ACTIONS AND INTERACTIONS OF TEMPERATURE, PH AND PHOTOPERIOD ON MERCURY BIOACCUMULATION BY NYMPHS OF THE BURROWING MAYFLY HEXAGENIA-RIGIDA, FROM THE SEDIMENT CONTAMINATION SOURCE

Based on a three-compartment system - water, natural sediment, Hexagen ia rigida nymphs - an experimental study was set up, using a complete factorial design, to quantify the actions and interactions of three ab iotic factors (temperature: 10, 18, and 26-degrees-C; photoperiod: 6, 12, and 18 h per day; pH: 5.0 and 7.5) on inorganic mercury (HgCl2) an d methylmercury (CH3HgCl) bioaccumulation by Hexagenia rigida (whole o rganism and gills). The two chemical forms of the metal were initially introduced into the sediment; the exposure duration was 15 d. Total H g burdens measured at the whole-organism level revealed a very high bi oaccumulation capacity of this burrowing mayfly species and important differences between the two contamination conditions of the sediment s ource, a factor close to 20 observed in favor of methylmercury, for si milar exposure conditions. Among the three abiotic factors taken into account, temperature and water-column pH played an important role on H g bioaccumulated by the nymphs, when considered in isolation and in in teraction. An increase in temperature from 10 to 26-degrees-C gave ris e to an increase in Hg bioaccumulation, with the higher differences cl ose to a factor of 1.7. On the other hand, acidification of the water column from 7.5 to 5.0 led to a decrease in the amounts of the metal a ccumulated by Hexagenia rig-ida. These effects were similar for the tw o Hg compounds, but they were more pronounced when the experimental un its were contaminated by methylmercury. This comparative analysis of t he amounts of metal bioaccumulated by whole organism and by the gills, estimates of nymph activity within the sediment, and results from ear lier lab studies have generated several hypotheses on the involved mec hanisms. We propose that ingested sediment is the predominant route of exposure and that the gut acts as a selective barrier that favors org anic Hg absorption.