Multiparameter cluster analysis of seasonal variation of water masses in the eastern Beibu Gulf

Using the hierarchical clustering method, we present comprehensive analyses of water masses and their seasonal variations based on substantial in situ observations in the eastern Beibu Gulf, a shallow sea with a depth mostly less than 100 m. Besides traditional physical parameters (salinity and temperature), chemical and biological parameters are also used for the water mass classification and analyses, which include dissolved oxygen, alkalinity, nitrite, reactive silicate, and chlorophyll a. Consequently, the eastern Beibu Gulf water is classified into five water masses in the study area, namely the diluted water, the mixed water, the surface water, the subsurface water, and the bottom water. The diluted water is located in the area along the Guangxi coast; the mixed water mainly occupies the northern study area; the surface water resides in the central and southern study area; the subsurface water exists just near the Gulf mouth; and the bottom water appears in the lower layer east of Bailongwei Island, which is different from the Beibu Gulf cold water mentioned in previous studies. The parameter properties and spatial distribution of most water masses in the eastern Beibu Gulf exhibit strong seasonal variability which is mainly modulated by circulations and solar radiation.