Elemental Concentrations of Water and Otoliths as Salinity Proxies in a Northern Gulf of Mexico Estuary

Abstract

To use elements as salinity proxies, relationships of elemental concentrations with salinity must be known and vary from freshwater to marine endmembers. To extend these proxies to biogenic carbonates, elements must incorporate into carbonate matrices proportional to environmental concentrations. Therefore, this study quantified calcium (Ca), magnesium (Mg), strontium (Sr), barium (Ba), and manganese (Mn) in Mobile Bay, Alabama from freshwater to marine endmembers. Two smaller drainages (Fowl River and Dog River) were quantified to determine if all freshwater elemental concentrations were consistent. The ratio of elements (Mg, Sr, Ba, Mn) to Ca were also measured in water and Red Drum otolith edges across the salinity gradient. Water Ca, Sr, and Mg had positive relationships with salinity, but Ca and Sr were lower in Fowl River freshwater. Barium exhibited a mid-salinity peak and decreased with further salinity increases. More Ba and Mn were present during low river discharge; however, Mn did not vary with salinity. In both water and otoliths, Sr:Ca increased and Ba:Ca decreased non-linearly with salinity increases, Mn:Ca exhibited no relationship with salinity, and Mg:Ca increased non-linearly in water. These results demonstrate that Sr:Ca and Ba:Ca may be used as salinity proxies within the Mobile Bay estuary and likely other estuaries in the region. However, given non-linearity of element:Ca salinity relationships, variable concentrations among species and estuaries from other studies, and variation in elemental ratios between drainages and flow regimes, species-specific element salinity relationships should be known in the estuary of interest prior to utilizing elemental salinity proxies.