A biological indication of a tintinnid species, Tintinnidium primitivum, of the Yellow Sea Bottom Cold Water

Abstract

Tintinnid ciliates, having distributional sensitivity to hydrological changes, have been applied as a useful tool to track the movements of water masses. The Yellow Sea Bottom Cold Water (YSBCW) is one of the most prominent summer oceanographic phenomena in the Yellow Sea (YS), a semi-closed marginal sea of the western Pacific. To examine the potential of tintinnid species as a biological indicator of the bottom cold water, the spatial distributions of the ciliates were investigated in the southeastern YS during the summers of 2010 and 2020. A dominant species, Tintinnidium primitivum, was found in the bottom cold waters in both years. The temperature and salinity (T-S) diagram for the dominant species distribution was plotted against the T-S range for the YSBCW at low temperatures between 6 ℃ and 12 ℃ and salinity between 32 and 33. Tintinnidium primitivum as a potential indicator species was seasonally monitored to trace vertical mixing along the 35°N line from 2019 to 2022. A bottom-oriented distribution was observed in every season except for winter. Winter vertical mixing was supported by water temperature and salinity profiles and the homogenous distribution of T. primitivum throughout the water column at Stns. 35-11 and 35-13. The absence of this species in response to the difference in the low-salinity surface water mass at Stn. 35-15 during winter indicated the invasion of shallow water mass from China. Using the sensitive indicator species, horizontal movements of the YSBCW from the 32°N to the 37°N were also monitored in the central trough of the YS. Distribution of the species showed southward movement in spring whereas its distribution extended toward the northern area of the 37°N in autumn. Therefore, the species sensitivity to hydrological changes can provide a valuable insight into the vertical and horizontal migration of the YSBCW. Detail distributional information on T. primitivum accumulated through further spatial and temporal investigations in the YS can contribute toward T. primitivum serving as an alternative tool when the water mass of the YSBCW cannot be clearly tracked by T-S measurement owing to water mixing and diffusion.