Thesis
Parameterisation of underwater light fields in the Arctic Ocean and associated impact on biological processes
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2023
- Thesis identifier
- T16726
- Person Identifier (Local)
- 201950634
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- 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 light levels to be sufficiently high that under-ice productivity contributions may exceed 10% of total Pan-Arctic primary production in spring, and may reach saturation levels in summer, accounting for >20% of total productivity. In open waters, calculations of primary production were found to be highly sensitive to the parameterisation of the diffuse attenuation coefficient of light. Comparing the results of various light field models designed for use in the Arctic showed a factor 12 difference in calculated water column productivity when using output irradiances to drive a model of primary production. Comparing modelled underwater spectral irradiance to the diel vertical migration (DVM) patterns of Arctic krill in early spring 2018 showed that the spectral distribution of light may act as a trigger mechanism for DVM. Results appear to indicate that although diurnal changes in the magnitude of downwelling irradiance largely drives bulk migration patterns, the population of krill also responded to changes in the ratio of green to blue light, driven by changes in lunar and solar elevations, preferring to occupy regions of the water column with a dominant blue colour of underwater light.
- Advisor / supervisor
- McKee, David
- Resource Type
- DOI
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PDF of thesis T16726 | 2023-10-10 | Público | Baixar |