Saturation indices of aqueous mineral phases as proxies of seasonal dynamics of a transitional water ecosystem using a geochemical modeling approach

The geochemistry and saturation indices (SI) of carbonate and non-carbonate mineral phases under standard thermodynamic conditions in an aqueous environment are largely influenced by the dynamic physico-chemical parameters. Based on the known feedback mechanisms of environmental parameters on the rates of mineralisation, the changes in environmental parameters such as pH, dissolved ionic concentrations and salinity were modelled as a function of their spatial and temporal distributions (dry and wet seasons) in a transitional aquatic ecosystem in South India, Pulicat lagoon. The investigation revealed the strong seasonal influence of these parameters on the magnitude and values of the SIs signalling dissolution or mineralisation. Hydroxyapatite and Talc were found to have higher positive SIs. Co-precipitation of calcite and aragonite were evident and found to be constrained by high salinities. Among the carbonates, dolomites and magnesium-calcites showed high SIs compared to calcite and aragonite, indicating that magnesium ions were incorporated into the mineral phases possibly due to the availability of nucleation sites through heterotrophic reactions. The formation of apatite was strongly influenced by pH changes, corresponding to low calcite precipitation in the wet season. High SI of hydroxyapatite, talc and silicate minerals indicate the low bioavailability of dissolved phosphate and silicates in the water during both the seasons. Strong evidences of seasonal control of pH, salinity and Total Dissolved Solids on the dissolution and mineralisation characteristics further influence nutrient bioavailability over varied spatio-temporal scales. The present study supports the potential application of the saturation indices as proxies to understand the complex biogeochemical dynamics of transitional water ecosystems.