Determining the effects of major cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) and pH on Scirpus mariqueter to Assess the Heavy Metal Biotoxicity of a Tidal Flat Ecosystem

The biotic ligand model (BLM) as a mechanistic bioavailability model has been used to assess the risk of metals on various ecosystems. In this study, the effects of major cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) and pH on root elongation of the salt marsh plant Scirpus mariqueter were investigated by laboratory experiments, and then a tidal flat sediment-BLM was developed successfully to assess heavy metal biotoxicity on a tidal flat ecosystem for the first time. The results showed that the increase of Na⁺ activities could decrease Cu²⁺ toxicity to S. mariqueter. A nonlinear relationship was observed between the median effective concentration (EC50) and H⁺ activity over the pH range (5.55-8.22). The results indicated that H⁺ did not compete with Cu²⁺ for the binding sites within the roots of S. mariqueter under pH < 7.0 (5.55-6.62), while Cu²⁺ and CuCO3(aq), as toxic species, competed for binding sites of the biotic ligands (BL) with Na⁺ under pH > 7.0 (7.03-8.22). The following conditional binding constants for the binding of Cu²⁺, CuCO3(aq), and Na⁺ to biotic ligands were obtained: log KCuBL = 6.60, = 6.20 and log KNaBL = 1.778. The developed Cu-BLM for S. mariqueter in this study was validated so that it could provide accurate predictions of Cu toxicity. This developed model could provide a scientific and reasonable basis for an environmental quality standard to assess risk in an estuarine tidal flat wetland.