Evaluating otolith Sr/Ca as a tool for reconstructing estuarine habitat use

There is no standard method to determine the applicability of otolith Sr/Ca ratio to reconstructing estuary use. We have developed a novel method to determine the response of otolith Sr/Ca to changes in water Sr/Ca and salinity in San Francisco Estuary ( California, USA). We perform correlated, spatially resolved Sr/Ca and Sr isotope measurements using otoliths from adult striped bass ( Morone saxatilis) in the San Francisco Estuary to estimate the otolith-water Sr/Ca partition coefficient (D(Sr) = 0.305 +/- 0.009). DSr did not vary significantly with salinity, and therefore the salinity-otolith Sr/Ca model was constructed by substituting the partition coefficient into the nonlinear salinity-water Sr/Ca mixing model for the system. The model demonstrates that the primary factor controlling the response of Sr/Ca to salinity is the Ca concentration in the freshwater source flowing into the estuary. A concentration of 60 ppm Ca is an approximate threshold below which estuary Sr/Ca increases rapidly to near the marine Sr/Ca at low salinities (5 parts per thousand-15 parts per thousand), thereby providing sharp delineation of estuary entrance, but little to no discrimination among higher salinity habitats. Our approach provides a general framework for assessing the potential utility of Sr/Ca in estuarine systems and specifically for the San Francisco Estuary.