TEMPERATURE AND PH EFFECTS ON CADMIUM AND METHYLMERCURY BIOACCUMULATION BY NYMPHS OF THE BURROWING MAYFLY HEXAGENIA-RIGIDA, FROM WATER COLUMN OR SEDIMENT SOURCE

An experimental approach, based on a complete experimental design, was set up in order to carry out a comparative analysis of cadmium (Cd) a nd methylmercury (MeHg) bioaccumulation in the burrowing mayfly nymphs of Hexagenia rigida (Ephemeridae) after 15 days' exposure to water co lumn or sediment compartment as initial contamination sources. Combina tions of two modalities of temperature-15 and 25 degrees C- and pH-5.0 and 7.5-enabled us to quantify the actions of these two abiotic facto rs and also their interactions on the metal bioaccumulation. Whatever the initial contamination source, a high level of metal bioaccumulatio n was observed after exposure to MeHg; Cd transfers, on the other hand , were very low. For similar theoretical exposure conditions, differen ces between the bioaccumulation capacities of the two metals were betw een 20 and 30, in favor of MeHg. Multiple regression did not reveal si gnificant interactions between MeHg and Cd towards their bioaccumulati on in the nymphs. When the microcosms were contaminated via the sedime nt source, increasing the temperature from 15 to 25 degrees C led to a n increase in MeHg bioaccumulation and a decrease of Cd bioaccumulatio n. After exposure from the water source, no significant amounts of Cd were measured in the nymphs at 25 degrees C even though significant me tal concentrations were observed at 15 degrees C. pH had no significan t effect on the bioaccumulation processes when the two metals were ini tially added to the sediment compartment; acidification of the overlyi ng water, however, gave rise to a decrease in MeHg and Cd concentratio ns in the nymphs, with marked interactions with temperature. The resul ts are discussed from the data available on metal partitioning in the biotopes and their bioavailability, uptake routes and also the structu ral and functional properties of the biological barriers involved (gil ls, gut wall).