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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 532 nm, 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. Key Points: Pigment ratios indicate low‐light acclimation of an Arctic underice phytoplankton bloomThis provides evidence for local phytoplankton growth in low-light conditions below Arctic pack iceIn situ absorption measurements have potential for assessing the phytoplankton photoacclimation state [ABSTRACT FROM AUTHOR]

Bio‐optical properties of surface waters were characterized off western and northern Spitsbergen in the summers of 2013, 2014, and 2015. We observed statistically significant year‐to‐year differences in spatial distribution of spectral absorption (a(λ)) and beam attenuation (c(λ)). Highest a(λ) and c(λ) were located in the frontal zone between water masses and co‐varied strongly with chlorophyll a fluorescence. Phytoplankton pigments dominated the absorption budget at 443 nm (50%). The contribution of chromophoric dissolved organic matter to total nonwater absorption was highest at 412 nm (42%), and detrital absorption contributed most at 550 nm (37%). Almost all inherent optical properties, except chromophoric dissolved organic matter, were highly correlated with the chlorophyll a concentration (Chla, R2 > 0.81). Relationships between Chla and the particulate and phytoplankton pigments absorption coefficients at 443 and 676 nm were characterized by significant determination coefficients (R2 > 0.73). The phytoplankton pigments line height absorption aLH(676) was found to be the most reliable optical proxy for determination of Chla, compared to total nonwater absorption, apg(676), and stimulated in situ chlorophyll a fluorescence intensity, IChla. In the presence of sea ice melt the water column was stratified and the vertical distribution of inherent optical properties was characterized by a surface minimum followed by a distinct subsurface maximum, aligned with a subsurface chlorophyll a maximum. We surmise that prevailing regional wind patterns affect sea ice and surface drift in central Fram Strait, and thus the location of sea ice meltwater, which affects the vertical stratification and occurrence of subsurface chlorophyll a maximum. Plain Language Summary: Arctic is rapidly changing due to anthropogenic climate change. One of the manifestations of that change is a northward advancement of warm Atlantic Water in the European Sector of the Arctic Ocean. This has important implications for marine ecosystem, including phytoplankton at its base. Ocean color remote sensing from space is a common tool to monitor phytoplankton in surface waters of the ocean. However, in situ optical and biological observations are needed for correct estimates of phytoplankton biomass from space. In this study we provide a comprehensive description of bio‐optical properties of surface waters within the Atlantic Water inflow region near Svalbard based on three years of observations. We found that phytoplankton itself is the most important factor influencing the underwater light field and therefore the signal that is detected from space. We constructed key bio‐optical relationships that are needed for validation of remote sensing products. We tested three different optical ways to estimate chlorophyll a concentration, which is a proxy for phytoplankton abundance. We found that year to year variability in optical properties in central Fram Strait is linked to wind patterns in the area, which affect the interaction between Atlantic Water and sea ice, and therefore stratification. Key Points: Bio‐optical properties within the Atlantic Water (AW) inflow region off Spitsbergen are mainly driven by phytoplanktonWinds affect advection of sea ice and melt waters, influencing stratification and thus occurrence of subsurface chlorophyll maximaPhytoplankton pigments line height absorption at 676 nm is the best optical proxy for estimating chlorophyll a concentration in the area [ABSTRACT FROM AUTHOR]