Absorbance of light by suspended particles in freshwater ecosystems: Factors controlling spectral UV-VIS absorbance

A model for spectral absorbance of natural particles was constructed for three freshwater ecosystems. The range of lake conditions included dates and depths where photochemical bleaching, iron concentration, oxygen saturation, and the redox state of iron would be expected to vary. Stream samples included variations in the organic vs. mineral composition of particles. The model accounts for the absorbance due to three optical components: photosynthetic pigments indexed to chlorophyll-a (Chl-a), particulate organic matter (POM), and particulate iron (PFe). When the model was tested with near-surface, deep anoxic iron-rich and recently-oxidized iron-rich samples, model accuracy decreased, suggesting a role for iron oxidation in the formation of particles and for photobleaching of POM and/or iron. The anoxic bottom of Lake Lacawac was rich in dissolved organic matter (DOM) and dissolved iron (DFe) and there is evidence that these become a source of POM and PFe to the rest of the lake.