Methylmercury exposure in a subsistence fishing community in Lake Chapala, Mexico: an ecological approach

<p>Abstract</p> <p>Background</p> <p>Elevated concentrations of mercury have been documented in fish in Lake Chapala in central Mexico, an area that is home to a large subsistence fishing community. However, neither the extent of human mercury exposure nor its sources and routes have been elucidated.</p> <p>Methods</p> <p>Total mercury concentrations were measured in samples of fish from Lake Chapala; in sections of sediment cores from the delta of Rio Lerma, the major tributary to the lake; and in a series of suspended-particle samples collected at sites from the mouth of the Lerma to mid-Lake. A cross-sectional survey of 92 women ranging in age from 18-45 years was conducted in three communities along the Lake to investigate the relationship between fish consumption and hair mercury concentrations among women of child-bearing age.</p> <p>Results</p> <p>Highest concentrations of mercury in fish samples were found in carp (mean 0.87 ppm). Sediment data suggest a pattern of moderate ongoing contamination. Analyses of particles filtered from the water column showed highest concentrations of mercury near the mouth of the Lerma. In the human study, 27.2% of women had >1 ppm hair mercury. On multivariable analysis, carp consumption and consumption of fish purchased or captured from Lake Chapala were both associated with significantly higher mean hair mercury concentrations.</p> <p>Conclusions</p> <p>Our preliminary data indicate that, despite a moderate level of contamination in recent sediments and suspended particulate matter, carp in Lake Chapala contain mercury concentrations of concern for local fish consumers. Consumption of carp appears to contribute significantly to body burden in this population. Further studies of the consequences of prenatal exposure for child neurodevelopment are being initiated.</p

Geographic variability of the concentration of Hg, Co, Fe and Ni in mussels Mytilus californianus (Conrad, 1837) from the coast of Baja California

The spatial distribution of the Hg, Fe, Co, and Ni pollution was determined in the Northwest coast of Baja California using the mussel Mytilus californianus as biomonitor. Samplings of mussels were done during February and July 1994 at eight sites located along 300 km of coast, from Punta Bandera (10 km south of the Mexico-USA border) to San Quintin (300 km south of the border). The spatial distribution of Hg showed a North-South gradient from Punta Bandera (0.331 µg g–1) to San Quintin (0.094 µg g–1). This gradient suggests that the Hg distribution is affected by anthropogenic activities nearby the Mexico-USA border. Cobalt showed a spatial distribution with significantly higher concentrations (P < 0.01) in Piedra Blanca (2.98 µg g–1) and San Quintin (2.26 µg g–1). Nickel showed an irregular spatial distribution with higher concentrations in Punta Bandera (2.07 µg g–1) and San Quintin (1.83 µg g–1). In general, Fe showed higher concentrations from Punta Bandera to Bajamar in February (515–504 µg g–1); except for Punta China (1142 µg g–1) that is characterized by the extraction of petrous material for the cement industry, the sampling sites in the central part of the study area showed lower concentrations. No significant correlations were observed among metals and the biological variables of the analyzed mussels. The cluster analysis among metals, grouped Hg with elements (Ag and Pb) associated with anthropogenic activities in a previous study. Cluster analysis showed three geographical zones that could be defined as polluted, transition and clean zone, respectively

Short-term temporal variability of Ag, Cd and Cu in Mytilus californianus and the effectiveness of this organism as a bioindicator

In order to study the short-term temporal variability of the concentration of Ag, Cd and Cu in Mytilus californianus and to determine the effectiveness of this organism as a bioindicator of these metals, 11 simultaneous samplings (every other day) were conducted from 24 August to 23 September 1995 at one polluted site and one unpolluted site at Todos Santos Bay in Ensenada, Baja California, Mexico. The concentrations of Ag and Cu (µg g–1 dry weight) were significantly greater at the polluted station (0.64–1.69 and 6.72–9.10, respectively) than at the unpolluted station (0.05–0.27 and 5.48–7.00, respectively), possibly due to the proximity of the polluted station to a wastewater discharge. The concentrations of Cd at the unpolluted station (1.37–5.25) were significantly greater than those at the polluted station (0.60–2.13), probably due to the association of Cd with upwelling, which is a common occurrence at the unpolluted station during the time the samplings were conducted. The relationships between the raw concentrations of the metals studied and the concentration normalized by the condition index indicate that M. californianus is a better indicator of Ag and Cd than of Cu

Bioaccumulation of metals in mussels from four sites of the coastal region of Baja California

This study analyzes the concentrations and variations of Cu, Mn, Zn, Al, Cd, Ag, As and Se among different size classes of Modiolus capax, from Punta Estrella, Bahía de los Ángeles and Santa Rosalía in the Gulf of California, and of Modiolus modiolus and Mytilus californianus, from Bahía Tortugas in the Pacific. During November 1988, mussels of these species were collected and 45 organisms were selected (three replicates of 15 organisms each). Their biometric variables were measured: large size (88–98 mm), medium size (70–87 mm) and small size (56–66 mm). The reference standards and procedural blanks were analyzed for each group of 15 samples. The results indicate that the geographic distribution and accumulation of metals in the different sizes are mainly influenced by the sources of input, bioavailability in each place and the biological condition of the organisms. The mussels presented Cu levels independent of size in the region, except at Santa Rosalía. Zn, Cd and Se showed strong regional variability, but no clear trend with respect to the size of the organisms. Al showed high regional variability and a tendency to accumulate in small organisms. As and Se presented higher concentrations in the large to medium mussels, regardless of the sampling site. Mn tended to accumulate in large and medium mussels and was dependent on the sampling site. In Bahía Tortugas, M. californianus accumulates Cu, Mn and Cd at a rate three times lower than M. modiolus at the same level of exposure. Within the Gulf of California, the geographic distribution of Mn, Al and Zn was similar, with higher concentrations in the northern part. This is probably because the mussels are exposed to these metals through the sedimentary material resuspended in the Colorado River Delta. Cd presented levels of 70 and 27 µg g–1 at Bahía Tortugas and Bahía de los Angeles, respectively; both sites are exposed to coastal upwelling. Cu presented relatively low levels (<35 µg g–1), except at Santa Rosalía (89 µg g–1), where Cu is mined. As, Se and Ag presented low variability in the study area, with low levels that reflect natural levels at the sites studied

Pesticides in coastal waters of the Gulf of California: Mussel watch program 1987-1988

A monitoring program was carried out in 1987 and 1988 using the mussel Modiolus capax as bioindicator to establish the contamination by organochlorine and organophosphate insecticides along the western coast of the Gulf of California. The results show that p,p′-DDE (5.78-105.3 ng g–1 dry weight) is the most bioavailable organochlorine toxic substance found in waters of the Gulf of California. The highest levels of this metabolite of the DDT insecticide were detected at the sites of Bahia de los Ángeles, San Rafael and San Francisquito, located in the central Gulf. The geographical distribution of p,p′-DDE was similar in 1987 and 1988. The results do not show a definite trend with regard to the temporal pattern of the contamination by pesticides. Heptachlorepoxide was detected in 18% and endrin in 8% of all the samples collected. Dieldrin, cis-chlordane, dacthal and endosulfan I were detected in less than 5% of the samples. The higher concentrations recorded in the central Gulf may originate in the agricultural valleys of Sonora and Sinaloa. The concentrations of pesticides detected in M. capax during the two years of the study were below the international limits established for the protection of aquatic biota

Heavy metals in coastal sediments of the Baja California (Mexico)-California (USA) border zone

Metal pollution and distribution in marine surface (0-5 cm) sediments of the Baja California border region were characterized during 1989. The total organic carbon, clay, silt and sand contents in the sediments and their correlation with the metals examined were also analysed. The results show higher concentrations (µg g–1 dry weight) of Al (7493), Mn (78.8) Cr (24.2) Zn (16.5) Cu (6.2) Cd (0.29) and Ag (0.30) in sediments of the 60-m isobath, with a gradient in their concentrations toward the shoreline. Analyses of simple correlation, principal components and similarity indicate that this behaviour is explained by the association of the elements with grain size and with silicoaluminate and organic matter contents. The possible influence of pollution and productivity phenomena as other processes that may control the distribution and origin of the metals in the study area, particularly Cd and Ag, is also noted. The results obtained show a decrease in Zn (15%), Cu (16%) Ag (20%) and Cd (22%) with respect to the values measured in coastal sediments of San Diego, California, in 1979

Geochemistry of Fe, Ti and Al as an indicator of volcanoclastic sedimentation in San Quintín coastal lagoon, Baja California, Mexico

The geochemistry of Fe, Ti and Al and the grain-size distribution were studied in 32 surficial sediment samples from San Quintín coastal lagoon (SQCL) in Baja California, in order to identify the presence of volcanoclastic sediment derived from the weathering and erosion of basaltic rocks from the San Quintín volcanic field (SQVF). The composition of the SQVF rocks is characterized by high Fe and Ti concentrations in comparison with the continental crust and Peninsular Ranges batholith. Similarly, the sediments from SQCL have significantly higher Fe concentrations (mean 3.73%, standard deviation [S] ±0.99) than other coastal sediments from Baja California and other regions. The high Fe concentrations normalized relative to the percentage of the <63-µm sediment fraction (Fenorm), indicate that the enrichment of Fe in some samples may in part be due to the presence of heavy minerals and/or rock particles, which were derived from the weathering and erosion of SQVF. The high concentrations of Ti (mean, 0.54%, S ±0.19) in the sediment are atypical for coastal and marine sediments. This enrichment as well as the high Ti/Al ratios at some sites within SQCL, especially Falsa Bay (FB), support the hypothesis of a volcanogenic influence. The sediment samples that have the highest proportion of amorphous Fe oxyhydroxides (≥50% of bulk Fe) were found in FB. This fact and the high Fenorm and Ti concentrations, as well as high Ti/Al ratios, suggest an association (at least in part) between the amorphous Fe oxyhydroxides and the volcanoclastic particles in some samples from FB; however, the high variability in geochemical characteristics of the sediment suggests a complex combination of hydrodinamic, mineralogical and diagenetic properties in SQCL

Mytilus californianus transplanted as upwelling bioindicators to two areas off Baja California, Mexico

 To study the spatial and temporal variability of bioavailable Cd, mussels (Mytilus californianus) were transplanted from a control zone to two upwelling areas off the northwestern coast of Baja California, Mexico. The mussels were placed at 5 and 15 m depth on buoy systems, at two sites selected for their different upwelling intensity. Punta Banda is characterized by intense upwelling events, with mean monthly temperatures of 13.00 ± 1.83ºC and abrupt drops in temperature to 9.7ºC, whereas upwelling off the Coronado Islands is less intense and mean monthly temperatures are 16.33 ± 2.15ºC, with abrupt drops in temperature to 11.0ºC. Mussel samples were collected regularly during upwelling (April to September) and non-upwelling periods (January to March). Mean Cd concentrations in soft tissue of transplanted mussels were higher at Punta Banda (17.23± 1.89 ∞g g|–1 dry weight) than at Coronado Islands (9.86 ± 1.37 ∞g g–1 dry weight). The analysis of correlation showed that the concentration of Cd in mussels from Punta Banda (r = 0.91) and Coronado Islands (r = 0.75) is related to the Bakun upwelling index. These results suggest that upwelling plays an important role in controlling Cd content in M. californianus from northwestern Baja California. Transplanted mussels proved to be excellent bioindicators of upwelling

Spatial distribution and enrichment of mercury in surface sediments off the northwest coast of Baja California, Mexico

An evaluation was made of the distribution and enrichment of Hg in surface sediments collected in August 1998 at 37 stations along the northwest coast of Baja California (NCBC). Mercury was analyzed by cold-vapour atomic absorption spectrophotometry. The percentages of mud ( < 63 µm) and organic matter were also measured. Mercury concentrations showed a homogeneous spatial distribution in the study area. The mean concentration of Hg (58.2 ng g–1) was similar to that found in the Southern California Bight. Mercury enrichment was observed at four stations located in the northern and central zones of the study area. In general, the Hg concentrations (30.4–96.5 ng g–1) were relatively low and pose no environmental concerns according to North American marine sediment quality guidelines. No correlations were found between Hg and mud or organic matter, suggesting that these variables do not influence the concentration and spatial distribution of this element in sediments from NCBC.

DDT in sediments from the northwest coast of Baja California (Mexico) and its biotransformation by Vibrio sp.

During December 2003, sediment samples were collected at 20 stations along the northwest coast of Baja California (Mexico). Analyses of DDT were performed by gas chromatography, percentage of particles <63 μm in diameter, organic carbon, and sulphate-reducing bacteria. A bacterium from station 49, which showed the highest concentration of DDT and its metabolites DDE and DDD, was isolated and identified biochemically and molecularly to belong to the genus Vibrio. In the laboratory this bacterium was tested in minimal salts medium plus DDT and incubated at 28ºC under aerobic and anaerobic conditions for 90 days. In general, the sediments showed low concentrations of DDT, ranging from undetected to 4.78 ng g–1, and the highest concentrations occurred at the deepest stations containing higher percentages of particles <63 μm and organic carbon concentrations. The isolated Vibrio strain showed an ability to biotransform DDT under aerobic and anaerobic conditions, presenting the o,p′-DDE and o,p′-DDD metabolites after 60 and 90 days of incubation in both conditions.