Future projections of temperature and mixing regime of European temperate lakes

The physical response of lakes to climate warming is regionally variable and highly dependent on individual lake characteristics, making generalizations about their development difficult. To qualify the role of individual lake characteristics in their response to regionally homogeneous warming, we simulated temperature, ice cover, and mixing in four intensively studied German lakes of varying morphology and mixing regime with a one-dimensional lake model. We forced the model with an ensemble of 12 climate projections (RCP4.5) up to 2100. The lakes were projected to warm at 0.10–0.11&thinsp;∘C&thinsp;decade−1, which is 75&thinsp;%–90&thinsp;% of the projected air temperature trend. In simulations, surface temperatures increased strongly in winter and spring, but little or not at all in summer and autumn. Mean bottom temperatures were projected to increase in all lakes, with steeper trends in winter and in shallower lakes. Modelled ice thaw and summer stratification advanced by 1.5–2.2 and 1.4–1.8 days&thinsp;decade−1 respectively, whereas autumn turnover and winter freeze timing was less sensitive. The projected summer mixed-layer depth was unaffected by warming but sensitive to changes in water transparency. By mid-century, the frequency of ice and stratification-free winters was projected to increase by about 20&thinsp;%, making ice cover rare and shifting the two deeper dimictic lakes to a predominantly monomictic regime. The polymictic lake was unlikely to become dimictic by the end of the century. A sensitivity analysis predicted that decreasing transparency would dampen the effect of warming on mean temperature but amplify its effect on stratification. However, this interaction was only predicted to occur in clear lakes, and not in the study lakes at their historical transparency. Not only lake morphology, but also mixing regime determines how heat is stored and ultimately how lakes respond to climate warming. Seasonal differences in climate warming rates are thus important and require more attention.</p

Planktonic events may cause polymictic-dimictic regime shifts in temperate lakes

Water transparency affects the thermal structure of lakes, and within certain lake depth ranges, it can determine whether a lake mixes regularly (polymictic regime) or stratifies continuously (dimictic regime) from spring through summer. Phytoplankton biomass can influence transparency but the effect of its seasonal pattern on stratification is unknown. Therefore we analysed long term field data from two lakes of similar depth, transparency and climate but one polymictic and one dimictic, and simulated a conceptual lake with a hydrodynamic model. Transparency in the study lakes was typically low during spring and summer blooms and high in between during the clear water phase (CWP), caused when zooplankton graze the spring bloom. The effect of variability of transparency on thermal structure was stronger at intermediate transparency and stronger during a critical window in spring when the rate of lake warming is highest. Whereas the spring bloom strengthened stratification in spring, the CWP weakened it in summer. The presence or absence of the CWP influenced stratification duration and under some conditions determined the mixing regime. Therefore seasonal plankton dynamics, including biotic interactions that suppress the CWP, can influence lake temperatures, stratification duration, and potentially also the mixing regime

Global lake thermal regions shift under climate change

Water temperature is critical for the ecology of lakes. However, the ability to predict its spatial and seasonal variation is constrained by the lack of a thermal classification system. Here we define lake thermal regions using objective analysis of seasonal surface temperature dynamics from satellite observations. Nine lake thermal regions are identified that mapped largely contiguously globally, and robustly even for small lakes. The regions differed from other global patterns, and so provide unique information. Using a lake model forced by 21st century climate projections we found that 12, 27 and 66% of lakes will change to a lower latitude thermal region by 2080-2099 for low, medium and high greenhouse gas concentration trajectories (Representative Concentration Pathways 2.6, 6.0 and 8.5) respectively. Under the worst-case scenario, a 79% reduction in the number of lakes in the northernmost thermal region is projected. This thermal region framework will facilitate the global scaling of lake-research

Climate change and freshwater zooplankton: what does it boil down to?

Recently, major advances in the climate–zooplankton interface have been made some of which appeared to receive much attention in a broader audience of ecologists as well. In contrast to the marine realm, however, we still lack a more holistic summary of recent knowledge in freshwater. We discuss climate change-related variation in physical and biological attributes of lakes and running waters, high-order ecological functions, and subsequent alteration in zooplankton abundance, phenology, distribution, body size, community structure, life history parameters, and behavior by focusing on community level responses. The adequacy of large-scale climatic indices in ecology has received considerable support and provided a framework for the interpretation of community and species level responses in freshwater zooplankton. Modeling perspectives deserve particular consideration, since this promising stream of ecology is of particular applicability in climate change research owing to the inherently predictive nature of this field. In the future, ecologists should expand their research on species beyond daphnids, should address questions as to how different intrinsic and extrinsic drivers interact, should move beyond correlative approaches toward more mechanistic explanations, and last but not least, should facilitate transfer of biological data both across space and time

Global lake responses to climate change

Climate change is one of the most severe threats to global lake ecosystems. Lake surface conditions, such as ice cover, surface temperature, evaporation and water level, respond dramatically to this threat, as observed in recent decades. In this Review, we discuss physical lake variables and their responses to climate change. Decreases in winter ice cover and increases in lake surface temperature modify lake mixing regimes and accelerate lake evaporation. Where not balanced by increased mean precipitation or inflow, higher evaporation rates will favour a decrease in lake level and surface water extent. Together with increases in extreme-precipitation events, these lake responses will impact lake ecosystems, changing water quantity and quality, food provisioning, recreational opportunities and transportation. Future research opportunities, including enhanced observation of lake variables from space (particularly for small water bodies), improved in situ lake monitoring and the development of advanced modelling techniques to predict lake processes, will improve our global understanding of lake responses to a changing climate

Silicon carbide polytype characterisation in coated fuel particles by Raman spectroscopy and 29Si magic angle spinning NMR

The silicon carbide layer of a batch of as-produced TRISO (tristructural isotropic) coated fuel particles with zirconia kernels was characterised by Raman spectroscopy and magic angle spinning nuclear magnetic resonance (MAS-NMR). The techniques were evaluated as a probe for the evolution of SiC local structure as a function of chemical vapour deposition processing. Nuclear magnetic resonance resolved 29Si resonances for multiple hexagonal or cubic silicon local environments, consistent with a mixture of 6H, 15R and 4H polytypes, within a majority (36%) 3C–SiC target structure. Polarised Raman spectroscopy by contrast, showed some evidence of hexagonal and cubic local environments but no evidence for clearly defined hexagonal or orthorhombic polytypes. It was clear from the Raman that there was significant scattering from q > 0 regions of the Brillouin zone, consistent with a loss of translational symmetry associated with stacking faults. Simulation and TEM images suggested that the signals observed in Raman and NMR correspond closer to a random arrangement of SiC layers in which structures similar to the various polytypes occur over short distances. As NMR is a probe of local environment, the signals obtained were similar to those that would come from a mixture of crystallites, each of a well-defined polytype. The NMR data was analysed quantitatively by fitting the spectra of known polytypes and by using a simple model to represent the random arrangement of layers in a heavily faulted crystal

Silicon carbide polytype characterisation in coated fuel particles by Raman spectroscopy and 29Si magic angle spinning NMR

The silicon carbide layer of a batch of as-produced TRISO (tristructural isotropic) coated fuel particles with zirconia kernels was characterised by Raman spectroscopy and magic angle spinning nuclear magnetic resonance (MAS-NMR). The techniques were evaluated as a probe for the evolution of SiC local structure as a function of chemical vapour deposition processing. Nuclear magnetic resonance resolved 29Si resonances for multiple hexagonal or cubic silicon local environments, consistent with a mixture of 6H, 15R and 4H polytypes, within a majority (36%) 3C–SiC target structure. Polarised Raman spectroscopy by contrast, showed some evidence of hexagonal and cubic local environments but no evidence for clearly defined hexagonal or orthorhombic polytypes. It was clear from the Raman that there was significant scattering from q > 0 regions of the Brillouin zone, consistent with a loss of translational symmetry associated with stacking faults. Simulation and TEM images suggested that the signals observed in Raman and NMR correspond closer to a random arrangement of SiC layers in which structures similar to the various polytypes occur over short distances. As NMR is a probe of local environment, the signals obtained were similar to those that would come from a mixture of crystallites, each of a well-defined polytype. The NMR data was analysed quantitatively by fitting the spectra of known polytypes and by using a simple model to represent the random arrangement of layers in a heavily faulted crystal