Confronting turbidity, the major challenge for satellite-derived coastal bathymetry

Monitoring the complex seafloor morphology that drives the functioning of shallow coastal ecosystems is vital for assessing marine activities. Satellite-derived bathymetry (SDB) can provide a crucial dataset for creating the bathymetry maps needed to understand hazards and impacts produced by climate change in vulnerable coastal zones. SDB is effective in clear water, but still has limitations in application to areas with some turbidity. Here, using the twin satellites Sentinel-2A/B, we integrate water quality information from the satellite with a multi-temporal compositing method to demonstrate a potential for comprehensively operational bathymetric mapping over a range of environments. The automated compositing method diminishes the turbidity impact in addition to inferring the maximum detectable depth and removing optically deep-water areas. Examining a wide range of conditions along the Caribbean and eastern coast of the U.S. shows detailed bathymetry as deep as 30 m at 10 m spatial resolution with median errors <1 m when compared to high-resolution lidar surveys. These results demonstrate that the model adopted can provide useful bathymetry in areas that do not have consistently clear water and can be extended across multiple geographic regions and optical conditions at local, regional, and national scales.

An order determination method in direct derivative absorption spectroscopy for correction of turbidity effects on COD measurements without baseline required

In chemical oxygen demand (COD) measurements based on ultraviolet-visible spectroscopy, turbidity interference represents a difficult technical problem. Common methods need the spectrum of real turbidity particles as the baseline, but it is inconvenient and vain to obtain because it changes with perturbation. Due to lack of theoretical guidance, the direct derivative method with no need of baseline was rarely used. Here in, we studied the absorption spectra of turbidity particles in series of samples and proposed an order determination method in the` direct derivative. In turbidity interference handling, the absorbance derivative of turbidity at each wavelength fell to zero by determining the appropriate derivative order. Two parameters were presented for order determination, which represented the overall size of particles and its scattering type based on the spectral shape and profile. Experiments showed that the formazine solution, as an extreme situation, had the largest parameters, so the derivative order was determined depending on it to ensure effective turbidity correctionin various real water samples. Through comparing the correction effects and COD prediction results of first-to-fourth order derivative spectra, the proposed method was verified to provide theoretical basis of direct derivative method and reduce blindness in the order determination.