During summer months, photolysis may lead to a decrease in the dissolved and colloidal chromophoric substances (photobleaching) particularly of stagnant systems with no significant influx from the terrestrial surrounding. Humicrich lakes and bogs lose much of their visible brown color. Photobleaching produces a large proportion of no longer chromophoric substances, but aliphatic carbon (and to a lesser degree CO2); the aromatic ring structures are opened.
Photobleaching is accompanied with oxygen consumption. Several balance studies show that the photolytic H2O2 production is linearly related to the photobleaching and may account for about half of the observed O2 uptake. The other half may be ascribed to organically mediated oxygen reductions. These may include formation of organic peroxides and their decomposition products.
Due to the degradation of the global ozone layer in the troposphere, and the consequent increase in UV-B radiation, the proportion of chromophoric organic carbon in aquatic systems is probably decreasing worldwide. As a consequence, one natural UV shield declines, the subsequent UV-penetration depth increases exponentially, and energy transmission changes drastically. The most vulnerable systems are high mountain lakes above the tree line, particularly those with catchments dominated by
bare rock and without higher vegetation. In these lakes, photobleaching is a serious problem, particularly in shallow systems in which the plankton cannot avoid UV radiation through vertical migration.
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