Contrasting optical properties of surface waters across the Fram Strait and its potential biological implications

Abstract

Underwater light regime is controlled by distribution and optical properties of colored dissolved organic matter (CDOM) and particulate matter. The Fram Strait is a region where two contrasting water masses are found. Polar water in the East Greenland Current (EGC) and Atlantic water in the West Spitsbergen Current (WSC) differ with regards to temperature, salinity and optical properties. We present data on absorption properties of CDOM and particles across the Fram Strait (along 79°N), comparing Polar and Atlantic surface waters in September 2009 and 2010. CDOM absorption of Polar water in the EGC was significantly higher (more than 3-fold) compared to Atlantic water in the WSC, with values of absorption coefficient, a CDOM (350), m −1 of 0.565 ± 0.100 (in 2009) and 0.458 ± 0.117 (in 2010), and 0.138 ± 0.036 (in 2009) and 0.153 ± 0.039 (in 2010), respectively. An opposite pattern was observed for particle absorption with higher absorption found in the eastern part of the Fram Strait. Average values of particle absorption (a P (440), m −1 ) were 0.016 ± 0.013 (in 2009) and 0.014 ± 0.011 (in 2010), and 0.047 ± 0.012 (in 2009) and 0.016 ± 0.014 (in 2010), respectively for Polar and Atlantic water. Thus absorption of light in eastern part of the Fram Strait is dominated by particles -predominantly phytoplankton, and the absorption of light in the western part of the strait is dominated by CDOM, with predominantly terrigenous origin. As a result the balance between the importance of CDOM and particulates to the total absorption budget in the upper 0-10 m shifts across Fram Strait. Under water spectral irradiance profiles were generated using ECOLIGHT 5.4.1 and the results indicate that the shift in composition between dissolved and particulate material does not influence substantially the penetration of photosynthetic active radiation (PAR, 400-700 nm), but does result in notable differences in ultraviolet (UV) light penetration, with higher attenuation in the EGC. Future changes in the Arctic Ocean system will likely affect EGC through diminishing sea-ice cover and potentially increasing CDOM export due to increase in river runoff into the Arctic Ocean. Role of attenuation of light by CDOM in determining underwater light regime will become more important, with a potential for future increase in marine productivity in the area of EGC due to elevated PAR and lowered UV light exposures.

Characteristics of chromophoric and fluorescent dissolved organic matter in the Nordic Seas

“…The influence of transformed AW generated in the Barents and Norwegian seas had impacted a CDOM (443) values in the Beaufort Gyre and Amundsen and Nansen basins, causing its decrease below 0.2 m −1 as reported by Gonçalves-Araujo et al (2018). The reported lower range of a CDOM (350) observed in AW during AREX2014 (2014: 0.14 ± 0.06 m −1 ) is in good agreement with data from the eastern part of Fram Strait at the 79 • N section reported by Granskog et al (2012), , and Pavlov et al (2015) and with data reported by Hancke et al (2014) south of the Polar Front in the Barents Sea. Kowalczuk et al (2017) observed similar a CDOM (350) values north of Svalbard.…”

“…The CDOM distribution in the area influenced by AW was reported by Stedmon and Markager (2001) in the central part of the Greenland Sea, and by Granskog et al (2012) and Pavlov et al (2015), who presented CDOM and particulate absorption distribution along a transect across the Fram Strait at 79 • N. Hancke et al (2014) studied the seasonal distribution of the CDOM absorption coefficient (a CDOM (λ)) in an area across the Polar Front in the central part of the Barents Sea. Seasonal studies on CDOM contribution to overall variability in inherent optical properties (IOPs) reported on sea ice (Kowalczuk et al, 2017) and in the water column during a spring under-ice phytoplankton bloom north of Svalbard .…”

“…Furthermore, PSWw was also limited to the uppermost 50 m of the water column with S ≤ 34.9. The water mass with similar temperature-salinity (TS) characteristics to PSWw but slightly different ranges was referred to in the literature for Arctic Surface Water, ASW (e.g., Pavlov et al, 2015;Gonçalves-Araujo et al, 2016), but due to the dominance in the area of water originating from the Atlantic Ocean the name PSWw from Rudels et al (2005) classification is used. We could find Arctic Atlantic Water (AAW) in our data set as a result of the mixing process of AW and PSW, in the range of 0 < ≤ 2 • C and 27.7 < σ θ ≤ 27.97 (Rudels et al, 2005).…”

“…Recent studies have reported intensification of AW inflow into the Arctic Ocean (Walczowski, 2014;Polyakov et al, 2017;Walczowski et al, 2017), further highlighting the importance of the European sector of the Arctic Ocean to better understand the complex interactions between inflowing AW and PSW. Optically these waters are contrasting, especially with respect to CDOM (Granskog et al, 2012;Pavlov et al, 2015; and FDOM (Jørgensen et al, 2014;Gonçalves-Araujo et al, 2016). In the absence of sea ice, favorable vertical mixing conditions and sufficient levels of solar radiation make it a very productive and important region from an ecosystem and socioeconomic standpoint, thus ensuring motivation for ongoing studies of the complex marine system in the area (Skogen et al, 2007;Olsen et al, 2009;Dalpadado et al, 2014).…”

“…A number of occasional synoptic surveys of CDOM and optical properties have been conducted in the different regions of the European Arctic Ocean and concentrated on the western part of the Fram Strait influenced by polar water outflow with the East Greenland Current (EGC) (Granskog et al, 2012;Pavlov et al, 2015;Gonçalves-Araujo et al, 2016). The CDOM distribution in the area influenced by AW was reported by Stedmon and Markager (2001) in the central part of the Greenland Sea, and by Granskog et al (2012) and Pavlov et al (2015), who presented CDOM and particulate absorption distribution along a transect across the Fram Strait at 79 • N. Hancke et al (2014) studied the seasonal distribution of the CDOM absorption coefficient (a CDOM (λ)) in an area across the Polar Front in the central part of the Barents Sea.…”