Controls on vertical nutrient distributions in the Sansha Yongle Blue Hole, South China Sea

Abstract

Blue holes are unique geomorphological units characterized by steep redox and biogeochemical gradients. The Yongle Blue Hole (YBH) is located on the largest atoll (Yongle Atoll) of the western Xisha Islands in the South China Sea (SCS). Although its depth was only just determined in July, 2016, the YBH has been known for centuries. The YBH is ca. 300 m in depth and has been recognized as the deepest known marine blue hole in the world. In this work, water column samples were collected from the YBH in March, 2017, and examined for dissolved inorganic nutrients, temperature, salinity, dissolved oxygen (DO), pH, and chlorophyll for the first time in order to better understand the nutrient cycling in this unique marine blue hole. The YBH water column is characterized by well-defined physical and chemical gradients with sharp transitions in salinity, temperature, density, DO and pH occurring at 80 m depth. With the disappearance of DO at 100 m depth, the hydrogeochemistry of the water column in the YBH dramatically changed from oxic to anoxic. Therefore, the YBH water column stratification existed mostly within the depth range of 80–100 m. Most physical and chemical parameters in the YBH remained relatively uniform below the depth of 160 m. Nutrient profiles in the water column varied distinctively in the YBH, with large shifts at the redoxclines. For example, surface waters ( 20 m) had nitrate concentrations that averaged 0.08 μmol/L, but increased to 8.59 μmol/L below 70 m and then decreased rapidly below 100 m. Nitrite were generally low with two peaks in the water column at 40 m (0.49 μmol/L) and 95 m (0.18 μmol/L). Changes in nitrate and nitrite were reflective of nitrification and denitrification within the 100 m depth of the YBH. Concentrations of ammonium was low within the upper 95 m, but increased to ca. 85 μmol/L below 160 m, where nitrate and DO decreased to near zero. The concentrations of phosphate and silicate were also very low within 70 m (0.04 and 1.21 μmol/L in average, respectively), increased rapidly below 70 m, and kept stable below 160 m at 4.9 and 152 μmol/L, respectively. Relatively constant distributions of ammonium, phosphate and silicate below 160 m was likely attributed to the stable conditions of low OM concentration, chlorophyll- a , temperature, salinity and pH. The N/P ratios were high in the surface layer (up to 300) and decreased with depth. The Si/N ratios were low in both surface and bottom layers, and the peak value of 15 appeared at 95 m. The Si/P ratios were high in the surface layer too, but also had peak value at 95 m (up to 70). All nutrient ratios (Si/N, Si/P, and N/P) were also stable below 160 m and were very close to the Redfield ratios. Vertical profiles of nutrients in the YBH were strongly linked to redoxclines and OM concentrations. In general, the YBH is an ideal place for examining the transport and transformation of biogenetic elements across steep redox gradients in the coastal margin.