Remote sensing reflectance (Rrs) of the surface water during 4 expeditions from spring to fall in 2019, Mackenzie Delta Region (CA)

Vertical profiles of downwelling irradiance (Ed) and upwelling radiance (Lu) were measured during legs 2, 3, and 4 using a Compact-Optical Profiling System (C-OPS) in an ICE-Pro frame from Biospherical Instruments, Inc. (for a detailed description see Morrow et al. 2010 (see further details)). Additionally, above-surface incident downward irradiance (Es(0+)) was measured at about two meters above sea level and was used to correct in-water Ed and Lu for changes in the incident light field during Lu profiling (Zibordi et al., 2019; doi:10.25607/OBP-691). All radiometric quantities were measured at 19 wavelengths spanning from 380 to 875 nm. In-water profiles were obtained from the boat using a 3 m long pole, deployed towards the sun to avoid shading from the boat. The data that were acquired with a tilt of more than 5 degrees were discarded (Hooker et al., 2013; doi:10.5194/bg-10-4511-2013). Due to the high absorption and scattering coefficients in the sampled waters and considering relatively large dimensions of the ICE-Pro, self-shading correction was not negligible. Absorption observed in the present study were mostly outside the limits examined by Gordon and Ding (1992; doi:10.4319/lo.1992.37.3.0491), suggesting the application of the correction questionable. To overcome this issue, we performed Monte-Carlo simulations using the SimulO software (Leymarie et al., 2010; doi:10.1364/AO.49.005415) for examining the self-shading correction factor on Lu at null depth (Gerbi et al., 2016 (doi:10.1175/JTECH-D-16-0067.1); Leymarie et al., 2018 (doi:10.3389/fmars.2018.00437)). The exact dimensions of the ICE-Pro were simulated and virtually placed at a depth of 0.5 m. A wide range of IOPs was considered to cover the conditions encountered in the field. The simulations provide a robust relationship between the computed self-shading and the quantity x = a + bb, where a is the total measured absorption coefficient (i.e., the contributions of pure water, CDOM, algal and non-algal particles) and bb is the total backscattering coefficient (i.e., the contributions of water molecules and particles). The shade-corrected upwelling radiance (Lu corrected) can be expressed as a function of the measured radiance (Lu measured) as: (1) L_u(corrected)=(L_u(measured))/((1-ε)), (2) ε=1-e^(-0.14(a+bb)), where (2) is fitted for solar zenith angles > 45° and bb (which was not measured in the field) was calculated using an empirical relationship from the Malina-cruise dataset (Doxaran et al., 2012 (doi:10.5194/bg-9-3213-2012); Massicotte et al., 2020 (doi:10.5194/essd-13-1561-2021)). Subsurface downward irradiance and upward radiance Ed(0-) and Lu(0-) were estimated with an iterative linear fitting of the log-transformed Ed(z) and Lu(z) vs depth z. Fitting was applied to successively greater depths until the correlation coefficient (r2) exceeded 0.99 or until the layer thickness reached 2.5 m (Bélanger et al., 2017; doi:10.1175/JTECH-D-16-0176.1). Remote Sensing Reflectance (Rrs) was calculated following Mobley (1999; doi:10.1364/AO.38.007442) with: Rrs(λ)=(0.54L_u (0^-,λ))/(E_s(0^+,λ)). To calculate the Rrs we used the R "Cops" package (https://github.com/belasi01/Cops) (Bélanger 2017: doi:10.1175/JTECH-D-16-0176.1).