Salinity has considerable effects on the toxicity of metals in estuarine waters. The effects of salinity are manifold, making it difficult to summarize for risk assessments. In this study, we separated and quantified the multiple effects of salinity on cadmium (Cd) in a toxicokinetic–toxicodynamic framework. The estuarine clam, Potamocorbula laevis, was used as a model organism. Cd bioaccumulation was measured using a stable-isotope-tracer technique; in parallel, toxicity tests were conducted. With the increase of salinity from 5 to 30, Cd uptake decreased monotonically. In contrast, the intrinsic sensitivity of organisms, measured by the toxicodynamic parameters, reached its minimum at intermediate salinities (i.e., 10 to 20). The overall salinity effects were dominated by the effects on Cd bioaccumulation; therefore, Cd toxicity decreased monotonically with the increases of salinity. The model developed in this study could provide predictions of no-effect concentration (1.7 to 34.9 μg L–1, end point mortality) and the median lethal concentration (LC50) of Cd at different salinities. In conclusion, we developed a framework for quantifying the multiple effects of salinity and a method for estimating no-effect concentration from acute toxicity tests, which can be used for better assessments of metal risks in estuarine waters.

In estuaries, salinity fluctuates rapidly and continuously, greatly affecting the bioavailability and thus toxicity of contaminants, especially metals, causing difficulties in deriving site-specific water quality criteria. We developed a method for predicting the toxicity of the metal cadmium (Cd) in estuarine waters of any salinity fluctuation scenario. Cd bioaccumulation and toxicity were measured in an estuarine clam Potamocorbula laevis under stable salinities (salinity = 5, 15, 25) and fluctuating salinities (5–25), using the toxicokinetic–toxicodynamic (TK–TD) framework. Cd bioaccumulation decreases with increasing salinity; whereas intrinsic Cd sensitivity of organisms reaches the minimum at an intermediate salinity around 20. At each specific Cd level, interpolating TK–TD parameters measured at the stable salinities well predicts the Cd bioaccumulation and toxicity under fluctuating salinities. To extend the model for various Cd levels, the biotic ligand model (BLM) was integrated into the TK–TD framework. The BLM-based TK–TD model was successfully applied to scenarios of simulated and monitored salinity fluctuations in estuarine waters, for which the median lethal concentrations and no-effect concentrations (2.0–3.1 μg L–1) of Cd were derived. Overall, we integrated the BLM and TK–TD models and provided a useful tool for predicting metal risks and deriving criteria values for salinity-fluctuating estuarine waters.