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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]

Optical properties of chromophoric (CDOM) and fluorescent dissolved organic matter (FDOM) were characterized in the Nordic Seas including the West Spitsbergen Shelf during June-July 2013, 2014, and 2015. The CDOM absorption coefficient at 350 nm, aCDOM(350) showed significant interannual variation (T test, p <0.00001). In 2013, the highest average aCDOM(350) values (aCDOM(350)D0.30±0.12m-1) were observed due to the influence of cold and low-salinity water from the Sørkapp Current (SC) in the southern part of the West Spitsbergen Shelf. In 2014, aCDOM(350) values were significantly lower (T test, p <0.00001) than in 2013 (average aCDOM(350)D0.14±0.06m-1), which was associated with the dominance of warm and saline Atlantic Water (AW) in the region, while in 2015 intermediate CDOM absorption (average aCDOM(350)D0.19±0.05m-1) was observed. In situ measurements of three FDOM components revealed that fluorescence intensity of protein-like FDOM dominated in the surface layer of the Nordic Seas. Concentrations of marine and terrestrial humic-like DOM were very low and distribution of those components was generally vertically homogenous in the upper ocean (0-100 m). Fluorescence of terrestrial and marine humic-like DOM decreased in surface waters (0-15 m) near the sea ice edge due to dilution of oceanic waters by sea ice meltwater. The vertical distribution of protein-like FDOM was characterized by a prominent subsurface maximum that matched the subsurface chlorophyll a maximum and was observed across the study area. The highest protein-like FDOM fluorescence was observed in the Norwegian Sea in the core of warm AW. There was a significant relationship between the proteinlike fluorescence and chlorophyll a fluorescence (R2 D0.65, p <0.0001, nD24 490), which suggests that phytoplankton was the primary source of protein-like DOM in the Nordic Seas and West Spitsbergen Shelf waters. Observed variability in selected spectral indices (spectral slope coefficient, S300-600, carbon-specific CDOM absorption coefficient at 254 and 350 nm, SUVA254, a* CDOM(350)) and the nonlinear relationship between CDOM absorption and the spectral slope coefficient also indicate a dominant marine (autochthonous) source of CDOM and FDOM in the study area. Further, our data suggest that aCDOM(350) cannot be used to predict dissolved organic carbon (DOC) concentrations in the study region; however the slope coefficient (S300-600) shows some promise in being used. [ABSTRACT FROM AUTHOR]