Recent reports on Arctic underice phytoplankton blooms have directed attention to primary production below the sea ice cover. Such underice blooms cannot be detected from space; thus, methods for autonomous underice measurements are critically needed to extend observations beyond ship-based surveys. One central aspect of the ecology of these blooms is whether they were advected from open-water areas or were able to develop below the ice cover under typically low light conditions. The photoacclimation state of the bloom can provide clues about the growth conditions and therefore its origin. Here we investigate the photoacclimation state of a Phaeocystis pouchetii-dominated underice bloom in the Arctic Ocean using ratios of photoprotective carotenoids (PPC) to photosynthetic carotenoids (PSC) and chlorophyll a. The pigment proxies indicate local growth under the ice pack. Furthermore, a method using in situ light absorption measurements to estimate the PPC:PSC ratio was in agreement with the pigment data. The slope of in situ phytoplankton absorption between 488 and 532nm, affected by both PPC and PSC, had a significant linear relationship to the PPC:PSC ratio, indicating that prediction of photoacclimation state can be obtained from absorption profiles. We also review, with regard to the pigment function, different ways of grouping pigments into PPC or PSC applied in previous studies. Although more validation data sets are needed to assess the impact of pigment packaging on the relationship between PPC:PSC and absorption measurement slopes, our study shows the potential for using in situ absorption measurements to collect information about phytoplankton physiology below sea ice. Plain Language Summary Phytoplankton blooms below sea ice cover, that is, underice blooms, can be advected by ocean currents below the ice pack from ice-free waters where they had sufficient light available for growth. They can also grow below the ice pack if sufficient light is transmitted through the ice, and they are able to adjust to the typically low light conditions. Algal pigments can indicate which of the two scenarios is more likely. Algal pigments capture sunlight for photosynthesis, but some of them are used to protect the algal cells from excess light. The ratio of photoprotective to photosynthetic pigments reveals the photoacclimation state ("short time adjustments") of the algae. In this study we investigate the photoacclimation state of an Arctic underice bloom based on pigment ratios, which indicate local growth below the ice pack. Furthermore, we validate a method to estimate the pigment ratios from light absorption measurements carried out with an instrument in the water column. Studies like these are important for developing methods to study remote areas. Ship-based surveys can only cover a restricted area, and satellite remote sensing does not provide biological information from ice-covered waters. Our study shows promising results for using water column instruments to study phytoplankton physiology.