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Since many estuarine zones are impacted by copper contamination, there is an on-going effort to develop Biotic Ligand Models (BLMs) predicting copper toxicity in transitional environments. In the first stage of this project, a critical analysis of the BLM framework identified some aspects of the model that required further investigation. In particular, a BLM for estuaries needed (a) a better characterization of the dissolved organic matter (DOC) and its effect on copper availability, and (b) the inclusion in the model’s equation of a salinity-correction factor modulating the relationship between copper accumulation on the biotic ligand and toxicity. The first issue was addressed by modelling the data produced using a Chelex resin method to determine the labile fraction of copper in samples of mixed riverine and estuarine waters. A refined and simplified BLM equation was then presented, accounting for both the DOC characteristics and the relevance of the osmotic gradient in modulating the relationship between copper accumulation and toxicity. A critical analysis of the literature on copper toxicity and salinity led to the hypothesis that copper-exposed fish are more sensitive to osmotic stresses, as copper interferes with their osmoregulatory pathways. In particular, the cytosolic isoform-2 of the enzyme carbonic anhydrase (CA2) was identified as an osmotic effector protein targeted by copper and involved in osmotic stress response pathways, hence representing a mechanistic link between the combined effects of copper exposure and osmotic stress. To test this hypothesis, two in vivo studies were performed, using the euryhaline fish sheepshead minnow (Cyprinodon variegatus) and applying different rates of salinity changes as a way of dosing osmotic stress. The results showed a disturbance in plasma ion homeostasis after the salinity transitions, but notably the magnitude of the disturbance was greater in the copper-exposed individuals, suggesting a sensitizing effect of copper on the responses of fish to osmotic stress. Gene expression data demonstrated that CA2 is targeted by copper and confirmed the role of the enzyme in osmoregulatory pathways, as further supported by a promoter analysis of the gene coding for zebrafish CA2, which revealed the presence of osmotic-stress related elements. Overall, these results suggest that CA2 is an osmotic effector protein whose response can be activated by a medium level of osmotic stress through a combination of transcriptional and post-translational control circuits.