| Summary: | Arctic under-ice phytoplankton blooms are initiated by a sudden increase in the transmission of photosynthetically active radiation (PAR; 400 – 700 nm) as a result of the formation of surface melt ponds in late spring. However, the more pronounced spatial variability in irradiance levels beneath ponded and white ice compared to snow-covered ice create difficulties in measuring light availability for primary production. In this thesis, the impact of spatiotemporal variability in transmitted irradiance on under-ice light field parameters is examined and later applied to produce the first estimate of late spring production in the ice-covered Hudson Bay. Phytoplankton production is estimated based on PAR availability at the ice bottom and its vertical attenuation with increasing water depth. I demonstrated that spatially averaged transmittance from large-scale continuous measurements provides more representative estimates of under-ice PAR relative to single point irradiance measurements due to large variations in transmitted PAR. Vertical irradiance profiles in the first meters of the water column are greatly influenced by these spatial variations and the horizontal spreading of light in the overlying ice cover. Therefore, it is recommended to derive the diffuse vertical attenuation coefficient from deeper depths. To further account for the shift from a diffuse to a more-downward directed light field with ongoing ice surface melt, these measurements should be performed with scalar radiometers with a spherical collector. Otherwise measured downwelling irradiance can be converted into scalar irradiance by using under-ice downwelling average cosine values, for which direct measurements are first reported in this thesis. Applying this improved parametrization of the apparent optical properties in the investigation of microalgal primary production in Hudson Bay, I estimate that 32% of annual biomass is produced during the sea ice melt period. Under-ice phytoplankton reach high production rates due to a large plasticity ...
|
|---|