Accumulation of total mercury and methyl mercury in insect larvae of hydroelectric reservoirs 1 Alain Tremblay and Marc Lucotte Abstract: Mean total mercury (Hg) and methyl mercury (MeHg) concentrations in insect larvae from hydrolelectric reservoirs were 3–5 times (up to 10 times) higher than in their counterparts from natural lakes. Taxonomic differences, which may be related to feeding behavior, and substrate type (lake sediment or flooded soils) together explained a large part of the total variability of total Hg (r 2 = 0.64) and MeHg (r 2 = 0.79) concentrations in insects. MeHg concentrations in flooded soils increased from 0.3–2 ng Hg⋅g dry weight –1 after 1–2 years of flooding to 15–2 ng Hg⋅g dry weight –1 after 16 years of impoundment. Total Hg and MeHg concentrations in insect larvae followed a similar pattern, but only weak correlations of both total Hg and MeHg concentrations were found between flooded soils and insect larvae, suggesting that factors other than MeHg content of flooded soils also affect the accumulation of these compounds in insects. Our results indicate that suspended particulate matter eroded from flooded soils by wave and ice action and bacterial activity enhanced by the release of labile carbon and nutrients from the flooded soils may indirectly transfer MeHg from flooded soils to insect larvae. Résumé : Les concentrations moyennes de Hg total et de MeHg de larves d’insectes aquatiques de réservoirs hydroélectriques sont de 3 à 5 fois (jusqu’à 10 fois) plus élevées que celles de lacs naturels. Les différences taxonomiques, qui peuvent être reliées aux modes d’alimentation des insectes et le type de substrat sur lequel ils vivent (sédiments lacustres ou sols inondés), expliquent une forte proportion de la variabilité totale des concentrations de Hg total (r 2 = 0,64) et de MeHg (r 2 = 0,79) des insectes. Nos résultats démontrent une augmentation de l’étendue des teneurs en MeHg des sols inondés passant de 0,3–2 ng Hg⋅g poids sec –1 après 1–2 ans de mise en eau à environ 15–25 ng Hg⋅g poids sec –1 16 ans après leur mise en eau. L’accumulation de Hg total et de MeHg dans les insectes suit une tendance similaire mais de faibles corrélations ont été obtenues entre la concentration de ces composés dans les sols inondés et dans les insectes et suggèrent que des facteurs autres que la teneur dans les sols affectent l’accumulation de ces composés dans les insectes. Nos résultats indiquent que l’érosion et la mise en suspension des sols inondés par l’action conjugée des vagues et de la glace, ainsi que l’augmentation de la production microbienne favorisée par le relargage d’éléments nutritifs des sols inondés, pourraient transférer indirectement le MeHg des sols inondés aux larves d’insectes. Introduction Numerous researchers working in remote temperate regions have documented elevated concentrations of mercury (Hg) in fish of hydroelectric reservoirs (Potter et al. 1975; Cox et al. 1979; Meister et al. 1979; Bodaly et al. 1984). Because Hg from both natural and anthropogenic sources is known to be widely dispersed via atmospheric transport and then accumu- lated in the humic horizon of soils following wet and dry depo- sition (e.g., Grondin et al. 1995; Lucotte et al. 1995), the release of Hg from flooded soils to the water column may be responsible for the high Hg levels observed in reservoir fish in the first few years after flooding. This is supported by the positive relationship between the rate of Hg accumulation in fish and the proportion of flooded land in reservoirs reported by Bodaly et al. (1984) and Hecky et al. (1991). At present, the hypothesis most often cited by researchers to explain the accumulation of Hg in fish of reservoirs is that with an increase in bacterial activity in the flooded soils and the resulting enhanced methylation of inorganic Hg, MeHg is transferred to the water column and is made available for aquatic organisms by passive diffusion (Hecky et al. 1991; Morrison and Thérien 1991) and (or) by the erosion and sus- pension of flooded soil particles (Louchouarn et al. 1993; Mucci et al. 1995). Because aquatic insect larvae are closely associated with the sediment, in many cases feeding on it, and represent a major proportion of the diet of many fish species (Doyon et al. 1996b), they may constitute an important pathway for the transfer of sedimentary Hg to the food web. Furthermore, in a recent study of 11 natural Quebec lakes, Tremblay et al. (1996b) reported a positive correlation between the methyl mercury (MeHg) concentrations in sediments and those in in- sect larvae. Therefore, we propose that insects may have a strong influence on the release of biologically important quan- tities of both total Hg and MeHg from flooded soils and would then transfer these contaminants along the food chain. The aim of this study is to understand the bioaccumulation of both total Hg and MeHg concentrations in insect larvae of hydroelectric reservoirs and natural lakes of northern Quebec. Can. J. Fish. Aquat. Sci. 54: 832–841 (1997) Received February 14, 1996. Accepted September 27, 1996. J13300 A. Tremblay 2 and M. Lucotte. Chaire de recherche en environnement, Université du Québec à Montréal, C.P. 8888, succursale Centre-Ville, Montréal, QC H3C 3P8, Canada. 1 Contribution of the Chaire de recherche en environnement (Hydro-Québec – Natural Sciences and Engineering Research Council of Canada – Université du Québec à Montréal). 2 Author to whom all correspondence should be addressed. e-mail: tremblaa@envir.hydro.qc.ca 832 © 1997 NRC Canada