Marine animals often accumulate various harmful substances through the foods they ingest. The bioaccumulation levels of these harmful substances are affected by the degrees of pollution in the food ...and of biomagnification; however, which of these sources is more important is not well-investigated for mercury (Hg) bioaccumulation. Here we addressed this issue in fishes that inhabit the waters around Minamata Bay, located off the west coast of Kyushu Island in Kumamoto Prefecture, Japan. The total Hg concentration (hereafter THg) and carbon and nitrogen stable isotope ratios (δ13C and δ15N) were analyzed in the muscle tissue of 10 fish species, of which more than five individuals were caught by gillnet. Except one species, each was separated into two trophic groups with respective lower and higher δ13C values ranging from −17‰ to −16‰ and −15‰ to −14‰, which suggested that the fishes depended more on either phytoplankton- and microphytobenthos-derived foods (i.e., pelagic and benthic trophic pathways), respectively. Linear mixed effects models showed that the Hg levels were significantly associated with both δ15N and the differences in the trophic groups. THg increased with δ15N (i.e., indicative of higher trophic levels), but the slopes did not differ between the two trophic groups. THg was significantly higher in the group with higher δ13C values than in those with lower δ13C values. The effect size from marginal R squared (R2) values showed that the variation in THg was strongly ascribed to the trophic group difference rather than δ15N. These results suggest that the substantial Hg bioaccumulation in the fishes of Minamata Bay is mainly an effect of ingesting the microphytobenthos-derived foods that contain Hg, and that the subsequent biomagnification is secondary.
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•Fish depended more on either phytoplankton- and microphytobenthos-derived foods.•Fish THg were significantly higher in high-δ13C trophic groups than in low-δ13C groups.•Fish THg were significantly associated with δ15N.•Variation in THg was strongly ascribed to the trophic group difference rather than δ15N.
This is the first study that evaluated the relative importance between the effects of primary producer origins of the diet and biomagnification on mercury bioaccumulation in Minamata Bay.
To better understand the methylation and demethylation of mercury (Hg) in the atmosphere, monomethyl mercury (MMHg) concentrations in wet deposition samples collected in the Minamata Bay area from ...September, 2009 to August, 2010 were determined. The concentrations of total Hg (dissolved Hg + particulate Hg) and dissolved reactive Hg were also measured. The volume-weighted mean concentrations of dissolved MMHg and total Hg were 0.061 and 5.9 ng L−1, respectively. Almost 90% of total Hg was in the dissolved phase and dissolved reactive Hg was the dominant Hg species in the wet deposition. The wet deposition fluxes of total Hg and reactive Hg increased in the rainy season (summer and fall), while the concentrations and wet deposition fluxes of MMHg were higher in winter and spring than in summer. Factors affecting this seasonal variation of MMHg were also considered. The MMHg in wet depositions may be decomposed by the increased levels of UV radiation in summer months, although MMHg can also be emitted from anthropogenic and biogenic sources and/or produced by photochemical reactions. Long-range transport from the Asian continent may also contribute to the seasonal variation of the wet MMHg deposition flux.
•Our study provides the first data of wet depositions of monomethyl mercury (MMHg) in Japan.•Both concentrations and depositions of MMHg are higher in winter and spring than in summer.•Our data indicate MMHg in wet depositions can be decomposed by strong UV-A in summer.•Long-range transport of air masses from the Asian Continent may contribute to enhanced MMHg in rainwater in winter and spring.
Methylmercury concentrations in Minamata Bay are high, but the cause is unclear. We conducted a basic study on the behavior of methylmercury in Minamata Bay seawater; the findings suggest that ...mercury methylation may occur throughout the year in Minamata Bay. Seawater temperature, salinity, and concentrations of dissolved organic carbon were the environmental factors that affected methylation, and the degree of methylation was closely related to bacterial community structure. The concentration of methylmercury in suspended particulate matter was highest 10 m below the surface and decreased with greater depths. We did not observe a correlation between methylmercury concentrations in suspended particulate matter and concentrations of dissolved methylmercury.
•Mercury methylation occurs in Minamata Bay throughout the year.•Temperature, salinity, and dissolved organic carbon are factors in methylation.•The degree of mercury methylation is related to bacterial community structure.•Methylmercury concentrations in suspended particulate matter vary by water depth.•These concentrations do not correlate with concentrations of dissolved methylmercury.
Methylmercury concentrations in fish from the historically polluted Minamata Bay remain higher than in fish from other coastal seas around Japan. To obtain a better understanding of this phenomenon, ...the chemical characteristics of pore water from Minamata Bay sediments were investigated. Samples were taken from two stations over a 1-year period. Total average values in the pore water at the two stations for dissolved total mercury and methylmercury concentrations were 6.64±4.93 and 2.69±2.07ng/l, respectively. The pore water was centrifuged at 1000rpm to 3000rpm. The highest ratio of dissolved methylmercury to dissolved total mercury exceeded 60% for pore water centrifuged at 3000rpm. Furthermore, because total average values of Log Kd of total mercury and methylmercury in sediment (St1 and St2) were 5.42 and 2.32 (Lkg−1), methylmercury in Minamata Bay sediment is more eluted than other mercury species.
•Average-diss-total-Hg concentration in pore water was 6.64±4.93ng/l.•Average diss-methyl-Hg concentration in pore water was 2.69±2.07ng/l.•The highest diss-methyl-Hg content to diss-total-Hg ratio exceeded 60%.•Average Log Kd for total-Hg for the sediments was 5.42.•Average Log Kd for methyl-Hg for the sediments was 2.32.
This study was carried out to evaluate the present-day chemical properties of methylmercury in surface sediment in Minamata Bay where a dredging project was completed 28years ago. Present-day ...sediment from Minamata Bay consists of sandy silt, and the average loss-on-ignition in surface sediment was 7.0±2.3%. The average methylmercury concentrations in the upper sediment layers were significantly higher than those in the lower sediment layers. Currently, the concentrations in sediments in Minamata Bay do not exceed the Japanese regulatory standard value for mercury. The average concentration of methylmercury in Minamata Bay surface sediment was 1.74±1.0ng/g on a dry weight basis (n=107). The methylmercury concentration in Minamata Bay surface sediment was almost 16 times higher than that in surface sediment from Isahaya Bay surface sediment, which was 0.11±0.045ng/g on a dry weight basis (n=5).
•The Minamata Bay sediment is sandy-silt; average value of ignition loss was 7.0±2.3%.•Average value of methyl-Hg in Minamata Bay surface sediment was 1.74±1.0ng/g.•Average value of methyl-Hg in Isahaya Bay surface sediment was 0.11±0.045ng/g.•A correlation between ignition loss and methyl-Hg in sediment was observed (r=0.69).
Between 1932 and 1968, industrial wastewater containing methylmercury (MeHg) and other mercury (Hg) compounds was discharged directly into Minamata Bay, Japan, seriously contaminating the fishery. ...Thousands of people who consumed tainted fish and shellfish developed a neurological disorder now known as Minamata disease. Concentrations of total mercury (THg) in recent fish and sediment samples from Minamata Bay remain higher than those in other Japanese coastal waters, and elevated concentrations of THg in sediments in the greater Yatsushiro Sea suggest that Hg has moved beyond the bay. We measured stable Hg isotope ratios in sediment cores from Minamata Bay and the southern Yatsushiro Sea and in archived fish from Minamata Bay dating from 1978 to 2013. Values of δ(202)Hg and Δ(199)Hg in Yatsushiro Sea surface sediments were indistinguishable from those in highly contaminated Minamata Bay sediments but distinct from and nonoverlapping with values in background (noncontaminated) sediments. We conclude that stable Hg isotope data can be used to track Minamata Bay Hg as it moves into the greater Yatsushiro Sea. In addition, our data suggest that MeHg is produced in bottom sediments and enters the food web without substantial prior photodegradation, possibly in sediment porewaters or near the sediment-water interface.
The Yatsushiro Sea in Japan is contaminated with mercury in wastewater discharge from the Chisso Company, which produced acetaldehyde from 1932 onwards. In this study, we investigated the current ...spatial distribution of total mercury (T-Hg) in sediments, both at the surface and at different depths, using 43 sediment cores and 22 surface sediment samples collected from the Yatsushiro Sea in August 2017. Altogether, 821 surface and core samples were analyzed for their T-Hg concentrations. Most of the mercury that was discharged from Chisso remained either in, or around the exit of, Minamata Bay, but some had been transported from Minamata Bay to the Nagashima Sea. We estimated that almost 51 and 6 tons of mercury had accumulated in the Yatsushiro and the Amakusa Sea areas, respectively. The amount of acetaldehyde produced in Chisso over time was correlated with the T-Hg concentrations in the sediments from the Yatsushiro Sea.
•51 and 6 tons of Hg had accumulated in the Yatsushiro and the Amakusa Sea areas.•Acetaldehyde production over time was correlated with the T-Hg in the sediments.•Average value of T-Hg in the sediment of Yatsushiro Sea area was 0.46 ± 0.44 mg/kg dry.•Average value of T-Hg in the sediment of Amakusa Sea area was 0.10 ± 0.04 mg/kg dry.
The distributions of the total mercury (T-Hg), methylmercury (MeHg), and ethylmercury (EtHg) concentrations in soil and their relationship to chemical composition of the soil and total organic carbon ...content (TOC, %) were investigated. Core samples were collected from hill slope on the right and left riverbanks of the Idrija River. Former smelting plant is located on the right bank. The T-Hg average in each of the core samples ranged from 0.25 to 1650 mg kg−1. The vertical T-Hg variations in the samples from the left bank showed no significant change with depth. Conversely, the T-Hg varied with depth, with the surface, or layers several centimeters from the surface, tending to show the highest values in the samples from the right bank. Since the right and left bank soils have different chemical compositions, different pathways of mercury delivery into soils were suggested. The MeHg and EtHg concentrations ranged from n.d. (not detected) to 444 μg kg−1 and n.d. to 17.4 μg kg−1, respectively. The vertical variations of MeHg and EtHg were similar to those of TOC, except for the near-surface layers containing TOC greater than 20%. These results suggest that the decomposition of organic matter is closely related to organic mercury formation.
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•The distributions of T-Hg, MeHg, and EtHg near the Idrija Hg mine were investigated.•The Hg values were compared to the chemical composition, TOC, and T-N of the soil.•The first data of EtHg in soil around the mine is reported.•The decomposition of organic matter is closely related to organic Hg formation.•Hg circulation among soil, air and plant with change of chemical forms was suggested.
After the outbreak of Minamata Disease, highly mercury-polluted sediment was dredged from Minamata Bay. Nevertheless, some studies indicated that trace mercury (Hg) migrated from Minamata Bay to the ...Yatsushiro Sea with the movement of sediments. Mercury was considered to adsorb on sediment particles and realized the transport. However, the influence of particle size as a crucial factor affecting adsorption is barely considered in the analysis of mercury migration. In this study, we develop a novel method based on particle size classification to comprehend both horizontal and vertical distribution of the total-Hg (T-Hg) concentration in sediments with different particle sizes in the Yatsushiro Sea. From the experiment result, it could be concluded that overall, in the horizontal direction, the T-Hg concentration became smaller and smaller along A and B lines from the sampling point Y28, indicating the mercury transport from Minamata Bay to the Yatsushiro Sea. Moreover, the T-Hg concentration increased first and then decreased in the vertical direction, suggesting that the migrated mercury was mainly deposited in the upper sediments. Furthermore, according to the classification experiment results, the smaller the particle size, the higher the T-Hg concentration. And, the sediment particles migrating from Minamata Bay to the Yatsushiro Sea are mainly very fine slit to fine silt.
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