In search of light: Estimating diffuse attenuation coefficient of downwelling irradiance and its variation in optically complex shallow water habitats using Sentinel-2 imagery

Frequent resuspension of sediments in the water column hampers photosynthesis in submerged aquatic vegetation due to decreased light, reduces gas exchange from sediment deposition and affects their anchoring ability. Remote sensing of light attenuation of the water column in form of diffuse attenuation coefficient of downwelling photosynthetically available radiation (K_dPAR) provides a spatial perspective to study light attenuation of water column. This study aims to use Sentinel-2 derived K_dPAR to characterize light variation and test its significance in relation to occurrence of eelgrass (Zostera marina) at Horsens and Roskilde fjords located in coastal waters of Denmark. Areas with maximum of K_dPAR (mean > 75^th percentile) coinciding with maximum and minimum variation (standard deviation >75^th and <25^th percentile respectively) were identified at the study sites. The standard deviation (SD) of K_dPAR and fraction of surface irradiance reaching bottom (PAR_Z) was used to predict occurrences of eelgrass using Logistic regression (LR) and Support Vector Machine (SVM). The LR and SVM (linear kernel) could predict eelgrass presence-absence with 65 % overall accuracy and precision of 63 % and 62 % respectively on the test set (n = 238). SD correlation with eelgrss occurrences (Point-biserial correlation = -0.11, p = 0.002) and with predicted probabilities of logistic regression (Pearson's r = -0.46) indicates increased K_dPAR variation reduces chances of eelgrass. The K_dPAR for this study was derived using Quasi Analytical Algorithm Version-5 (QAA-V5) and Case-2 Regional Coast Colour (C2RCC) and validated against in-situ matched observations (n = 113) for duration of March to October of 2016-2018.