Photochemical production of ionic and particulate aluminum and iron in lakes

Photochemical liberation of allochthonous organically bound aluminum (Al) and iron (Fe) and in-lake hydroxide precipitation are important sources of these metals to lake sediments. Mass budgets of ionic Al and Fe (Ali, Fei), organically bound Al and Fe (Alo, Feo), and particulate Al and Fe (Alp, Fep) were measured for two western Czech Republic forest lakes (Plesne and Certovo) in the 2000-2003 hydrological years. The lakes were net sinks of Ali, Alo, and Feo and net sources of Alp and Fep. The average Alo and Feo inputs from terrestrial sources (66-110 and 12-17 mmol m(-2) yr(-1), respectively, on a lake area basis) were reduced 45% and 25% in the lakes. Mass budgets of dissolved organic carbon, particulate organic C, and Al species indicated that only a minor part of the observed in-lake retention of Alo could be explained by coagulation and sedimentation of organic matter, or from Ali hydrolysis and formation of Alp. Laboratory experiments with a short-time irradiation (approximately 300 nm, approximately 800 W m(-2)) of water from inlets to Plesne Lake showed the importance of photochemical processes in the liberation of Al and Fe from Alo and Feo. After 12 h of irradiation, Alo and Feo concentrations decreased 54 +/- 6% and 70 +/- 16%, respectively, compared to those of the dark controls. The photoliberated Alo and Feo increased the Ali and Fei concentrations reciprocally, on a 1:1 mass basis. The subsequent hydrolysis of Ali and Fe1 in lakes forms insoluble hydroxides, increasing the sediment concentrations of Al and Fe.