Small mountain lake opens a window to the past
Water brownification due to humic substances has been a long-standing phenomenon in freshwater ecosystems across the northern hemisphere. It appears that the intense surface water brownification observed in the last 30 years was preceded in many catchments by brownification lasting hundreds or even thousands of years. Current climate change is proposed as one of the main causes of the progressive brownification. While the influence of climate on brownification is very difficult to demonstrate on decadal time scales, the millennial time scale allows sufficient distance to better understand this significant environmental change.
Paleoecologists from the Department of Botany and the Institute of Environmental Science, Faculty of Science, CU, decided to look into the history of Prášilské jezero, whose waters are now naturally brown and, according to historical observations, were already brown before anthropogenic acidification by acid rain. "We suspected that this might have been the case in the distant past, which was eventually confirmed and the key influence of humic substances on the organisms inhabiting the lake was demonstrated throughout practically the entire Holocene," says Anna Tichá, who was responsible for the analysis of the diatom remains. "We already had indications of the shading of the lake bottom from our earlier research, which revealed the collapse of Eurasian quillwort populations in Prášilské jezero. Brownification was a clear adept for the source of this shading, but we lacked more detailed information on changes in the lake ecosystem to reconstruct its evolution," said Alice Moravcová.
Reconstructions of pH and phosphorus concentration based on the species composition of diatoms preserved in the sediments provided the necessary insight into changes in lake water chemistry. According to the diatoms, the steepest decline in pH occurred ~5.5-4.2 thousand years ago. However, major changes occurred even earlier, approximately ~6.8 thousand years ago, when the community began to show signs of low phosphorus availability and a concomitant increase in sediment-stored phosphorus. "Due to the high sensitivity of some chironomid (non-biting midge) species to oxygen deficiency, we were able to trace very early signs of brownification as early as 10.7 thousand years ago, when more pronounced phases of bottom anoxia first appeared in the lake," adds Daniel Vondrák. The biotic indicators were complemented by Prof. Richard Chiverrell from the University of Liverpool by geochemical analysis of the sediment using X-ray fluorescence. This analysis focused on elements and element ratios sensitive to biogeochemical processes associated with water brownification and catchment development (e.g., P, Ti, Al/Rb, Fe/Ti, Mn/Ti, Si/Ti).
In addition to reconstructing the impacts of brownification on the lake ecosystem, the authors' main objective was to reveal the factors that have influenced its progress over millennia. "In the case of the first major input of humic substances to the lake ~10.7 thousand years ago, we were able to rely on our previous vegetation reconstructions based on the analysis of pollen grains found in the sediment. It was during this period that the basin of Prášilské jezero was afforested and the increased production of organic matter served as a source of humic substances," explains the reason for the beginning of the brownification process Assoc. Prof. Petr Kuneš. Anna Tichá adds: "We could not explain at first what started happening in our record ~6.8-4.2 thousand years ago. In this period, the geochemical indicators suddenly diverge from each other, and it is also the only period when the sedimentary phosphorus record does not match the one estimated from diatom species composition." But then the solution of these discrepancies helped to reveal the cause of what were probably the highest humic concentrations in the lake's history. Previously well-drained and therefore oxygenated soils then began to become saturated with water, leading to alternating oxic and anoxic conditions in the soil profiles. These are the most favourable conditions for leaching of humic substances, but also of many elements, which "confused" the lake geochemical record of the authors of the study. At the same time, they noted the development of peaty soils in the basin of the lake. The high input of dissolved organic matter probably subsided ~4.2 thousand years ago, when the soilsbecame so saturated with water that they developed stable anoxic conditions and the leaching of organic matter from them ceased to be as effective as before.
The onset of the main phase of brownification in the small mountain lake ~6.8 thousand years ago corresponds in time to the overall shift to a wetter climate documented in the northern hemisphere ~6 thousand years ago (the so-called Mid-Holocene climate transition). Sedimentary records from many basins in Canada and Scandinavia indicate the development of peatlands and/or more intense brownification of lake waters with the onset of this period.
"The discovery of similar mechanisms in the small mountain lake promises interesting possibilities for comparing developments in thoroughly studied boreal regions and the less studied Central European area. The fate of the Bohemian Forest lakes during the Holocene is thus no longer of interest only on a regional scale, e.g. for the needs of conservation strategy, but may help to clarify the processes affecting the entire northern hemisphere," concludes Associate professor Petr Kuneš, who was responsible for the sediment sampling of several Bohemian Forest lakes seven years ago. Anna Tichá adds: "Understanding the drivers of the brownification that has been changing the face of surface waters for the last 30 years seems to be crucial, but also very difficult, because various natural and anthropogenic influences are mixed on a short time scale. Only a survey of brownification over thousands of years could reveal the shift to a wetter climate and subsequent spread of peatlands in the catchment as the main factor acting on this timescale." It seems that the Bohemian Forest lakes will once again speak to our understanding of the global environmental change, as they did years ago in the study of the impacts of acid rain, for which it has become a site of global importance.
Tichá, A., Vondrák, D., Moravcová, A., Chiverrell, R., & Kuneš, P. (2022). Climate-related soil saturation and peatland development may have conditioned surface water brownification at a central European lake for millennia. Science of The Total Environment, 159982. https://doi.org/10.1016/j.scitotenv.2022.159982