Modelling lake-water photochemistry: Three-decade assessment of the steady-state concentration of photoreactive transients (OH, CO3- and 3CDOM∗) in the surface water of polymictic Lake Peipsi (Estonia/Russia)

Over the last 3–4 decades, Lake Peipsi water (sampling site A, middle part of the lake, and site B, northern part) has experienced a statistically significant increase of bicarbonate, pH, chemical oxygen demand, nitrate (and nitrite in site B), due to combination of climate change and eutrophication. By photochemical modelling, we predicted a statistically significant decrease of radicals OH and CO3- (site A, by 45% and 35%, respectively) and an increase of triplet states of chromophoric dissolved organic matter (3CDOM∗; site B, by ∼25%). These species are involved in pollutant degradation, but formation of harmful by-products is more likely with 3CDOM∗ than with OH. Therefore, the photochemical self-cleansing ability of Lake Peipsi probably decreased with time, due to combined effects of climate change and eutrophication. In different environments (e.g. Lake Maggiore, NW Italy), ecosystem restoration policies had the additional advantage of enhancing sunlight-driven detoxification, suggesting that photochemical self-cleansing would be positively correlated with lake water quality.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Connection between 3 and 4 decade time series of water chemistry and photochemistry. ► Significant increase with time of main water-chemistry parameters in Lake Peipsi. ► Significant time decrease of modelled [OH] and [CO3-] in one sampling site. ► Significant but smaller time increase of modelled [3CDOM∗] in the other site. ► Overall decrease of the ability of Lake Peipsi water to photodegrade pollutants.