Estimating Sea Ice Algal Biomass in Svalbard Fjords Using Underwater Hyperspectral Imaging: Integrating Laboratory and Field Calibration Approaches

Abstract

Sea-ice algae are an integral part of the polar marine ecosystems, but they remain under-sampled due to the limitations of invasive sampling techniques, the large spatial heterogeneity of their biomass and logistical challenges conducting research in remote regions. This study provides an assessment of an underwater hyperspectral imager (UHI) on a simple L-arm as a tool to better monitor sea ice algal community biomass based on data sets from in situ sampling of two Svalbard fjords as well as laboratory cultures of ice algae. We use normalized difference indexes (NDIs) to quantify chlorophyll a (chl a) from UHI surveys, using both standardized radiance and transmittance for comparison. Our results demonstrate that while both approaches work in a laboratory setting, the transmittance-based NDIs capture the in situ patchiness of chl a more effectively, revealing differences of up to 4 mg/m2 at fine scales (2 cm) that are unachievable with traditional sampling methods. We developed a new approach for NDI calibration, where a mixed species sample from natural sea ice was cultivated to determine an optimal NDI which then was applied to in situ UHI-surveys, reducing the need for extensive invasive ice coring. We also show the importance of calibrating NDIs to case-specific algal species, since even a minor shift in composition can reduce NDI effectiveness by nearly 50%. This study adds to the potential of using UHIs paired with NDIs for small-scale characterization and quantification of sea-ice algal chl a, while showcasing a new and simpler approach for calibration of bio-optical models.