南海三沙永乐龙洞水深约300 m, 是世界上已知最深的海洋蓝洞, 100 m以下水体属于无氧环境, 使其成为研究从有氧到无氧的转变及无氧环境下生源要素迁移转化过程的理想场所. 2017年3月在永乐龙洞开展了现场调查和样品采集, 对溶解态无机营养盐进行了分析, 并结合温度、盐度、溶解氧(DO)等环境参数, 讨论了营养盐浓度与结构的垂直分布特征及影响因素, 初步探讨了营养盐循环过程. 结果表明, 不同的营养盐在蓝洞内有迥异的变化规律, 最大转变发生在氧化还原跃层. 表层营养盐浓度均较低, 但随着深度的增加, 各营养盐浓度体现不同的峰值分布. 例如, 硝酸盐浓度峰值(8.59 mol/L)出现在90 m深处, 而亚硝酸盐在40和95 m处出现双峰分布(分别为0.49和0.18 mol/L). 铵氮浓度在95 m之后迅速升高, 磷酸盐和硅酸盐浓度则从70 m开始升高, 150 m后其浓度均不再增加, 分别稳定在85, 4.9和152 mol/L左右. 营养盐浓度的变化直接影响了其结构的分布, N/P比与Si/N比和Si/P比呈现相反的变化趋势. N/P比在表层较高(接近300), 整体随深度而降低; Si/N比在表层和底层都较低, 在95 m出现15的峰值; Si/P比也是表层较高, 但在95 m也出现达70的峰值. 在160 m以下, 各营养盐比例均保持稳定, 并接近Redfield比值. 永乐龙洞营养盐垂直分布的变化表明其循环过程与DO、有机物和微生物等之间存在密切联系.
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.