In hydrological optics, "optical closure" means consistency between the apparent optical properties (AOPs) determined from radiometric measurements and those derived from radiative transfer modelling based on concurrently measured Inherent optical properties (IOPs) and boundary conditions (sea and sky states). Good optical closure not only provides confidence in the data quality but also informs on the adequacy of the radiative transfer parameterization. Achieving optical closure in highly absorptive coastal waters is challenging due to the low signal-to-noise ratio of radiometric measurements and uncertainties in the measurements of IOPs, namely the spectral absorption and backscattering coefficients. Here, we present an optical closure assessment using a comprehensive set of in situ IOPs acquired in highly absorptive coastal waters optically dominated by chromophoric dissolved organic matter (CDOM). The spectral remote sensing reflectance, Rrs(λ), was modeled using the software HydroLight (HL) with measured IOPs and observed boundary conditions. Corresponding in-water Rrs(λ) was derived from radiometric measurements made with a Compact Optical Profiling System (C-OPS; Biospherical). The assessment revealed that the inclusion of inelastic scattering processes in the model, specifically Sun-induced CDOM Fluorescence (fDOM) and Sun-Induced Chlorophyll Fluorescence (SICF) from Chlorophyll-a ([chl]), significantly improved the optical closure and led to good agreement between measured and modeled Rrs (i.e., for 440 ≤ λ ≤ 710 nm with no inelastic processes: R2=0.90, slope=0.64; with inelastic processes: R2=0.96, slope=0.90). The analysis also indicated that f_DOM and SICF contributed a substantial fraction of the green-red wavelength Rrs in these waters. Specifically, fDOM contributed ~18% of the modeled Rrs in the green region and SICF accounted for ~20% of the modeled Rrs in the red region. Overall, this study points out the importance of accounting for fDOM in remote sensing applications in CDOM-dominated waters.

Combinaison of optic and acoustic remote sensing to retrieve benthic reflectance for shallow water ecosystem cartography

The retreating sea ice in the coastal Arctic Ocean significantly impacts the nearshore ecosystems. The increasing sediment load in the coastal waters due to increased riverine discharge and waves-driven re-suspension has resulted in increased turbidity in many arctic coastal zones. However, cryospheric, geomorphologic, hydrologic and ocean exposure settings (i.e., the coastscapes) induce varying turbidity levels across the pan-Arctic coastal zone. The change in water turbidity reduces the availability of photosynthetically active radiation (PAR) and, consequently, the primary production of benthic vegetation. Here we present a regional analysis of PAR and turbidity of the Arctic coastscapes at the pan-Arctic scale. We have employed satellite ocean colour data to compute the suspended particulate matter (SPM) and PAR reaching the bottom (PAR(zb)) for eight contrasting coastscapes, varying from shallow estuarine coasts to deep nearshore waters adjacent to rocky cliffs. The trends of SPM and PAR(zb) for the MODIS time series (2003-2020) were calculated for each region. Variability in SPM was correlated to PAR(zb) for different sites to evaluate the effect of turbidity on the benthic PAR. We found that increasing sediment load affect the PAR(zb) observed on the coastal seafloor. However, in some coastscapes, other factors, such as absorption by coloured dissolved organic matter (CDOM) or phytoplankton, play a more significant role in determining the magnitude of PAR passing through the water column. Environmental drivers of SPM, CDOM and phytoplankton variability will be further explored.