| The overarching objective of this research was to develop new quantitative insights into how absorption and scattering affect solar irradiance in aquatic ecosystems. Solar energy penetration largely determines the biological productivity of aquatic ecosystems by controlling the heat budget, light availability for the autotrophs at the base of the foodweb, and exposure to harmful ultraviolet radiation (UV). Colored dissolved organic matter (CDOM) is often responsible for most of the UV attenuation in lakes. Although many correlative models relating UV penetration to CDOM absorption (or its correlate, dissolved organic carbon concentration) have been published, these models are specific to the sites from which they have been established. I tested the hypothesis that a quantitative optics approach based on the inherent optical properties (IOP absorption and scattering) would show significant effects of absorption by particulate matter (e.g., by plankton) and scattering (e.g., by suspended sediments) on UV attenuation. This approach was applied to two systems where correlative models are inadequate: freshwater ice and turbid waters. My optical analysis of the ice cover of aquatic ecosystems from the boreal, subarctic and arctic zones showed that CDOM is substantially excluded from ice during freeze-up. Synchronous fluorescence analysis indicated that larger CDOM molecules were excluded more effectively from the ice than smaller molecules and that the exclusion factor (water/ice) was non-linearly related to CDOM absorption coefficients in the underlying waters. Despite elevated scattering in the ice resulting from crystal structure, UV attenuation was often lower than in the underlying waters where CDOM absorption dominated the attenuation. Since northern regions are among those most severely affected by climate change and stratospheric ozone depletion, ice optical properties are crucially important. I refined and validated this optics approach based on the IOPs by a study of the turbid waters in Lake Biwa, Japan. By extending into the UV waveband a model developed for photosynthetically available radiation (PAR) by Kirk (1991, Limnol. Oceanogr. 36: 455--467) it was possible to model and predict the attenuation coefficients for downward irradiance (Kd, an apparent optical property, AOP) from the measured absorption and scattering coefficients. In waters of relatively high turbidity, CDOM absorption explained only 40% of UV attenuation, while particle absorption and scattering were responsible for 36% and 24%, respectively. This model taking into account the effects of all dissolved and particulate components allows a better understanding of the factors affecting solar irradiance attenuation in aquatic ecosystems and allows more accurate prediction of the modifications linked to global change and resource utilization. |
