We describe an Underwater Hyperspectral Imager (UHI) deployed on an instrument-carrying platform consisting of two interconnected mini-ROVs (Remotely Operated Vehicle) for the mapping and monitoring of Arctic macroalgal habitats in Kongsfjorden (Svalbard) during the Polar Night. The mini-ROV-UHI system is easy to transport, assemble and deploy from shore, even under the dark, icy and cold conditions of the Arctic Polar Night. The system can be operated by two persons, keeping the operational costs low. In vivo hyperspectral reflectance of collected specimens of brown, red and green macroalgae was measured with a spectrometer in the lab to provide a spectral library for supervised pigment group classification based on UHI photomosaics. The in situ UHI-photomosaics provided detailed information of the areal coverage of the seafloor substrate (16%), as well as brown (51% habitat cover), red (18%), and green (14%) macroalgae, with spatial resolution in the range of cm and spectral resolution of 2 nm. The collected specimens from the mapped area were also used for species identification and health state evaluation. This innovative UHI sampling method provides significant information about macroalgal distribution and physiology, and due to its flexibility in terms of deployment, it is applicable to a variety of environments.

Arctic macroalgae species have developed different growth strategies to survive extreme seasonal changes in irradiance in polar regions. We compared photophysiological parameters such as the light saturation parameter (E_k) and pigment composition of green, red, and brown macroalgae collected in January (Polar Night) and October 2020 (end of the light season). Macroalgae in January appeared healthier (morphologically) and had longer lamina (new growth) than those in October. E_K values for red, and brown algae were higher with lower maximum quantum yield of PS II fluorescence (F_v/F_m) in January versus October. Furthermore, in January, new tissues in kelp species had higher E_K than the older tissue. Higher E_K and lower F_v/F_m during the Polar Night indicates that the photosynthetic apparatus is active but slow. Furthermore, we discuss Chlorophyll (Chl) a emission spectra under blue and green excitation light to determine the ratio of Chl a in photosystem II (PS II) vs photosystem I (PS I). Absorbance spectra of P. palmata was used to interpret the emission spectra. The observed spectral shifts in the absorbance and reflectance spectra of different macroalgae is discussed. Photophysiological methods provide health information complementary to future mapping and monitoring of macroalgae. These results reveal that macroalgae grow new tissue in darkness.

In situ spectral reflectance initially captured at high spatial resolution with underwater hyperspectral imaging (UHI) is effective for classification and quantification in oceanic biogeochemical studies; however, the measured spectral radiance is rarely used as an absolute quantity due to challenges in calibration of UHI instruments. In this paper, a commercial UHI instrument was calibrated for radiometric flat field response and pixelwise immersion effect to support in situ measurement of absolute spectral radiance. The radiometric and immersion factor calibrations of the UHI instrument were evaluated quantitatively through comparative experiments with a spectroradiometer and a spectrometer. Results show that the immersion factor of the center pixel of the tested UHI instrument was 1.763 in pure water at 600 nm, and the averaged difference in immersion factor between the center and edge pixel of the UHI instrument in the visible light band was only 1∼3% across its half angle field of view of 35° in air. The new calibration coefficients were further used to calculate the spectral radiance of transmitted sunlight through ice algae clusters in sea ice measured by the UHI instrument during an Arctic under-ice bio-optical survey.

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.

The Eurasian Basin Transect 2026 cruise (EBT2026; ID 2026007011) was conducted 23 July–10 September 2026. Its geographical coverage included the Nansen and Amundsen Basins, the adjacent Lomonosov Ridge and Southeastern Makarov Basin, including the full width of the Transpolar Drift current. Throughout the cruise, a comprehensive set of physical, chemical, and biological observations was collected from multiple components of the Arctic system, including the atmosphere, ocean, sea ice, and sea floor.