Assessment of the temporal retention of mercury and nutrient records within the mangrove sediments of a highly impacted estuary

The role of soil organic matter quality and mineralogy controlling the highest mercury concentration of the Brazilian mangroves

The ecological role of estuarine areas on organic matter storage is well known. However, the contributions of soil organic matter (SOM) quality and mineralogy as geochemical filters remain unclear. It requires further investigations in pursuit of contaminant retention understanding, such as mercury (Hg), one of the greatest threats to mangrove areas. We evaluated the highest Hg pollution case of the Brazilian mangroves to investigate the role of SOM and minerals composition in controlling this heavy metal fate. Soils were sampled from different forests: Laguncularia racemosa, coexistence of species, and Rhizophora mangle distributed along the Botafogo estuary. Redox potential (Eh), pH, electrical conductivity (ECe), SOM content, and granulometry were performed. Total mercury (THg) and its distribution were also determined. SOM was analyzed by the evaluation of its thermal stability and molecular composition, while mineralogy was investigated due to XRD and microscopy procedures. We identified a severe contamination, in which THg concentration achieved values up to 14.3 mg kg-1, 161-fold higher than the local background. Besides the distance from the source, THg variation along the contaminated forests was controlled by the natural heterogeneity of C groups provided by different species domains. It clearly generates different scenarios for Hg retention in estuarine areas, especially where Rhizophora mangle develops, considering their characteristics to release more refractory C, such as tannin and lignin, responsible for blocking this contaminant. Simultaneously, but with a lower contribution, pyrite (Fe sulfide) acted as a retainer, also controlling Hg fate on the soil.

An assessment of metal concentrations in leaves, roots, and associated sediments of mangrove plant (Avicennia marina) in the Myeik area, Myanmar

The Myeik region in Myanmar hosts extensive mangrove forests, however, increasing human activities like mining and agriculture have caused heavy metal pollution. This study investigates the role of Avicennia marina in metal dynamics and contamination status in Myeik through metal concentration analysis in leaves, roots, and sediments. Bioconcentration factors (BCF) and translocation factors (TF) were calculated, and ecological indices were used to assess contamination levels and ecological risks. Sediments had higher metal concentrations than plant parts, with nonessential elements (As, Cd, Co, Cr, Pb) averaging 83.2 ± 21.4, 3.16 ± 0.58, 25.6 ± 4.3, 55.1 ± 14.2, and 87 ± 18 mg/kg, and essential elements (Cu, Fe, Mn, Ni, Zn) averaging 21.0 ± 8.0, 18,283 ± 1614, 719 ± 312, 81 ± 30, and 108 ± 32 mg/kg. A. marina showed high BCF for Cd and Cu, and essential elements (Ni, Zn, Mn) exhibited higher TF than nonessential elements (As, Pb, Co). Ecological indices indicated significant contamination and risks due to As, Cd, and Pb, mainly sourced from mining and industrial activities. The contamination levels are ranked as Inlaymyine > Kyweku > Lighthouse. A. marina's metal uptake and translocation were influenced by local environmental factors and physiological responses. Mangrove sediments retained more metals than mudflats, showing habitat-specific differences. These findings highlight the potential of A. marina as a bioindicator of heavy metal pollution and its significant contribution to natural bioremediation processes in the Myeik.

Assessing sediment quality at tropical mangrove areas for using as reference sites in ecotoxicological bioassays

The sediment of five mangrove in Pernambuco/Brazil was investigated to find a reference site for toxicity bioassays. Sediment characteristics, metal levels, and toxic effects on the nauplius of the copepod Tisbe biminiensis were studied. The sediment was composed by terrigenous muds and siliciclastic sands with medium to high organic matter contents. The FeAs association show most of the high concentrations and positive correlation among its constituents in the south. In the north, the Ca-Sr-La association is higher and exhibits positive correlation among its components that usually found in carbonate rocks. Very intense toxic effects were observed, mainly in the south, with >70 % reductions in development. As and Hg were positively correlated with mortality and decrease in development. The sediment quality at studied mangroves prevent their use as a reference site in bioassays. These could be linked to the direct/perennial contribution of geogenic sources associated with anthropogenic environmental impacts.

Human activity has increasingly affected recent carbon accumulation in Zhanjiang mangrove wetland, South China

Mangrove wetlands are an important component of blue carbon (C) ecosystems, although the anthropogenic impact on organic C accumulation rate (OCAR) in mangrove wetlands is not yet clear. Three sediment cores were collected from Zhanjiang Gaoqiao Mangrove Reserve in Southern China, dated by 210Pb and 137Cs, and physico-chemical parameters measured. Results show that the OCARs in mangroves and grasslands have significantly increased by 4.4 and 1.3 times, respectively, since 1950, which is consistent with the transformation of organic C sources and the increase of sedimentation rate. This increment is due to increased soil erosion and nutrient enrichment caused by land use change and the discharge of fertilizer runoff and aquaculture wastewater. This study provides clear evidence for understanding the changes in organic C accumulation processes during the Anthropocene and is conducive to promoting the realization of C peak and neutrality targets.

Spatiotemporal response of water quality in fragmented mangroves to anthropogenic activities and recommendations for restoration

Mangroves have received substantial attention for their pivotal role as ecological barriers between land and sea, owing to their capacity to effectively capture considerable quantities of terrestrial pollutants. Mangrove fragmentation has been a widespread global trend. There is limited information on the water quality status of these small scattered mangrove patches in coastal sub-developed areas, coupled with a paucity of efficient and intuitive assessment methodologies. To address this gap, the Water Quality Index (WQI) was introduced to evaluate the spatiotemporal characteristics of mangrove water quality. The major sources of pollution and anthropogenic activities that affect mangrove water quality were identified. The results revealed an average WQI value of 44.1 ± 13.3 for mangrove patches, consistently indicating a "low" water quality classification throughout all seasons. Both the size and natural conditions impact the water quality of mangroves. The large artificial patch (WQI: 56.4 ± 7.61) and the natural patch (WQI: 46.6 ± 13.6) exhibited relatively superior water quality, while the WQI value of a size-equivalent artificial patch compared with the natural patch is 38.6 ± 11.8. Aquaculture was the primary human activity that adversely affected the water quality of mangroves, and the potential sources of pollution were rainfall runoff and river discharge. These findings elucidate the unfavorable water quality characteristics and dominant pollution of fragmented mangroves, and validate the applicability of the WQI method for long-term evaluation of the water quality in mangrove patches. This study provides a basis for decision-making in water quality assessment and management of coastal wetlands and marine ecosystems. Scientific guidance to the management for mangrove protection and restoration was offered, such as regulating aquaculture activities, controlling non-point source pollution, implementing mangrove reforestation by using native species in historical mangrove sites.

Outwelling of nutrients into the Pasur River estuary from the Sundarbans mangrove creeks

The Pasur River estuary (PRE), the largest estuary in the Sundarbans mangrove area, provides vital fishery resources and supports millions of livelihoods in the southwestern coastal region of Bangladesh. This study focused on the tidal and run-off effects on the outwelling of nutrients from the Sundarbans mangrove creeks to the PRE. Spatial and temporal variations of nutrient and chlorophyll-a concentrations were assessed by water sampling at 11 stations in the study area from January to December 2019. Dissolved inorganic nutrients and chlorophyll-a were analyzed by standard methods using a spectrophotometer. In the tidal mangrove creeks, dissolved inorganic nitrogen, phosphate, and silica concentrations were significantly higher (p < 0.05) during the spring tide than those during the neap tide, suggesting that these nutrients were flushed from the mangrove area by the inundation and tidal mixing of the spring tide. In general, chlorophyll-a (mean ± SD) concentrations in the PRE and the tidal mangrove creeks were 5.62 ± 1.30 μg/L and 9.03 ± 0.59 μg/L in the wet season, respectively. During the dry season, the chlorophyll-a decreased to 4.37 μg/L ± 0.68 and 4.94 ± 1.52 μg/L in the PRE and the tidal mangrove creek, respectively. The amount of nutrients outwelled from the mangrove creeks to the estuary was 1.53 ± 0.67 mg/L DIP, 0.001 ± 0.0004 mg/L DIN, and 1.38 ± 0.48 mg/L dissolved silica. DIP, silica, and chlorophyll-a concentrations were significantly higher (p < 0.05) during the spring tide compared to the neap tide, but salinity was not significantly (p > 0.05) different between the two tidal levels. This study showed that the mangrove creeks formed an important link in transporting nutrients from the mangrove forest to the estuary.