State Key Laboratory of Marine Resources Utilization in the South China Sea, School of Ecology and Environment, School of Marine Science and Engineering, Hainan University, Haikou, China
2.
Collaborative Innovation Center of Marine Science and Technology, Hainan University, China
Funds:
The Major Science and Technology Plan of Hainan Province under contract No. ZDKJ2021008; the Hainan Provincial Natural Science Foundation of China under contract No. 623RC456; the Hainan Province Science and Technology Special Fund under contract Nos ZDYF2021SHFZ064 and ZDYF2022SHFZ056; the Collaborative Innovation Center of Marine Science and Technology in Hainan University under contract No. XTCX2022HYC19.
The exchange of inorganic nutrients at the coastal sediment-water interface (SWI) plays a crucial role in regulating the nutrient budget in overlying water. The related studies mainly focus on the mid to high-latitude regions, leaving a significant gap in the quantitative assessment of nutrient exchange and environmental controls at the SWI in low-latitude coastal regions. We quantitatively assess the exchange of inorganic nutrients at the SWI in three tropical bays (Dongzhai harbor, Xiaohai lagoon, Qinglan harbor). Sediments act as a source of ammonium, phosphate, and silicate, but for nitrate, sediments can be both a source and sink, although with substantial spatial and temporal variations in their fluxes. Labile organic matter is a critical regulator for the fluxes of inorganic nutrients at the SWI. The sedimentary nutrients input with high N/P ratio will alter the nutrient stoichiometry to mitigate the nitrogen limitation in coastal waters. However, the internal sediment release in these tropical bays plays a relative weak role in contributing to the nutrient addition in comparison with the other external nutrient sources including riverine input, submarine groundwater discharge, and atmospheric deposition. According to the global compilation on SWI nutrient fluxes, we propose that water column primary production and external inputs to interpret the variation in exchange and fluxes of nutrients at the SWI in different ecosystems. Such a conceptual understanding of these chain biogeochemical processes involving external nutrient input, primary production, particulate organic matter settling, and the accumulation and release of inorganic nutrients in sediments will be helpful for the scientific-based pollution prevent and control in coastal waters.
Figure 1. Map of sampling stations at Dongzhai harbor (DZG) in winter and summer, Xiaohai (XH) and Qinglan harbor (QLG) in summer
Figure 2. The spatial distribution of envrionmental parameters in the overlying water and sediments during the four curises. Subfigures (a–i) denote the water depth, temperature, salinity, dissolved Oxygen (DO), porosity, TOC, TN, C/N ratio and Chl a, respectively. The solid and dashed lines represent the median and average of values, respectively.
Figure 3. Spatiotemporal distribution of nutrient concentration in bottom water. Subfigures a−b, c−d, e−h, and i−l represent the cruises in winter DZG, summer DZG, XH, and QLG, respectively. Subfigures from the first to fourth column represent the ${\mathrm{NO}}_x^- $-N、${\mathrm{NH}}_4^+ $-N、DIP、DSi concentration, respectively, unit in μmol L−1. ND represents No data.
Figure 4. Spatiotemporal distribution of nutrient concentration in porewater. Subfigures a−d, e−h, i−l, and m-p represent the cruises in winter DZG, summer DZG, XH, and QLG, respectively. Subfigures from the first to fourth column represent the $ {\mathrm{NO}}_x^-$-N、$ {\mathrm{NH}}_4^+ $-N、DIP、DSi concentration, respectively, unit in μmol L–1.
Figure 5. Spatial distribution of nutrient diffusion fluxes in sediment-water interface, unit: μmol m−2 day−1 (a-b and c-d represent the cruises of Dongzhai Habor in winter and summer, respectively, e-h represent the cruise of Xiaohai Lagoon, i-j represent the cruise of Qinglan Habor in summer, are ${\mathrm{NO}}_x^- $-N、${\mathrm{NH}}_4^+ $-N、DIP、DSi, respectively)
Figure 6. Pearson’s correlation analysis between nutrient flux at the sediment-water interface and the detected environmental parameters
Figure 7. The conceptual diagram of the effects of external inputs and primary production on nutrient exchange at the SWI in (a) shallow water and (b) deep water ecosystems.
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