Assessing Arctic under sea-ice light regimes and ice algal bio-optical properties using ROV-based hyperspectral imaging

Abstract We used a Remotely Operated Vehicle (ROV) system in the northern Barents Sea, equipped with an upward-facing underwater hyperspectral imager (UHI) to map ice algae patchiness and biodiversity through measurements of transmitted downwelling radiance L d (λ) per image pixel. Each image pixel contained L d (λ) spectra with 2 nm spectral resolution. The integrated L d [photosynthetically active radiation (PAR), from 400 to 700 nm] under the ice varied by a factor of 5 from 1270 to 6330 mW m −2 sr −1 (6–30 µmol photons m −2 s −1 sr −1 ) due to changing ice thickness, snow cover and other related factors. Furthermore, the L d (PAR) transmitted through ice with an algae biofilm represented the photosynthetically usable radiance (PUR), denoted L PUR , corresponding to the total absorption of photons by ice algae (denoted, AQ total ) in addition to the light attenuation by sea ice. From the L d (λ), we created a relative photosynthetic biomass index of chlorophyll a (Chl a) concentration varying from 0 to 1. Subsequently, we calculated the L d (λ) difference spectra (ice pixel spectra–algae pixel spectra) which reflected the absorbance spectra of the algal biofilm dominated by diatoms and resembled the absorption spectra of Nitzschia frigida . In addition, we determined light-harvesting pigments (Chl a, Chl c and fucoxanthin) as well as photoprotective carotenoids diato-diadinoxanthin through pigment analysis, in combination with pulse-shape analysis using imaging flow cytometry to characterise the ice algae community, confirming a diatom-dominated bloom. These ice algae were acclimated to high-light conditions, determined by a light saturation parameter, E K , of 37 µmol photons m −2 s −1 and indicated by the detection of diadinoxanthin. Combining the laboratory with the UHI data highlighted the potential for discriminating algal classes based on different absorption spectral peaks from UHI transmittance data.