LiDAR Remote Sensing for Vertical Distribution of Seawater Optical Properties and Chlorophyll-a From the East China Sea to the South China Sea

The traditional way to detect the vertical structure of seawater optical properties and chlorophyll-a is mainly through shipboard discrete observations or Biogeochemical-Argo profiling floats, which requires considerable time to cover a limited area. In this study, the vertical distribution of seawater optical properties and chlorophyll-a concentration across two different optically-contrasted sea areas from the East China Sea (ESC) to the South China Sea (SCS) were obtained for the first time using a shipboard integrated Mie-Raman-fluorescence lidar for large-scale observations, with a total observation distance of over 3700 km. More than 74,000 lidar profiles were obtained from September 5 to September 15, 2020. In general, the lidar-estimated inherent optical properties (IOPs) and chlorophyll-a values decreased from turbid water in the ECS to clear water in the SCS. Subsurface scattering layers were often observed at depths ranging from 10 to 20 m along the SCS coast. Subsequently, the lidar-derived results were compared against in situ measurements. In addition, the diurnal hourly variation in IOPs and chlorophyll-a by lidar at a fixed coastal station was monitored for the first time, which was relatively lower in the early morning and midday yet was higher in the evening, while the relative tide height showed the reverse change trend, which revealed that the tide possibly impacted the diurnal variation in IOPs and chlorophyll-a on the SCS coast. Overall, our results indicate that the lidar remote sensing technique is effective and feasible to monitor large-scale and long-term subsurface phytoplankton structure over different optically-contrasted sea regions, and integration of multiple detection mechanisms will enhance the monitoring capacity.