in the sediments. Sediment is a large reservoir of pollutants and plays an essential environmental role due to its capacity to store or release pollutants from or to the water column (Ip et al., 2007; Wang et al., 2016; Sun et al., 2019). When the hydrodynamic conditions (wave, flow, transport and bioturbation) changed, pollutants in sediment re- leased into water column, and that led to the secondary pollution of the water quality (He et al., 2017; Bastakoti et al., 2018). Benthic inter- actions are important reasons for heavy metal pollution (Blasco et al., 2000; Yi et al., 2018; Liu et al., 2019), and sediment-water interface (SWI) plays a vital role during those processes. The properties of SWI affect the cycle, transfer and preservation of materials in ecosystem. In other words, the fluxes of materials at the SWI are not only influenced by the material concentration gradient between overlying water and porewater, but the content and composition of organic matter, bottom water oxygen concentration and penetration depth, activities of benthic organisms and bacteria etc. (Blasco et al., 2000; Vopel et al., 2012; Kaiser et al., 2013). In this sense, the estimation of pollutant fluxes at the SWI is significant to assess the biogeochemical cycling of materials, sedimentary environment and the quality of ecosystem. Daya Bay is a typical coastal semi-closed bay severely affected by the excessive anthropogenic activities in southern China. With in- dustrial development and population expansion, a large amount of urban sewage and industrial effluent was discharged into the bay. Changes in the water/sediment quality and ecosystem health of Daya Bay need more attention. There has been a lot of research on heavy metal pollution in Daya Bay, which was mainly related to heavy metal distribution and sources (Gu et al., 2016; Zhao et al., 2016; Qu et al., 2018), and their ecological effects etc. (Qiu, 2015; Liu et al., 2018). There is still a lack of research on the heavy metal characteristics in porewater and their fluxes at the SWI, which was discussed among seasons and different areas in Daya Bay in the present study. Further- more, it is more significant that the exchange capacities of heavy metals were estimated in the bay to survey the potential threat of sediment to water environment. The main objectives of this study were (1) to in- vestigate the distribution of heavy metals (Cr, Mn, Fe, Ni, Cu, Zn, Cd and Pb) and to assess their ecological risks in porewater and overlying https://doi.org/10.1016/j.marpolbul.2019.110808 Received 22 August 2019; Received in revised form 2 December 2019; Accepted 6 December 2019 ⁎ Corresponding author at: South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China. E-mail address: xphuang@scsio.ac.cn (X. Huang). Marine Pollution Bulletin 150 (2020) 110808 Available online 10 December 2019 0025-326X/ © 2019 Published by Elsevier Ltd. T