Hydrographic features of the Yongle blue hole in the South China Sea and their influential factors

Abstract

A study of the hydrographic features of the Yongle blue hole (YLBH) in the South China Sea has been conducted using an extensive set of hydrological data collected in October, 2016. Observations include water column measurements of temperature, salinity, density, chlorophyll-a (Chl-a), dissolved oxygen (DO), turbidity, grain size of suspended particulate matter (SPM) and tidal currents. The variability of tidal currents indicates that the YLBH has no large-scale connection with the adjacent ocean. Four thermoclines were observed in the YLBH at depths of 3, 10, 50 and 80-110 m. The water column was most stratified at 50 m depth and was homogeneous below 155 m depth. A subsurface Chl-a maximum was present at 10-20 m depth, and an additional peak of Chl-a coincided with the deepest thermocline at 90 m depth. A high concentration of DO was present at the surface with values up to 7 mg/L, and concentrations decreased sharply at the thermoclines becoming 0 mg/L below approximately 90 m. Turbidity seems to vary with Chl-a, namely high values at depths of 10-20 m and approximately 90 m. The SPM in the YLBH had two major components in terms of grain size (5.28-38.55 and 145-500 μm). The volume concentration of the coarser component was 2-3 orders of magnitude higher than the finer component. A θ-S diagram indicates that the water properties below 80 m were distinct from the upper water column. The thermoclines above 80 m exhibit diurnal and seasonal variability, suggesting that the vertical circulation could reach as deep as a permanent thermocline at 80 m depth, that restricts water exchange between the upper water column and water below 80 m. In addition, the supply of DO from the atmosphere was inhibited by the permanent thermocline, which contributes to the formation of anoxic water below approximately 90 m. Concentration of the finer component of SPM dominated the turbidity signal in the YLBH, suggesting that the finer component was mainly composed of sediment, and the coarser component might be dominated by algae and marine snow.