Derivation of copper water quality criteria in the Bohai Sea of China considering the effects of multiple environmental factors on copper toxicity

Highlights

• Temperature has the most comprehensive influence on Cu toxicity in the Bohai Sea.

• The copper WQCs of the Bohai Sea in different seasons were derived.

• The WQC values decrease significantly with the increase of seasonal temperature.

• The WQC value in winter is much higher than in other seasons.

Abstract

Copper has become one of the most important heavy metal pollutants in the environment because of its wide application and high toxicity, but research on water quality criteria (WQCs) on copper is limited, especially the derivation of seawater WQC. In addition, the toxicity of copper in the seawater system is affected by various environmental factors. Therefore, establishing a WQC that meets the characteristics of the regional environment is a top priority. The correlations between four factors of temperature, salinity, pH, dissolved organic carbon (DOC) and the toxic effect values of copper were analyzed in this study, and the temperature was determined as the most influential factor among the four factors in the Bohai Sea. A specific correlation between temperature and the toxic effects of copper was identified, and WQCs were derived based on the identified correlation and the variations of the Bohai Sea's temperature in different seasons by species sensitivity distribution (SSD) method. Under the condition of the winter, spring, autumn, and summer with an average water temperature of 0.09, 15.96, 17.83, and 24.87 °C, the obtained short-term water quality criteria (SWQCs) were 44.29, 4.70, 4.31, and 3.33 μg/L; the long-term water quality criteria (LWQCs) were 18.14, 1.93, 1.77 and 1.36 μg/L. The findings indicated the importance of introducing specific environmental conditions during the derivation process. This work could provide valuable information for pollution prevention and aquatic life protection in the Bohai Sea and provide a valuable reference for the derivation of criteria in other regions alike.

Introduction

Due to the wide range of pollution sources and the substantial volumes, heavy metals pose a huge potential threat to the entire ecological environment system (Han et al., 2007; Gao et al., 2016; Zhang et al., 2007). Among the heavy metals, the potential hazard risk of copper to aquatic organisms ranked first according to the assessment results of researchers from the UK Centre for Ecology & Hydrology in 2014 (Donnachie et al., 2014) and also in a recent systematic review on toxicity of metals to corals (Nalley et al., 2021). Copper is more toxic to organisms than other common heavy metal pollutants such as zinc and cadmium. Liu et al. found that the toxic effects value of copper on Cynoglossus semilaevis Günther embryos and larvae were less than 50 μg/L and much lower than Zn, Cd, Pb, and even lower than Hg (Liu et al., 2006). For some crustaceans, the toxicity of copper is generally higher than that of other heavy metal pollutants. Li et al. through tests with five heavy metal ions on Neomysis awatschensis, found that the sensitivity of N. awatschensis to five heavy metals is Cu > Hg > Cd > Zn > Pb (Li et al., 2006). In addition, Chen et al. found that the safe concentration of copper was lower than the fishery water quality standard of China among the four heavy metals Cd, Mn, Zn, and Cu. and special attention should be paid during seedling and breeding (Chen et al., 2021). And in the global water environment pollution, copper pollution has always been on the rise, and it has been studied as the main heavy metal pollutant all year round (Li et al., 2019; Li et al., 2022). Therefore, it is necessary to establish relevant water quality criteria (WQC) for copper to protect the local ecosystem.

Copper in the environment mainly exists in the form of an ionic state, and its migration and transformation processes are complex and diverse, including various physical, chemical, and biological processes in water bodies (Wijaya et al., 2016). In the seawater system, the existing forms and biological toxicity of copper are mainly affected by four factors: water temperature, salinity, pH, and DOC (EPA, 2016). Some researchers found that the combinations of NaCl with copper significantly made the Zebrafish embryo's hatch success decrease (Santos et al., 2021), when the temperature changed from 10-15 °C to 25–30 °C, and the toxicity of metal ions increased rapidly (Braginskiy and Shcherban, 1978). While as the pH decreased, the Cu absorption rate constant of Dreissena polymorpha gradually decreased (Le et al., 2020), and the addition of DOC provided a significant protective effect against Cu toxicity to the B. plicatilis in all salinities (Cooper et al., 2015). Since changes in the species toxicity values will affect the magnitude of the criteria values, the establishment of criteria should consider the impact of different environmental factors.

Compared with the United States and other countries, the water quality criteria research in China started late (Wu et al., 2011; Hou et al., 2015). It was not until 2020 that the Ministry of Ecology and Environment of China released the first freshwater WQC of cadmium. Recently, the study of WQC under freshwater systems has considered the impact of environmental factors. Researchers have selected specific environmental factors such as hardness or temperature to modify the toxicity values and further derive the WQC. (Li et al., 2021; Park et al., 2018; Ding et al., 2021). But in the seawater system, most researches are still focused on the derivation of some basic criteria value without an in-depth consideration of the impact of environmental factors (Duran and Beiras, 2013; Beiras and Schonemann, 2021). This is also an urgent problem to be solved in the establishment of seawater WQC.

The Bohai Sea is considered both an internal sea and China's most northern offshore seawater area. Due to this special geographical factor, environmental changes in the Bohai Sea have obvious temporal and spatial characteristics. The temperature of the entire region varies greatly at different periods of the year, and the salinity of the estuary areas also varies greatly from the rainy season to the dry season. This is one of the reasons for its important functions in tourism, aquaculture, and seawater resources abundances. However, due to the development of industries in the Bohai Rim, the environment in the Bohai Sea was gradually deteriorating, some seasonal variations have been reported that the water quality of the Bohai Sea was polluted to varying degrees in all seasons, and copper was one of the main pollutants. (NCS.MNR, 2018). At present, there are few studies on WQCs in China's seas. Therefore, it is necessary to take copper as an example to establish a WQC that meets the environmental characteristics of the Bohai Sea to provide a reference for the derivation of WQCs in other seas.

The objective of the present work is to: (1) identify the most influential environmental factor on copper toxicity among temperature, salinity, DOC, and pH; (2) modify the toxicity value based on the environmental factors of the Bohai Sea in different seasons; (3) derive water quality criteria values under specific environmental conditions.