Internal Waves Induced the Increase of Nutrients and Chlorophyll-a Concentrations in the Dongsha Atoll Continental Shelf

English Abstract:

Many studies have already reported that the internal waves were generated frequently in the Luzon Strait and propagated westward to reach or break down on the Dongsha Atoll continental shelf. We have conducted several cruises to measure hydrological (T, S, D) and biogeochemical (nutrients, Chl-a) conditions from two transects covering the slope, shelf break and shelf zones of Dongsha Atoll in the northern South China Sea (SCS). We found that the surface water off Dongsha Atoll was generally depleted with nutrients and its DIN/DIP ratio was quite low and similar to the typical ratio of surface water in the northern SCS. However, the concentrations of nutrients, Chl-a and particulate organic carbon (POC) were enhanced after internal waves (IW) induced upwelling and lifted cold and nutrient rich water to the surface. Meanwhile, the DIN/DIP ratio in the euphotic zone was significantly elevated and approached to that in the aphotic zone after a distinct event of internal waves. Such features can be seen in both eastern transect (M1-M6) and northeastern transect (NM1-NM4) particularly on the shallow zones near the Atoll. The effects of a down-welling (W2 Station) event on biogeochemical distributions can be evaluated through a short time-series observation of T, DO and nutrients at 50 m during the passage of IW. A 30-hour sequential observation shows that the upwelled IW can transport nutrients into surface layer and down-welled IW can transport DOC and POC into subsurface layer and affect carbon budget significantly. In addition, the moored station (IW Station) shows simultaneous changes of T and Chl-a at 40, 100 and 150m, indicating a significant effect of IW on T and phytoplankton biomass. The POC derived from the enhanced nutrients in surface did not likely accumulate on shelf sediments (<50 m), which may imply a transport of Chl-a and POC toward north-western regions. Preliminary data also show that the area was likely a sink of atmospheric CO₂ along the eastern transect (M1-M6).