Calcium attenuates strontium toxicity and bioaccumulation in three freshwater invertebrates

Strontium (Sr) is increasingly detected in freshwater ecosystems influenced by mining and oil and gas development. Despite this, toxicity data remains limited for supporting the development of protective water-quality guidelines. Risk evaluation for Sr is further complicated by the shared uptake pathways of strontium and calcium (Ca), whereby Ca has been shown to modify Sr toxicity in a variety of freshwater organisms. In this study, we quantified the influence of Ca concentration ([Ca]) on Sr toxicity and bioaccumulation in three aquatic invertebrates (Lumbriculus variegatus, Chironomus dilutus, and Hyalella azteca) using 4-day acute bioassays conducted at [Ca] of 4, 20, and 80 mg/L. Toxicokinetic modeling for L. variegatus demonstrated a 4.7-fold decrease in Sr uptake rate (kᵤ) across the Ca gradient, while elimination rate constants (kₑ) remained unchanged, indicating that Ca primarily mitigates Sr toxicity by suppressing uptake rather than enhancing elimination. Median lethal concentrations (LC50s) increased three- to nine-fold from low- to high-Ca waters, with H. azteca exhibiting the greatest intrinsic sensitivity and the weakest Ca-mediated protection. Bioaccumulation responses (internal residues) in L. variegatus and C. dilutus followed similar Ca-dependent trends. Supplemental experiments showed that the counter-anion (Cl- versus SO42-) for added Sr did not influence toxicity. However, long-term pre-exposure to low-Ca conditions reduced Sr sensitivity in L. variegatus, indicating acclimation effects. The [Sr] and [Ca] evaluated span from natural freshwater conditions to highly mineralized systems, demonstrating that Sr toxicity is substantially elevated in low-Ca environments and supporting the consideration of ambient Ca in Sr guideline development and ecological risk assessments.