Arsenic (V) bioconcentration kinetics in freshwater macroinvertebrates and periphyton is influenced by pH

Arsenic is an important environmental pollutant whose speciation and mobility in freshwater food webs is complex. Few studies have characterized uptake and efflux rates of arsenic in aquatic benthic invertebrates. Further, we lack a fundamental understanding of how pH influences uptake kinetics in these organisms or how this key environmental variable could alter dietary exposure for primary consumers. Here we used a radiotracer approach to characterize arsenate accumulation dynamics in benthic invertebrates, the influence of pH on uptake in a subset of these organisms, and the influence of pH on uptake of arsenate by periphyton - an important food source at the base of aquatic food webs. Uptake rate constants (K_u) from aqueous exposure were modest, ranging from ∼0.001 L g^-1d^-1 in three species of mayfly to 0.06 L g^-1d^-1in Psephenus herricki. Efflux rate constants ranged from ∼0.03 d^-1 in Corbicula fluminea to ∼0.3 d^-1 in the mayfly Isonychia sp, and were generally high. Arsenate uptake decreased with increasing pH, which may be a function of increased adsorption at lower pHs. A similar but much stronger correlation was observed for periphyton where K_u decreased from ∼3.0 L g^-1d^-1 at 6.5 pH to ∼0.7 L g^-1d^-1 at 8.5 pH, suggesting that site specific pH could significantly alter arsenic exposure, particularly for primary consumers. Together, these findings shed light on the complexity of arsenic bioavailability and help explain observed differences reported in the literature.