The impact of climate change on lake-water photoactivity: chemistry modifications vs. extreme drought conditions

Earth climate has highly varied during our planet history, but in the last century it has undergone a fast change because of anthropic impacts. The related environmental modifications, such as global warming and atmospheric precipitations variations, are of high environmental concern because they can lead to ecosystems alterations. For instance, lakes can undergo rather fast and important variations of the water features, such as their physical parameters (water depth and temperature) and chemical characteristics (e.g., pH, alkalinity, ions concentrations and the dissolved organic matter content). For this reason, small and medium-sized lakes have been defined as 'sentinels of climate change'. The photochemical processes that take place in surface waters, such as lakes and rivers, can be indirectly affected by climate change, because chemistry and photochemistry of water bodies are strictly connected. These photo-processes include the formation of the so-called Photo-Produced Reactive Intermediate (PPRIs) that can react with water pollutants, such as xenobiotic compounds (namely, pharmaceuticals, pesticides, etc.) and cause their degradation. These reactions can act as self-depuration processes for lake water, but in some cases, they can also produce toxic by-products. The main PPRIs are hydroxyl and carbonate radicals (¿OH and CO3¿−, respectively), the excited triplet states of chromophoric dissolved organic matter (3CDOM*), and singlet oxygen (1O2). The PPRIs steady-state concentrations (resulting from the formation-deactivation budget) are highly affected by lake-water chemistry and by its variations, in particular by the concentrations of photosensitizing compounds (DOM, nitrate and nitrite) that produce PPRIs upon sunlight absorption, and by water depth. Xenobiotics photodegradation depends upon PPRIs steady-state concentrations, thus the self-depuration processes can be affected by climate change as well. The main goal of this work was to assess the possible long-term trends of photochemistry in lakes located in the European Alps (Ticino and Tyrol regions) and in southern Australia. The choice of these far-away areas was accounted for by the different kind of changes undergone by the relevant aquatic ecosystems. The time trends of lake-water chemistry were used as input data for a photochemistry-modelling software (HO_Software) to compute the PPRIs steady-state concentrations and obtain the time trends of modelled photochemistry. Different scenarios were found in alpine lakes: a few of them showed negligible chemical variations over the years and, therefore, constant photochemistry; in contrast, changes in water chemistry of other lakes (such as the nitrate content decrease) accounted for variations in PPRIs concentrations. However, a water-chemistry change did not always affect photochemistry. Overall, trends in alpine lakes appear to be affected by recovery from acid rains more than by climate issues. The studied Australian lakes have been affected by prolonged drought during 2008-2010, which has induced important water-chemistry variations and water-level fluctuations (mostly due to evaporative water concentration). The HO_Software output data suggested that photochemistry underwent changes as well because of the drought, and the time trend of the PPRIs steady-state concentrations showed statistically significant variations over the years, with important differences between drought and and non-drought periods.