Attenuation of in situ UV radiation in Mackenzie Delta lakes with varying dissolved organic matter compositions

In 2004, ultraviolet-B (UVB) and ultraviolet-A (UVA) attenuation were measured in Mackenzie Delta lakes spanning gradients in water renewal rate, dissolved organic carbon (DOC) concentration, and dissolved organic matter (DOM) composition. DOM compositions (ratio of chromophoric DOM (CDOM) to non-chromophoric DOM) in Delta lakes are complex, evolving seasonally via flooding, dilution, macrophyte production, photobleaching, and bacterial metabolism. Attenuation was more strongly related to CDOM absorption coefficients (a(330); UVB r(2) = 0.69, p < 0.0001; UVA r(2) = 0.58, p < 0.0001) than to DOC concentrations due to variable DOM compositions. Attenuation in one set of lakes was well related (linear models) to a330 and total suspended solids (UVB R-2 = 0.80, p < 0.0001; UVA R-2 = 0.81, p < 0.0001). When these models were applied to other Delta lakes, however, attenuation of UVB was overestimated in 17 of 19 cases and attenuation of UVA was overestimated in all 18 cases. This bias indicates that models are not transferrable among Delta lakes, and likely cannot be applied in other circumpolar delta lakes with similarly complex DOM compositions. Although attenuation is high in Delta lakes (KdUVB 17.1-33.4 m(-1); K(d)UVA 7.7-19.2 m(-1)), plankton and photoreactive solutes may be exposed to high levels of ultraviolet radiation (UVR) because Delta lakes are shallow, UVB and UVA penetrate the top 19% and 31% of water columns, respectively, and day lengths are extended during open water. Thus, climate change effects on DOM compositions may significantly alter in situ UVR environments in circumpolar delta lakes.