Can the biotic ligand model predict cu toxicity across a range of pHs in softwater-acclimated rainbow trout?

This study examined the effects of pH (5.0 - 8.5) on the toxicity of waterborne Cu to juvenile rainbow trout (Oncorhynchus mykiss) in soft water under flow-through conditions. Relationships between 96 h or 30 day Cu toxicity and 24 h lethal Cu accumulation on the gills (24 h LA_50-Acute or 24 h LA_50-Chronic) were examined in the context of predictions made using the biotic ligand model (BLM). Acute toxicity was relatively constant across pHs except for a 2- to 3-fold higher LC₅₀ at pH 5.0. In the chronic exposure, the fish had similar tolerance to Cu from pH 5.0 to 8.0, but were 3- to 4-fold more tolerant at pH 8.5. This pattern was not captured by the current BLM which predicts that acute and chronic LC₅₀ values should increase progressively from pH 5.0 to 8.5, with much greater values than those observed at the higher pH range. BLM-based water quality criteria would not be protective for trout at pH 8.0 or 8.5 in acute exposure to Cu in soft water. The measured 24hLA_50-Acute and LA_50-chronic at pH 8.5 were higher, and 24 h LA_50-Chronic at pH 5.0 was lower than those at the other pHs. This study indicates that gill Cu bioaccumulation does not explain toxicity at high or low pH, and the BLM needs revision to adequately predict Cu toxicity to trout in soft water.