Biogeochemical characterization of a Mediterranean shallow lake using stable isotopes: Laguna del Cristo (NW Iberian Peninsula)

The present multi-isotopic study (δ¹⁸O–δD_water, δ³⁴S–δ¹⁸O_{dissolved-sulphate}, δ¹³C_{dissolved-inorganic-carbon}, δ¹³C–δ¹⁸O_{shells-modern-gastropods}, δ¹³C_plants, and δ¹³C_{sedimentary-organic-matter}) is aimed at assessing the hydrogeochemical changes and biogeochemical dynamics in a Mediterranean shallow lake fed by a Quaternary–Tertiary aquifer, the “Laguna del Cristo” (NW Iberian Peninsula), a system sensitive to climate fluctuations, between 2010 and 2011. Lake water is of the bicarbonate type, and there are no major pollutants. δ¹⁸O-δD_water values plot on a local evaporation line (δD=5.29δ¹⁸O–12.29) indicating that evaporative enrichment had a significant impact on lake water isotopic features. Periods of high water levels are characterized by lower δ³⁴S–δ¹⁸O_{dissolved-sulphate} and δ¹³C_{dissolved-inorganic-carbon} values and suggest sulphate derived from weathering of sulphides in the catchment area, delivered to the lake by surface run-off or via groundwater, and in situ decay of organic matter. During lower water levels, sulphate reduction and enhanced primary productivity lead to higher δ³⁴S_{dissolved-sulphate} and δ¹³C_{dissolved-inorganic-carbon} values. Evaporation induced enrichment in ¹⁸O_sulphate, ¹³C_{dissolved-inorganic-carbon} and ¹³C–¹⁸O_{shells-Galba-Gyraulus}. δ¹³C_plant confirms the C3 photosynthetic pathway. Enrichment in ¹³C submerged aquatic plants indicates that HCO₃^? is the main carbon source, except for ¹³C-depleted Potamogeton. The TOC, δ¹³C_org values, and TC/TN ratios in sediments all confirm the autochthonous character of organic matter contribution. This study provides a baseline for isotopic research into shallow, flow-through lakes fed by siliciclastic aquifers, and stresses the importance of evaporation and refilling (direct precipitation and groundwater discharge) in controlling the solute chemistry and stable isotopic composition in temperate regions with contrasting seasonal climates. The results also provide a snapshot of modern lake isotope variability that can be applied to paleoenvironmental reconstructions.