The Role of Iron and Dissolved Organic Carbon in the Absorption of Ultraviolet Radiation in Humic Lake Water

Abstract

Absorption of solar ultraviolet radiation (UVR) in aquatic ecosystems is primarily controlled by dissolved organic carbon (DOC). The role of iron (Fe) has also been suggested to contribute to UVR attenuation either directly or by interactions with DOC. Here we present findings from three laboratory manipulations of Fe and DOC on changes to the dissolved UVR absorption (ad,320) in a mid-latitude, dimictic, humic lake. In a laboratory simulation of lake turnover where anoxic, hypolimnetic water was oxygenated ad,320 significantly increased from 23.3 to 81.7 m−1 (p<0.0001). In a second laboratory experiment, addition of ferrous Fe to deoxygenated lake water increased ad,320 upon reoxygenation up to a concentration of 1.0 mg l−1 Fe, where a solubility saturation threshold may have been reached. In situ lake experiments were designed to simulate release of UV absorbing substances from anoxic sediments by placing 20-l carboys (open at the bottom, sealed at the top) onto the lake bottom. UV absorption at 320 nm increased over time for samples from within the experimental carboys. Finally, samples from several lake profiles and sediment experiments were analyzed for ad,320, total Fe, and DOC. UV absorption of dissolved substances at 320 nm and total Fe concentration both increased with depth, however DOC remained relatively constant over depth. Furthermore, total Fe and spectral slope showed tight coupling up to 1 mg l−1 total Fe in our survey analysis. Our results provide evidence for the importance of anoxic sediments as a source of ferrous iron and UV absorbing substances and suggest a role for ferric iron in increasing UVR and PAR absorption in lake water. We suggest that as this ferrous Fe oxidizes, its absorptive properties increase, and it may bind with dissolved organic matter, enabling it to remain in solution and thus increasing the dissolved absorption of lake water for extended periods of time.