Antarctic pack ice serves as habitat for microalgae which contribute to Southern Ocean primary production and serve as important food source for pelagic herbivores. Ice algal biomass is highly patchy and remains severely under-sampled by classical methods such as spatially restricted ice coring surveys. Here we provide an unprecedented view of ice algal biomass distribution, mapped (as chlorophyll a) in a 100 m by 100 m area of a Weddell Sea pack ice floe, using under‐ice irradiance measurements taken with an instrumented remotely operated vehicle. In this talk we present the instrumentation of the ROV, technical challenges in operating a ROV under sea ice, and provide an introduction of methods to derive ice algal biomass from under-ice optical measurements, and linking these to concomitant in-situ surface measurements of snow depth, ice thickness, and estimated sea ice freeboard levels.