A novel method for tracing coastal water masses using Sr/Ca ratios and salinity in Nanwan Bay, southern Taiwan

A new method using two tracers, namely, the high-precision Sr/Ca ratio and salinity, has been successfully applied to quantitative determination of mixing proportions of freshwater, intruding surface seawater and upwelled water in a semi-enclosed bay, Nanwan Bay, southern Taiwan, where strong upwelling is induced by tidal forcing. The Sr/Ca ratio of the coastal water is essentially determined by the mixing ratio of the offshore surface water and the upwelled water, which usually have different Sr/Ca ratios. On the other hand, variability of coastal water salinity is strongly influenced by fresh water input. A Sr/Ca-salinity (Sr/Ca-S) diagram can thus be used to decipher the endmember contributions. During the 1994 dry season it was found that the Nanwan water was composed mainly of two components: 75.0% upwelled water from depths of 100e200 m offshore, and 25.0% offshore surface water. In the wet season an additional 2.0e2.5% fresh water, primarily from rainfall and surface runoff, was added to the bay. Validity of this model is confirmed by the agreement between the observed d18O value of the bay water and that calculated from model-derived mixing proportions using the endmember compositions. Once the mixing proportions were determined, the freshwater input rate provided a scaling factor for estimating the turnover rate of the bay water. The case study has thus demonstrated that the Sr/Ca-S relationship may serve as a novel tool for distinguishing different water masses and assessing the upwelling strength in a coastal environment.