Underwater light environment in Arctic fjords

Abstract

Abstract. Most inhabitants of the Arctic live near the coastline, including fjord systems where socio-ecological coupling with coastal communities is dominant. It is therefore critically important that the key aspects of Arctic fjords be measured as well as possible. Much work has been done to monitor temperature and salinity, but an in-depth knowledge of the light environment throughout Arctic fjords is lacking. This is particularly problematic knowing the importance of light for benthic ecosystem engineers such as macroalgae, which also play a major role in ecosystem function. Here we document the creation and implementation of a high resolution (~50–150 m) gridded dataset for surface photosynthetically available radiation (PAR), diffuse attenuation of PAR through the water column (KPAR), and PAR available at the seafloor (bottom PAR) for seven Arctic fjords distributed throughout Svalbard, Greenland, and Norway, during the period 2003–2022. In addition to bottom PAR being available at a monthly resolution over this time period, all variables are available as a global average, annual averages, and monthly climatologies. Throughout most Arctic fjords, the interannual variability of monthly bottom PAR is too large to determine any long term trends. However, in some fjords, bottom PAR has increased in spring and autumn, and decreased in summer. While a full investigation into these causes is beyond the scope of the description of the dataset presented here, it is hypothesised that this shift is due to a decrease in seasonal ice cover (i.e. enhanced surface PAR) in the shoulder seasons, and an increase in coastal runoff (i.e. increased turbidity/decreased surface PAR) in summer. A demonstration of the usability of the dataset is given by showing how it can be combined with known PAR requirements of macroalgae to track the change in time of the potential distribution area for macroalgal habitats within fjords. The dataset (Gentili et al., 2023a) is available on PANGAEA at: https://doi.org/10.1594/PANGAEA.962895. A toolbox for download and working with this dataset is available in the form of the FjordLight R package, which is available via CRAN (Gentili et al., 2023b), or may be installed via GitHub: https://face-it-project.github.io/FjordLight (last access: 8 December 2023).

Light-mediated temperature susceptibility of kelp species (Agarum clathratum, Saccharina latissima) in an Arctic summer heatwave scenario

“…Niedzwiedz and Bischof (2023a), Bartsch et al (2016) and Düsedau et al (n.d.) described the reduction of the kelp maximum distribution depth in meltwater plume dominated fjord systems, indicating reduced kelp primary production in darkening Arctic fjords. Schlegel et al (2023) highlighted that the interannual in situ PAR variability is too large to project clear long-term developments of Arctic fjord light conditions, strongly depending on season, timescale, and boundary conditions. The high variability in in situ PAR in combination with the strong response of kelps to light variations indicates a high variability of temperate kelp presence along Arctic coastal areas.…”

“…Increased temperatures lead to an early season breakup of sea ice (increased light availability; Nicolaus et al, 2012;Payne and Roesler, 2019), while higher terrestrial runoff (Bintanja and Andry, 2017;Milner et al, 2017;Bintanja, 2018) decreases light availability in summer (Gattuso et al, 2020;Konik et al, 2021). Thereby, Schlegel et al (2023) describe a high interannual variability of light availability. As photoautotrophic organisms, kelps are dependent on the underwater light conditions, driving their depth distribution (Fragkopoulou et al, 2022).…”