The effects of silicate mineral dissolution on seagrass functioning: Assessing olivine-based approaches to marine carbon dioxide removal

The active removal of carbon dioxide (CO2) from the atmosphere may be the only viable means of mitigating the worst effects of global warming. Known as carbon dioxide removal (CDR), these approaches involve a variety of techniques to sequester atmospheric CO2. One technique is marine enhanced rock weathering (mERW), which deploys natural silicate minerals (olivine) along the coastal zones, that dissolve, generate alkalinity and induce atmospheric CO2 drawdown upon air-sea equilibration. While the geochemistry of olivine dissolution is well known, the effects on foundation species are relatively unexplored. Here, we established a 16-week ecotoxicology experiment that exposed two tropical seagrasses (Thalassia testudinum and Syringodium filiforme) to one of three treatments: (1) surficial dose of fine-grained olivine; (2) surficial dose of clean aragonite sand; (3) control - no dose. During treatment exposure, seagrass structural and physiological characteristics were measured. Olivine or sand dosing had no effect on shoot density, leaf morphometrics, or photosynthetic yield. Leaf productivity, belowground biomass and carbon / nitrogen content were also unaffected. However, final aboveground biomass was lower in the olivine treatment relative to sand, but only for T. testudinum. Tissue trace metal concentrations (nickel and manganese) were elevated with olivine dosing for both species, while other metals showed mixed results. Neither olivine nor sand dosing influenced air-water CO2 flux. These results suggest that while olivine dosing had minimal effects on tropical seagrasses, elevated trace metals pose a concern, thus it may be necessary to regulate olivine dosing to avoid potential impacts on higher trophic levels.