The effect of hexavalent chromium on the critical thermal maximum and body burden ofClioperla clio (Plecoptera:Perlodidae)

Abstract

Clioperla clio (Plecoptera:Perlodidae) nymphs exposed to eight levels of hexavalent chromium experienced 0.03% (control) to 76.0% lethality after 96 hr at 9.0°C and had an LC50 of 101.3 mg/L (95% C.I.=88.9 – 118.1 mg/L). Nymphs exposed to sublethal levels of Cr⁺⁶ in EPA hardwater at 9.0°C had statistically significant reduction in upper temperature tolerance measured as critical thermal maximum (CTM_CONTROL >$CTM_{LC_{10} }$ >$CTM_{LC_{30} } = CTM_{LC_{50} }$, range 31.46–29.49°C). The “ecological death” endpoint criterion resulted in low variability for controls (C.V.=2.4%). Exposed nymphs digested with nitric acid had mean body burdens ranging from 1.37 (μg total Cr/g dry weight) for those exposed to 5.25 mg/L, to 26.71 for the 143.25 mg/L exposure group. As Cr⁺⁶ exposure increased, body burden of nymphs increased linearly, and CTM significantly decreased. Since the 96-hr LC50 greatly exceeds Cr⁺⁶ measured in the field and the critical thermal maxima are at least 10°C greater than the adult emergence temperature, it is unlikely that environmentally realistic combinations of Cr⁺⁶ and temperatures will have adverse impacts on mature nymphs ofClioperla clio in nature if adverse conditions are of short duration.