Heavy metal pollution status and deposition history of mangrove sediments in Zhanjiang Bay, China

Abstract

Mangroves have high ecological service value and play an important role in achieving carbon neutrality. However, the ecological services provided by mangroves are gradually declining due to the threat of various pollutants, especially heavy metal (HM) pollution. Researching the status and history of heavy metals (HMs) in mangrove sediments improves understanding of how mangroves can be protected. We studied the Zhanjiang Bay mangroves, which is an important part of the Zhanjiang Mangrove National Nature Reserve, and is greatly affected by human activities, as an example. We analyzed the pollution levels and deposition process of HMs and their influencing factors. In the mangrove surface sediments of Zhanjiang Bay, Pb, Cr, and Co were slightly to moderate enriched and Cu was moderate-heavy enriched. Zn and Hg showed moderate-heavy enrichment and heavy enrichment, respectively. Cu, Zn, Hg, and Co were 1.5, 1.88, 2.66, and 1.27 times the local background, respectively. Cu, Zn, and Hg mainly originated from anthropogenic sources, and Co may have derived from natural sources. The HM data of mangrove sedimentary cores in Zhanjiang Bay were obtained for the first time; high HM content appeared in the deep layer of the cores but not in the mangrove surface sediments. Both cores were well documented for the effects of natural and anthropogenic factors on the deposition process of HMs. Among them, the high enrichment of Hg in 1909–1921 may be closely related to human activities rather than natural formation. Before 1954, dense mangroves favored the accumulation and gradual increase of HMs. The intense human activity in 1945–1963 during the period of economic construction and reclamation of the Nansan Island Engineering led to significant increases in Cu, Zn, Pb, Cr, and Co, which reached their highest values in more than one hundred years. After 1963, anthropic factors such as deforestation and conversion to aquaculture sharply reduced mangrove area. Reclamation engineering enhanced tidal erosion. Natural factors such as precipitation, storm surge, sea-level rise, and high tides have accelerated the erosion and dilution of mangrove sediments in Zhanjiang Bay. These factors have led to the gradual reduction of HM content in sedimentary cores.

Integrated water quality dynamics in Wadi Hanifah: Physical, chemical, and biological perspectives

“…High electrical conductivity typically indicates a greater concentration of dissolved ions, which could suggest pollution from various sources or natural differences in mineral content along the Wadi. The wide range of electrical conductivity, indicative of ion concentration, aligns with Tang et al [ 39 ] , suggesting diverse pollution sources or mineral content.…”

Increasing eutrophication driven by the increase of phosphate discharge in a subtropical bay in the past 30 years

“…Additionally, increasing intrusion of higher salinity water from the outer bay can retain the contaminants in the inner bay (Lao et al, 2022). Suixi River at the top of the upper bay is the main river entering into Zhanjiang Bay, which can directly carry pollutants discharged from agricultural production, domestic sewage and industrial production into the bay (Shi et al, 2015;Li et al, 2020;Lao et al, 2022;Tang et al, 2022). Additionally, due to the different topography between the upper and lower bays in Zhanjiang Bay, the tidal range and tidal current are quite different (Wang et al, 2021c).…”

“…The eutrophication in Zhanjiang Bay was evaluated using the eutrophication index (EI), the formula as follow (Huang et al, 2022;Ke et al, 2022):…”

“…Generally, if the value of EI > 1, the seawater in the bay is considered eutrophic. A total of five classifications can be divided to assess the eutrophication, which is widely used to evaluate eutrophication of coastal waters (Andersen et al, 2017;Lao et al, 2021;Huang et al, 2022;Ke et al, 2022), that is as following: EI < 1 represents no eutrophication, 1.0 ≤ EI < 2.0 represents light eutrophication, 2.0 ≤ EI < 5.0 represents moderate eutrophication, 5.0 ≤ EI <15.0 represents high eutrophication, EI ≥ 15.0 represents severe eutrophication.…”

Quantitative and Predictive Evaluation of Heavy Metal Pollution in Bay Ecosystems of Northern China: Characteristics, Accumulation, and Risk Assessment

“…China produced 14.8 million tons of bivalve mariculture in 2020, more than 69.31% of total mariculture, of which more than 45% originates from northern China . However, China has experienced HM pollution due to industrial and agricultural development, especially in bays. − Bivalves accumulate high amounts of HMs from the sediment and seawater during feeding and survival . Bivalves have the ability of tolerance and adaptability to HMs, which could transfer HMs through the food chain.…”