Sensitivity of the glochidia (larvae) of freshwater mussels to copper: Assessing the effect of water hardness and dissolved organic carbon on the sensitivity of endangered species

The assessment of the potential impact of waterborne contaminants on imperilled freshwater mussels is needed. Acute copper toxicity was assessed in a standardized soft water (hardness 40–48 mg CaCO3 equivalents L−1) using the larvae (glochidia) from three common and six (Canadian) endangered mussel species. The resulting 24 h EC50s ranged from 7 to 36 μg Cu L−1, with the EC50s of two endangered species <10 μg Cu L−1. Acute copper sensitivity was also determined in Ptychobranchus fasciolaris, a species that employs conglutinates (packets of glochidia) in its reproductive strategy. Conglutinates were found to provide significant protection from acute copper exposure as the EC50 of the encased glochidia was more than four-fold higher than freed glochidia (72.6 μg Cu L−1 vs. 16.3 μg Cu L−1). The glochidia from two endangered species, Epioblasma triquetra and Lampsilis fasciola, were used to examine the influence of water hardness and dissolved organic carbon (DOC) on copper sensitivity. Exposures in moderately-hard water (165 mg CaCO3 L−1) demonstrated that an increase in water hardness resulted in a significant reduction in copper sensitivity. For example, in L. fasciola the 24 h EC50s were 17.6 (14.2–22.6) μg Cu L−1 and 50.4 (43.5–60.0) μg Cu L−1 in soft water and moderately-hard water, respectively. The addition of DOC (as Aldrich Humic Acid) also resulted in a significant decrease in Cu sensitivity, such that a 10-fold increase in the EC50 of E. triquetra was observed when the reconstituted soft water was augmented with 1.6 mg DOC L−1. To determine if current water quality regulations for copper would protect all glochidia, the USEPA's Biotic Ligand Model (BLM) was used to derive water quality criteria for these exposures. While BLM-derived criteria for the soft water exposures indicate that protection would be marginal for the sensitive endangered species, the criteria derived for the DOC exposures suggest that the natural complexity of most natural waters in Southern Ontario (Canada) will provide glochidia with protection from acute copper exposure.