Underwater irradiance attenuation and photobleaching of chromophoric dissolved organic matter in shallow Arctic lakes of the Mackenzie Delta, NWT.

In situ attenuation of ultraviolet-B (UVB; 310-320 nm subset), ultraviolet-A (UVA) and photosynthetically active radiation (PAR) generally declined over the 2004 open-water season in a set of shallow Mackenzie Delta floodplain lakes. Average 1% photic depths were < 30 cm for UVB and < 50 cm for UVA, indicating that part of the water column remained sunscreened from UV. Average euphotic and lake depths were approximately equal (~2 m), indicating sufficient PAR for photosynthesis at all depths. In 2005, dissolved organic matter (DOM) quality was examined using optical indices. Both spectral slope (decline in DOM absorbance as wavelength increases) and specific ultraviolet absorbance (average DOM aromaticity) were strongly correlated with sill elevation. UVA dominated in situ chromophoric DOM photobleaching, indicating that future increases in UVB fluxes are unlikely to exert as great an influence on in situ Mackenzie Delta optical environments as will increased chromophoric DOM from climate change.