Responses of<i>Isonychia bicolor</i>to Alkaline pH: An Evaluation of Survival, Oxygen Consumption, and Chloride Cell Ultrastructure

Abstract

The toxicity of alkaline pH to the mayfly, Isonychia bicolor, was evaluated in field and laboratory artificial stream bioassays, respiration analyses, and transmission electron microscope studies of chloride cell ultrastructure. In continuous-exposure, 96-h bioassays, field and laboratory LC50 values ranged from pH 9.54 to 10.37, respectively. While intermittent pH bioassays involving 8- or 4-h exposures per day failed to reduce alkaline pH toxicity, 1-h intermittent exposures were less toxic (P &lt; 0.05) than continuous exposures. Acute (4-h) exposures to lethal (pH 11.0) and sublethal (pH 10.0) levels caused reductions (P &lt; 0.05) in oxygen consumption relative to that at pH 8.0. Oxygen consumption at pH 11.0 was greater (P &lt; 0.05) than that at pH 10.0. Coniform chloride cells continuously exposed to pH 11.0 for 96 h accumulated fluid near the porous plate, and these fluid accumulations displaced cytoplasm from the apical region of the cell complexes. Chloride cells intermittently exposed to pH 11.0 (1 h per day for 96 h) appeared similar to those continuously exposed to pH 11.0, indicating that the damaging effects of 1-h exposures exceeded the rehabilitory effects of daily, 23-h recovery periods. Structural recovery (95%) occurred within 8 d following the cessation of intermittent exposure to pH 11.0.