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

Abstract

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 100–200 m offshore, and 25.0% offshore surface water. In the wet season an additional 2.0–2.5% 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 δ 18O 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.