Seasonal Variations of Water Mass Mixing off Southwest Coast of Taiwan

Detailed hydrographic surveys for water columns off Pingtung coastal zone, southwestern Taiwan, were preformed using stable oxygen isotope compositions, salinity and temperature data to determine the source and mixing of water masses during summer and winter periods. Results show that horizontal oxygen isotope distributions clearly display the extent of river plume derived from high freshwater input during the summer period. Vertical seawater oxygen isotope values exhibit systematic variations with depth from surface to bottom. Generally, oxygen isotope data of summer coastal waters are relatively depleted to those of the same depths in winter. Except the top surface layer in summer, coastal waters off the Pingtung have a temperature-salinity relationship between the Kuroshio and South China Sea. North Pacific Intermediate and Deep Waters can also be identified by oxygen isotope and salinity. Two layers of relatively depleted oxygen isotope values are found along the Kaoping Canyon in depths ranging from 400~600m and 1200m, respectively, in the summer of 2001. However, only the lower one still remained the depleted oxygen isotope nature in the winter season. Those light oxygen isotope signals of coastal waters in both summer and winter periods provide encouraging evidences of submarine groundwater discharge from the aquifers of the Pingtung Plain and deserve a further study.