Sr Isotope Ratios (87Sr/86Sr) in Water and Fish Otoliths as Estuarine Salinity Tracers: Case Studies from Three NW African Rivers
2022
Höpker, Sebastian N. | Wu, Henry C. | Lucassen, Friedrich | Sadio, Oumar | Brochier, Timothée | Nuworkpor, Ishmael Y. | Kasemann, Simone A. | Merschel, Peter | Westphal, Hildegard
Variations of strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) in river systems are increasingly utilised to geochemically trace origin and movement patterns of migratory fish species. Accretionary calcified structures, such as otoliths, preserve ⁸⁷Sr/⁸⁶Sr signatures of the surrounding water during a fish’s lifetime. In this study, we present ⁸⁷Sr/⁸⁶Sr measurements of water samples and catfish otoliths collected in the estuaries of the Sine-Saloum (Senegal), the Gambia River (The Gambia), and the Volta River (Ghana) to assess their systematics and relationships with salinity. The three rivers possess distinct hydrological properties resulting in variable degrees of correlations between ⁸⁷Sr/⁸⁶Sr and salinity. The Gambia River (⁸⁷Sr/⁸⁶Sr of ~ 0.71209) proved exceptionally preconditioned for the approach due to well-defined geochemical end-members, allowing for quantitative estimates of salinity based on otolith ⁸⁷Sr/⁸⁶Sr measurements. The Volta River (~ 0.71392) presents a more complex case due the possible influence of multiple water sources to the main channel, while the inverse salinity gradient and excessive evaporation in the Sine-Saloum estuary (~0.70915) impede any significant correlations between ⁸⁷Sr/⁸⁶Sr and salinity. Bulk otolith ⁸⁷Sr/⁸⁶Sr values in the Gambia River and Volta River clearly depicted a mixed influence of seawater and riverine compositions, strongly encouraging the application of this approach for geochemical fingerprinting of critical NW African species.
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