Regime shifts in lake ecosystems: Testing theory with long-term observational data and large scale experiments (LakeShift)
Final Report Abstract
The project was dedicated to assessing the future scenarios of the abrupt mixing regime shifts in northern temperate lakes in response to the gradual increase of the atmospheric heat input. Heated by the sun, lakes generally tend to be warmer at the surface with colder waters beneath. Since warmer water is lighter, the density increase with depth (stratification) isolates the bulk of lake water from direct contact with the atmosphere. This fundamental process determines the entire ecology of lakes and how they are likely respond to climate change or pollution. Whether mixing at the lake surface is able to destroy the density stratification emerged as a key question of limnology at the dawn of lake science. Until now, however, this fundamental question could not be answered based only on information about the lake shape and geographical location. Instead, lakes were classified either as polymictic (regularly mixed) or predominantly stratified based on rough depth estimates, or regionally-specific empirical relationships between lake depth and area. More accurate estimations were only possible using complex numerical models applied to a certain lake. We developed a unifying scaling criterion to predict whether lakes mix regularly or stratify seasonally. It is the first universal, quantitative relation to classify these important characteristics. We analyzed the effect of water transparency due to phytoplankton on the thermal structure of small to medium-sized lakes. We demonstrated that stratification duration and the mixing regime of lakes may respond strongly to seasonal changes in phytoplankton biomass. One of the major findings of our study was the importance of the plankton dynamics in late spring and early summer for the thermal structure and mixing regime. Superimposed by climatic trends in air temperature, the sudden changes in the spring plankton abundance are able to change completely the seasonal mixing regime of lakes with drastic consequences for water quality. Using modeling approach and future climate scenarios we developed future projections for lake mixing regime shift in European lakes. The simulations predicted that summer stratification should increase in strength and duration in all lakes due to an earlier start in spring, in turn caused by higher winter and spring air temperatures. Mean lake temperatures were projected to increase at 0.10 - 0.11 K decade^-1 in comparison to the projected increase in air temperature of 0.16 K decade^-1. In view of the crucial role of water transparency and solar radiation for the lake mixing, we performed the first comprehensive study on seasonal temperature patterns and recent trends in thermal regime of lakes on the Tibetan Plateau. Surprisingly, we found that the mean lake temperatures of Tibet did not change since 1970ties, nor did the ice cover or stratification. The reason is that the increase in air temperature was offset by the decrease in radiation, probably due to increasing humidity. The study demonstrated that air temperature trends are not directly coupled to lake temperatures and underscores the importance of solar radiation for the thermal regime of high-altitude lakes.
Publications
- (2015). Upwelling of deep water during thermal stratification onset—A major mechanism of vertical transport in small temperate lakes in spring?. Water Resources Research, 51(12), 9612-9627
Pöschke, F., Lewandowski, J., Engelhardt, C., Preuß, K., Oczipka, M., Ruhtz, T., & Kirillin, G.
(See online at https://doi.org/10.1002/2015WR017579) - (2016). Generalized scaling of seasonal thermal stratification in lakes. Earth-Science Reviews, 161, 179-190
Kirillin, G., & Shatwell, T.
(See online at https://doi.org/10.1016/j.earscirev.2016.08.008) - (2016). Planktonic events may cause polymictic-dimictic regime shifts in temperate lakes. Scientific reports, 6, 24361
Shatwell, T., Adrian, R., & Kirillin, G.
(See online at https://doi.org/10.1038/srep24361) - (2017). Extreme weather event triggers cascade towards extreme turbidity in a clear-water lake. Ecosystems, 1-14
Kasprzak, P., Shatwell, T., Gessner, M. O., Gonsiorczyk, T., Kirillin, G., Selmeczy, G., Padisák J. & Engelhardt, C.
(See online at https://doi.org/10.1007/s10021-017-0121-4) - (2017). Seasonal thermal regime and climatic trends in lakes of the Tibetan highlands. Hydrology and Earth System Sciences, 21(4), 1895-1909
Kirillin, G., Wen, L., & Shatwell, T.
(See online at https://doi.org/10.5194/hess-21-1895-2017)
