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

• Published in: Journal of geophysical research. Oceans Vol. 124; no. 3; pp. 1964 - 1987
• Main Authors: Kowalczuk, Piotr, Sagan, Sławomir, Makarewicz, Anna, Meler, Justyna, Borzycka, Karolina, Zabłocka, Monika, Zdun, Agnieszka, Konik, Marta, Darecki, Mirosław, Granskog, Mats A., Pavlov, Alexey K.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-03-2019
• Subjects: Absorption; absorption budget; Absorption coefficient; Absorption spectra; Absorptivity; Anthropogenic climate changes; Anthropogenic factors; Arctic Ocean; Attenuation; Chlorophyll; Chlorophyll a; Climate change; Coefficients; Density stratification; Dissolved organic matter; Distribution; Fluorescence; Geophysics; Ice melting; Inflow; inherent optical properties; Marine ecosystems; Meltwater; Ocean color; Ocean colour; ocean optics; Oceans; Optical properties; Organic matter; Phytoplankton; Pigments; Plankton; Remote monitoring; Remote sensing; Sea ice; Spatial distribution; Spectral absorption; Statistical analysis; Straits; Stratification; Surface chemistry; Surface water; Svalbard; Vertical distribution; Water circulation; Water column; Water inflow; Water masses; Wind; Wind patterns; Arctic Ocean; Fram Strait
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2018JC014529

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 phytoplankton
Winds affect advection of sea ice and melt waters, influencing stratification and thus occurrence of subsurface chlorophyll maxima
Phytoplankton pigments line height absorption at 676 nm is the best optical proxy for estimating chlorophyll a concentration in the area