Despite occupying no more than 0.06% of the earth’s surface, mangrove ecosystem actually played an important role in water purification, wind/waves protection and carbon restoration (De Lacerda & Linneweber, 2002; FIELD et al., 1998; Hamilton & Casey, 2016). Due to anthropogenic activities' negative effects, e.g., urbanization and aquaculture, coastal areas for mangrove forests growth have been significantly decreased by ~ 40% worldwide over the past half-century (Herbeck et al., 2020). Fortunately, with the increase of environmental protection awareness and the development of social economy of developing countries, especially for China, the conservation and restoration of mangrove forests in coastal wetlands were rapidly established since 1990s (Herbeck et al., 2020). However, the continuous discharge of pollutants from anthropogenic activities accomplished by industrial development might be still a seriously potential threat to mangrove forests, especially for that in restoration processes (Lewis et al., 2011; Maiti & Chowdhury, 2013). Recently, scarce information is presented concerning the potential ecological threats of anthropogenic pollutants on mangrove forests with varying restoration years.
As representative pollutants, heavy metals (HMs), mainly discharging from industrial activities into estuary areas (Algül & Beyhan, 2020), presented a significant risk to mangrove ecosystem (Feng et al., 2017; Tam & Wong, 2000; Zhang et al., 2014). The particularity of mangrove ecosystem was benefit for fine sediments, organics and minerals deposition, indirectly promoting the accumulation of HMs in the relevant sediments (Prasad & Ramanathan, 2008; Shao et al., 2009). In addition, the geochemical fractions of HMs in sediments might be transformed with acid dissolution and complexation effects (Kostka & Luther III, 1994; Zhang et al., 2017). It is generally agreed that bioavailable HMs in sediments could be easily accumulated by mangrove forests, resulting in a serious risk to plant health. This was mainly attributed to two reasons: (i) anions, e.g., Cl−, SO42−, migrated with tidal process could complex with HMs (Hirsch et al., 1989; Weggler et al., 2004); (ii) protons, organic acids and amino acids secreted by microorganisms and mangroves availed for soil acidification and lead to the increase amount of HMs in bioavailable HMs fractions (Wang et al., 2021). Furthermore, mangrove forests also played a significant part in the uptake of HMs from sediments, but this effect was significantly different due to the differences of mangrove growth stages (Feng et al., 2017). However, there is still a lack of relevant information on the relationships between HMs distribution characteristics and external pollutants or physicochemical properties of mangrove sediments in restoration stages.
The Greater Bay Aera (GBA), located in (sub)tropical regions, including Hong Kong, Macao and nine cities in Guangdong. There are obvious differences in industrial distribution and economic level due to varying development emphasis among these cities. Comparatively, cities on the East Bank of the GBA presented with more electronic manufacturing factories than that on the West Bank, resulting in certain differences in pollutant discharge amount during the industrial activities. In addition, there are certain differences in the treatment standards of industrial wastes among these cities with varying urban environmental policies (Yang et al., 2012), resulting in the different fractions and concentrations of HMs discharged into surrounding mangrove ecosystem. In addition, planting years of mangroves in various regions significantly varied due to the differences in restoration time, which might affect the tolerance of mangroves to HMs pollutants (Bai et al., 2011). Therefore, it is of great significance to understand the differences of human activities and urban development on the HMs distribution features of mangrove wetlands in the GBA.
Therefore, five representative mangroves sites with certain differences in planting years and industrial activities were selected for sediments sampling in the GBA. The study's objectives were: (i) to determine the relationship between sediment HMs concentration and urban effluent nutrients (e.g., AP, NH4+, NO3−); (ii) to estimate the influences of varying physiochemical properties on the geochemical fractions of HMs in sediments; (iii) to evaluate the effects of industrial development and environmental protection on HMs accumulation in mangrove sediments.