Response of three aquatic sentinel species to metal trace contamination in two rivers in the Abitibi region, Québec
Metallothionein (MT), an intracellular metal binding protein, is recognized for its role in the regulation of essential and non-essential trace metals. The mechanisms of synthesis of this protein have been studied in the laboratory but much less work has been carried out in the field, particularly in lotic environments. According to the literature, metallothionein protects tissues from metal damage, and may be used as a direct measure of the biochemical state of the affected tissue. Metallothionein shows high affinity for group IB and IIB metals ions (e.g. Cd, Cu, Zn), and consequently can sequester and detoxify metals. The use of MT as a biomarker for elevated levels of trace metals in aquatic environments has therefore been proposed as a tool in ecotoxicological studies. Changes at the biochemical level offer distinct advantages as biomarkers, since molecular alterations are normally the first detectable responses to environmental changes. The present study focused on two objectives, the first of which was to identify which trace metals (silver "Ag", cadmium "Cd", copper "Cu", nickel "Ni", lead "Pb" and zinc "Zn") may induce MT synthesis in three sentinel species collected in two northern Quebec rivers (Colombière River, close to Val d'Or, and Matagami River close to the town of Matagami) in a mining region (Abitibi, Québec). The three sentinel species were the adult bivalve Pyganodon grandis, the larval mayfly Hexagenia limbata, and the adult great northern pike Esox lucius. The second objective was to define variables that could influence the uptake of these trace metals and consequently the concentrations of MT in the sentinel species. The selection of these sentinel species was based on the following criteria: their relative abundance in the aquatic systems studied; their relative immobility; the ease of sampling; metal tolerance; metal bioaccumulation capacity; dynamics of metal accumulation; capacity to synthesize metallothionein; and availability of physiological and behavioural data. In 2001, physico-chemical conditions (e.g.: temperature, oxygen, transparency, phosphorus, chlorophyll a, etc.) were monitored during two sampling campaigns at 21 stations. Trace metals were measured in water and sediment in all stations, and in tissues of the sentinel species that were collected when present at the station (P. grandis - gills; H. limbata - whole body; E. lucius - kidney). Dialysis cells were used to collect water samples for the determination of concentrations of total dissolved metals, major anions and cations, humic and fulvic acids, and dissolved organic carbon. At each sampling site, divers collected three replicate sub-samples from two sediment cores for metal analyses. The cores were extruded in the boat and samples taken from the uppermost 0.5 cm, i.e. from the oxidized layer. The two rivers differ in their morphometry, their physico-chemistry, and their biology. The Allard River is larger (200 m vs. 7 m) and deeper (30 m vs. 3 m) than the Colombière River. The Colombière River is slightly more acid (pH: 6.2 - 6.7) and less productive (14 - 32 μg total P L-1) than the Allard River (pH: 6.7 - 7.4; 24 - 45 μg total P L-1). However, the rivers present some similarities such as slow current conditions and limited water transparencies. The range of Ag, Cu and Ni concentrations in water and sediment was greater in the Colombière River, whereas Pb concentrations were higher in the Allard River and Cd concentrations were similar in the two rivers. Zinc was the dominant metal in both rivers, but variations were more significant in water in the Colombière River. Although trace metal and MT concentrations were highly variable among species, stations and rivers, no consistent spatial gradients were detected in animals collected downstream from mining activities. As was the case for the ambient environment, Zn was also the dominant metal in organisms of both rivers. Zinc concentrations were similar in mayflies from the two rivers, but higher in molluscs and lower in pike collected from the Colombière River than in comparable animals from the Allard River. Following Zn, Cu was the second metal in importance in species from the two rivers, whereas the concentrations of Ag were low in all organisms and were undetectable in pike. Cadmium was more concentrated in mayflies and molluscs from the Colombière River than in those from Allard River, and also higher in the pike collected in the contaminated zone of the Colombière River. The concentrations of Ni in all species were comparable between rivers, but were more elevated in mayflies than in the other sentinel species. The range of Pb concentrations was comparable between rivers for all species, but concentrations were lower in the pike (kidney). The concentration of MT was higher in the pike than in the two other species. Mayflies had higher MT levels in the Allard River, while MT concentrations were higher in molluscs of the Colombière River. The relationships between metals and MT were very variable between rivers and species. Nevertheless, MT concentrations were most strongly related to those of accumulated Cd in the three sentinel species. The relationships between MT and Cd for all three species were stronger in the Allard River than in the Colombière River. In the case of P. grandis, because the number of stations where the bivalves could be collected from the Colombière River was low (n=4), no statistically significant relationship was observed between MT and Cd in the tissues, in spite of the high correlation coefficient (r=0.98). For the bivalves, manganese (Mn) concentrations in the sediment were negatively correlated with accumulated Cd concentrations and with MT levels. In H. limbata, in addition to Cd, other metals such as Cu (-) and Zn (+) also exhibited weak relationships with MT. Finally, in bivariate analyses, Pb concentrations in pike liver were significantly correlated with those of MT in this species, but based on multiple regression models, no variable other than Cd affected the level of MT in E. lucius.