Concentrations and ratios of Sr, Ba and Ca along an estuarine river to the Gulf of Mexico – implication for sea level rise effects on trace metal distribution

Abstract. Strontium and barium to calcium ratios are often used as proxies for tracking animal movement across salinity gradients. As sea level rise continues, many estuarine rivers in the world face saltwater intrusion, which may cause changes in mobility and distribution of these metals upstream. Despite intensive research on metal adsorption and desorption in marine systems, knowledge of the spatiotemporal distribution of these elements along estuarine rivers is still limited. In this study, we conducted an intensive monitoring of Sr and Ba dynamics along an 88 km long estuary, the Calcasieu River in South Louisiana, USA, which has been strongly affected by saltwater intrusion. Over the period from May 2013 to August 2015, we collected monthly water samples and performed in-situ water quality measurements at six sites from the upstream to the river mouth, with a salinity range from 0.02 to 29.50 ppt. Water samples were analyzed for Sr, Ba, and Ca concentrations. In-situ measurements were made on salinity, pH, water temperature, dissolved oxygen concentration, and specific conductance. We found that the Sr and Ca concentrations and the Sr / Ca ratio all increased significantly with increasing salinity. The average Sr concentration at the site closest to the Gulf of Mexico (site 6) was 46.21 μmol L⁻¹, which was about 130 times higher than that of the site furthest upstream (site 1, 0.35 μmol L⁻¹). The average Ca concentration at site 6 was 8.19 mmol L⁻¹, which was about 60 times higher than that of site 1 (0.13 mmol L⁻¹). The average Sr / Ca ratio at site 6 (8.41 mmol mol⁻¹) was about 3 times the average Sr / Ca ratio at site 1 (2.89 mmol mol⁻¹). However, the spatial variation in Ba concentration was marginal, varying from 0.36 μmol L⁻¹ at site 6 to 0.47 at site 5. The average Ba / Ca ratio at site 1 (4.82 mmol mol⁻¹) was about 54 times the average Ba / Ca ratio at site 6 (0.09 mmol mol⁻¹), showing a clear negative relation between the Ba / Ca ratio and increasing salinity. All the elemental concentrations and ratios had considerable seasonal variations, with significant differences among sampling months for the Sr, Ba concentrations and the Ba / Ca ratio (p < 0.01). The results from this study suggest that concentrations of Sr and Ca in the world's estuaries will very likely increase in the future as sea level rise continues. For low-gradient estuarine rivers such as the Calcasieu River in South Louisiana, USA, water chemistry upstream would experience substantial Sr and Ca enrichment, which could affect aquatic environments and biological communities.