An invasive bivalve with the potential to reconstruct chronologies of geomarkers in a large South American basin.

The Sr/Ca and 87 Sr/86 Sr ratios are used as natural indicators (geomarkers) in fish to reveal migratory patterns, due to significant relationships between these ratios measured in the water and in the fish calcified structures (otoliths, bones). The aim of this study was to assess the potential use of the Limnoperna fortunei shell as a proxy for monitoring the spatial and temporal variability of Sr/Ca and 87 Sr/86 Sr in water. These ratios were compared in water samples and bivalve shells proceeding from eight sites of four hydrogeological regions of the La Plata Basin (Argentina and Uruguay), collected in two hydrological periods (winter and summer) in order to depict the pattern of geographical and temporal variability and to evaluate the relationship between both matrices. Significant positive linear relationships were found between shell edge and water for 87 Sr/86 Sr (R2 = 0.96-0.97, p < 0.005) and Sr/Ca (R2 = 0.83-0.86, p < 0.05) in summer and winter. Slopes did not differ significantly from 1 (p > 0.05) for 87 Sr/86 Sr, while they differed from 1 for Sr/Ca in both seasons (p < 0.05), indicating that the elemental ratio would be influenced by endogenous/exogenous factors. No significant seasonal differences were found for both natural tags (p > 0.05), except for the lower Uruguay River (87 Sr/86 Sr) and outer estuary (Sr/Ca). These results indicate that the L. fortunei shell could be a valuable tool for monitoring spatio-temporal variations in water 87 Sr/86 Sr. Shell Sr/Ca shows high potential utility as a geomarker in freshwater, but additional assessments are needed to understand the impact of temperature and growth on this ratio.