Actions and interactions of temperature, ph and photoperiod on mercury bioaccumulation by nymphs of the burrowing mayfly Hexagenia rigida, from the sediment contamination source

Abstract

AbstractBased on a three‐compartment system — water, natural sediment, Hexagenia rigida nymphs — an experimental study was set up, using a complete factorial design, to quantify the actions and interactions of three abiotic factors (temperature: 10, 18, and 26°C; photoperiod: 6, 12, and 18 h per day; pH: 5.0 and 7.5) on inorganic mercury (HgCl2) and methylmercury (CH3HgCl) bioaccumulation by Hexagema rigida (whole organism and gills). The two chemical forms of the metal were initially introduced into the sediment; the exposure duration was 15 d. Total Hg burdens measured at the whole‐organism level revealed a very high bioaccumulation capacity of this burrowing mayfly species and important differences between the two contamination conditions of the sediment source, a factor close to 20 observed in favor of methylmercury, for similar exposure conditions. Among the three abiotic factors taken into account, temperature and water‐column pH played an important role on Hg bioaccumulated by the nymphs, when considered in isolation and in interaction. An increase in temperature from 10 to 26°C gave rise to an increase in Hg bioaccumulation, with the higher differences close 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 accumulated by Hexagenia rigida. These effects were similar for the two Hg compounds, but they were more pronounced when the experimental units were contaminated by methylmercury. This comparative analysis of the amounts of metal bioaccumulated by whole organism and by the gills, estimates of nymph activity within the sediment, and results from earlier lab studies have generated several hypotheses on the involved mechanisms. We propose that ingested sediment is the predominant route of exposure and that the gut acts as a selective barrier that favors organic Hg absorption.