Impact of experimental light regimes on Antarctic fast-ice algal communities observed by combining ice coring and imaging spectroscopy

Antarctic landfast sea ice is an integral component of coastal and nearshore ecosystems, providing bottom-ice habitats that harbour some of the most productive marine microalgal communities on Earth. Ice algal biomass is driven by the spatio-temporal dynamics of snow and sea-ice covers, which control light transmission and strongly influence algal photophysiology. In this study, five 25 m2 translucent coloured panels (red, green, blue, black, and grey) were deployed on the ice surface at Cape Evans, Antarctica, up to 30 days prior to the 2019 spring sampling season. We investigated bottom fast-ice algal communities under natural and experimentally altered light regimes by integrating pigment analysis, relative species abundance, and imaging spectroscopy across vertical ice-core sections (0–12 cm, sampled at 3 cm intervals). Under the experimental light regimes, we observed an increase in algal biomass towards the upper ice layers (3–12 cm), accompanied by a clearer upwards gradient in photoprotective to photosynthetic carotenoid ratios. Although pigmentation did not differ significantly among the coloured light treatments, species-specific preferences for particular spectral shades were evident, highlighting the complex response patterns of algal community structure and function under varying light regimes. The broad range of induced community photoadaptations also enabled us to test novel spectral indices that can serve as non-invasive proxies of algal biomass (e.g., Chlorophyll a, R2 = 0.85) and photoacclimation (e.g., photoprotective to photosynthetic carotenoid ratio, R2 < 0.66). These indices, applied to in-situ under-ice hyperspectral imaging data, produced observations consistent with traditional ice-core sampling when comparing natural and experimentally altered light regimes. This study advances the ability to observe shifts in ice algal biomass and photoacclimation state remotely, along with revealing photobiological insights that may have broader ecological implications for the Antarctic fast-ice ecosystem.