The study examined 22 surface-sediment samples collected from the Maowei Sea, an inland bay in the northern Beibu Gulf with an average water depth of about 2.5 metres. Ten sites were sampled in spring and 12 in autumn. Researchers used a cable-deployed grab sampler, targeted the uppermost 0–2 centimetres, and divided each sample for analysis of copper, lead, zinc, cadmium, chromium, arsenic and mercury, as well as organic carbon, sulfides and petroleum hydrocarbons. The shallow, semi-enclosed bay is connected to Qinzhou Bay through a narrow southern channel. The Qinjiang and Maoling Rivers deliver about 27.73 billion cubic metres of freshwater and 86,400 tonnes of suspended sediment annually, creating a strong terrestrial pathway into the bay. Approximately 80–85% of annual precipitation occurs from April to September, when increased river discharge can carry additional particles and associated contaminants toward the coast.

Laboratory procedures followed the Chinese marine sediment-monitoring specification GB 17378.5-2007 and included quality-control materials for the analyzed parameters. Organic carbon was measured by potassium dichromate oxidation-reduction capacity, sulfides by the methylene blue spectrophotometric method, and petroleum hydrocarbons by ultraviolet spectrophotometry after extraction with n-hexane at a detection wavelength of 225 nanometres. Metals were prepared by nitric acid and perchloric acid digestion, with nitric acid and hydrogen peroxide digestion used for mercury; arsenic was measured following acidic digestion. Copper, lead and chromium were analysed by flameless atomic absorption spectrophotometry, cadmium and zinc by flame atomic absorption spectrophotometry, and arsenic and mercury by atomic fluorescence spectrometry. Certified reference materials and standard solutions supported quality assurance. Reported recoveries ranged from 81.4% to 84.3% for petroleum hydrocarbons and from 73.0% to 96.1% across the seven metals, although recovery tests were not conducted for organic carbon or sulfides.

Most parameters followed a distinct north-to-south gradient. Organic carbon, sulfides, petroleum hydrocarbons, copper, lead, zinc, cadmium and arsenic were consistently enriched in northern and northeastern coastal sediments, while mercury and chromium were comparatively uniform across the study area. The northern sector therefore appeared to be the principal accumulation zone for both organic matter and trace metals. Concentrations generally declined toward the more open southern waters, a pattern consistent with the combined influence of terrestrial runoff, restricted exchange in the inner bay and stronger hydrodynamic processing farther south. Sediment concentrations were also substantially higher than those measured in overlying seawater, particularly for copper, lead, arsenic and chromium. This sediment–water contrast indicates that deposited fine particles and associated contaminants are retained over longer periods, making surface sediment a reservoir as well as a record of past inputs.

Seasonal differences were modest in the north but more evident in the central and southern parts of the Maowei Sea. Organic carbon ranged from 0.18% to 0.62% in spring, with a median of 0.27%, and from 0.26% to 0.54% in autumn, with a median of 0.36%. Petroleum hydrocarbons ranged from 14.6 to 30.2 micrograms per gram in spring, with a median of 21.36 micrograms per gram, and from 15.3 to 45.6 micrograms per gram in autumn, with a median of 25.61 micrograms per gram. Sulfides ranged from 22.30 to 45.80 micrograms per gram in spring and from 22.40 to 52.30 micrograms per gram in autumn. The authors linked generally higher autumn values in the central and southern bay to wet-season transport: increased precipitation and river discharge can deliver more suspended particles, organic matter and associated contaminants, after which seasonal circulation redistributes them through the bay.

The heavy metals also varied between seasons. Copper ranged from 4.40 to 16.70 micrograms per gram in spring and 8.80 to 18.20 micrograms per gram in autumn; lead ranged from 3.80 to 14.10 and 6.80 to 10.40 micrograms per gram, respectively. Cadmium ranged from 0.08 to 0.25 micrograms per gram in spring and 0.18 to 0.32 micrograms per gram in autumn, while arsenic ranged from 3.17 to 9.46 micrograms per gram in spring and 2.52 to 6.33 micrograms per gram in autumn. Zinc ranged from 17.70 to 36.70 micrograms per gram in spring and 14.30 to 33.20 micrograms per gram in autumn. Mercury remained below 0.1 microgram per gram and chromium below 15.0 micrograms per gram. Zinc and copper had the highest average concentrations in both seasons. Lead ranked above chromium in spring, whereas chromium ranked above lead in autumn, suggesting that relative source contributions may vary seasonally.

Comparisons with sediment-quality benchmarks produced a reassuring overall picture but did not eliminate every local concern. All measured parameters fell below Class I values in China’s national marine sediment standard, and all single-factor pollution indices were below 1. Under the National Oceanic and Atmospheric Administration’s Effects Range-Low and Effects Range-Median guidelines, most metals were below the ERL value, which is generally associated with a low probability of adverse biological effects. Arsenic at one station, Q12, reached 9.46 micrograms per gram, slightly above the ERL value of 8.2 micrograms per gram. The researchers said this exceedance does not by itself demonstrate ecological impairment, but it identifies an area that warrants continued observation. Relative to local historical background levels, cadmium and mercury showed clear enrichment, while copper, cadmium, lead and mercury were approximately two- to four-fold higher. This combination indicates anthropogenic contributions even though the concentrations remained within regulatory limits.

Correlations among sediment variables provided clues about retention processes, but did not uniquely identify pollution sources. Organic carbon, sulfides, copper, zinc, cadmium and arsenic were significantly positively correlated, consistent with an association between metal accumulation, organic-rich material and reducing sediment conditions. Moderate relationships between petroleum hydrocarbons and cadmium or lead were consistent with partly overlapping inputs, although correlation alone cannot establish a common emission source. The pattern may reflect the adsorption and complexation of metals in organic-rich sediments and the accumulation of hydrophobic petroleum compounds. Terrestrial runoff appears to be the principal delivery pathway, while organic matter and local reducing conditions regulate retention. The researchers noted that source-specific analysis, including lead isotopes and sediment grain size, would be needed to apportion the contributions more robustly.

Comparisons with other coastal sediments placed the Maowei Sea within a moderate global range but showed relatively low concentrations for many metals. Mean levels were lower than those reported in Maowei Sea mangroves, the Futian mangrove in Guangdong, Shenzhen’s urbanized coast and the Nansha mangroves. Cadmium was higher than values previously reported for the open Beibu Gulf, while copper, lead, zinc and chromium were lower. At the international scale, average concentrations were generally below those in Can Gio, Vietnam, apart from cadmium, although lead exceeded values reported for parts of Iran and the Arabian Gulf. The authors identified the eastern Maowei Sea and the Qinzhou Bay entrance as localized hotspots for lead and petroleum hydrocarbons, where port operations, shipping and adjacent industrial activity may contribute. They also noted that dredged material introduced by recent infrastructure in Qinzhou Bay could be carried into the Maowei Sea by flood-tide currents, although they regarded this as likely minor relative to riverine input.

The 2021 survey provides a reference point rather than a prediction of future conditions. Its limited seasonal coverage, 22 samples and focus on the uppermost two centimetres of sediment constrain how broadly the results can be generalized across time, sediment depth and local environmental conditions. Nevertheless, the dataset offers a pre-operation baseline against which later changes in contaminant concentrations, land-derived inputs and transport pathways can be evaluated as the Pinglu Canal and other coastal developments alter the region. The clearest priorities for monitoring are nearshore areas affected by runoff, port activity and engineering disturbance, especially for cadmium, lead and petroleum hydrocarbons. The study’s central result is a balance between broadly low contamination and concentrated human signatures: the Maowei Sea’s surface sediments remained below China’s Class I criteria in 2021, but local enrichment and strong spatial heterogeneity show that measured levels varied substantially among sites.

Subject of Research: Seasonal and spatial variation of sediment contaminants in China’s Maowei Sea

Article Title: Seasonal and spatial variations of multiple environmental parameters in surface sediments of Maowei Sea, northern Beibu Gulf

Article References: Qian, N., Miao, B., & Chaolu (2026). Seasonal and spatial variations of multiple environmental parameters in surface sediments of Maowei Sea, northern Beibu Gulf. Frontiers in Marine Science, 13, Article 1909234. https://doi.org/10.3389/fmars.2026.1909234

Image Credits: AI Generated

DOI: 10.3389/fmars.2026.1909234

Keywords: Maowei Sea, surface sediments, heavy metals, petroleum hydrocarbons, sediment contamination, terrestrial inputs, Beibu Gulf, Pinglu Canal, coastal pollution, environmental baseline, Seasonal, spatial