Bio-optical Properties of Surface Waters in the Atlantic Water Inflow Region off Spitsbergen (Arctic Ocean)

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(lambda)) and beam attenuation (c(lambda)). Highest a(lambda) and c(lambda) were located in the frontal zone between water masses and co-varied strongly with chlorophyll a fluorescence. Phytoplankton pigments dominated the absorption budget at 443nm (50%). The contribution of chromophoric dissolved organic matter to total nonwater absorption was highest at 412nm (42%), and detrital absorption contributed most at 550nm (37%). Almost all inherent optical properties, except chromophoric dissolved organic matter, were highly correlated with the chlorophyll a concentration (Chla, R-2>0.81). Relationships between Chla and the particulate and phytoplankton pigments absorption coefficients at 443 and 676nm were characterized by significant determination coefficients (R-2>0.73). The phytoplankton pigments line height absorption a(LH)(676) was found to be the most reliable optical proxy for determination of Chla, compared to total nonwater absorption, a(pg)(676), and stimulated in situ chlorophyll a fluorescence intensity, I-Chla. 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.