Parameterisation of underwater light fields in the Arctic Ocean and associated impact on biological processes

• Main Author: Connan-McGinty, Stacey
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: 2023
• Subjects: Arctic Ocean; polar night
• Online Access: https://doi.org/10.48730/sw9a-pz91; https://stax.strath.ac.uk/concern/theses/jw827c24h

Accurate characterisation of underwater light is an integral component in modelling the dynamics of marine ecosystems, particularly primary production and animal migration patterns. Existing methods of estimating light fields either rely on satellite data, in situ measurements or radiative transfer models that only operate when the sun is above the horizon. These methods are of limited use in Arctic waters, particular during Polar Night due to extended periods of extremely low light levels and prolonged periods when the sun remains below horizon. Estimating underwater light in the region is further hindered by the optical complexities introduced by widespread and seasonally varying snow and ice cover, and many current ecosystem models either simplify these under-ice light fields or excluding them entirely, potentially disregarding biologically significant light levels.This work presents a model of spectrally resolved underwater light that demonstrates the ability to simulate light levels over the full year into the period of Polar Night and is validated by in situ data. Downwelling spectral irradiance in the photosynthetically active radiation (PAR, 400 – 700nm) range is calculated in both open and ice-covered water columns and includes multiple reflection amplification effects of above surface irradiance between snow and cloud. Validation of downwelling broadband irradiance in open waters shows a mean absolute error of <6 μmol m–2 s–1 over the water column and comparison of modelled to measured transmission through snow and ice shows a mean absolute error of 2% transmittance. During overcast conditions, multiple reflection amplification between snow and cloud induced an average increase of 65% in above surface downwelling irradiance. Application of the model on a Pan-Arctic scale in 2018 demonstrated the potential for >20% of above surface irradiance to penetrate through thin ice ( <0.5m). When coupled to a broadband PAR model of primary production, model results showed the potential for under-ice ...