Increasing prevalence of warm monomictic lakes in France over six decades under climate change

The climatic signal of Marine Isotope Stage (MIS) 11 is well-documented in marine and ice-sheet isotopic records and is known to comprise at least two major warm episodes with an intervening cool phase. Terrestrial records of MIS 11, though of high resolution, are often fragmentary and their chronology is poorly constrained. However, some notable exceptions include sequences from the maar lakes in France and Tenaghi Philippon in Greece. In the UK, the Hoxnian Interglacial has been considered to correlate with MIS 11. New investigations at Hoxne (Suffolk) provide an opportunity to re-evaluate the terrestrial record of MIS 11. At Hoxne, the type Hoxnian Interglacial sediments are overlain by a post-Hoxnian cold-temperate sequence. The interglacial sediments and the later temperate phase are separated by the so-called ‘Arctic Bed’ from which cold-climate macroscopic plant and beetle remains have been recovered. The later temperate phase was deposited during an episode of boreal woodland and is associated with the artefacts, a diverse vertebrate fauna and molluscs. New amino acid geochronological data and biostratigraphical considerations suggest that the post-Hoxnian sequence correlates with late substages of MIS 11. The paper further investigates the correlation of the sequence at Hoxne with the palynological sequences found elsewhere in Europe and adjacent offshore areas.

Today, land and ocean observations are crucial for monitoring climate change. The method of GNSS reflectometry is an opportunistic way to provide low-cost observations of many geophysical parameters. However, although this method has been the subject of numerous research studies, work is still in progress to improve its possibilities and fields of application. This paper focuses on GNSS reflectometry using carrier phase measurements for water altimetry measurements. The difficulties in implementing such a method lie in the need to collect a coherent signal and to solve the integer ambiguity value. In this context, the implementation of innovative signal processing is described, including the correlation of the reflected signals in dedicated software and the prolongation of the coherent integration time to enhance signal coherency. These processes were applied to data collected over Carcans-Hourtin Lake in France to compute the height of the reflection surface which was then compared to in situ GNSS buoy height measurements. The results show that at 300 ft (91.44 m), the differences between the lake heights measured with the buoy and with the reflectometry data can reach less than 1 cm for the L1, E1 and E5 GNSS signals. In addition, the slope of the geoid estimated with the reflectometry data is very consistent with that of the RAF20 geoid model, with a difference of up to less than 2 mm/km.

Vertical profiles of dissolved oxygen (DO) and water temperature (WT) measured bi-monthly for 36 years (1980–2015) near the deepest part of a warm monomictic lake were analyzed with special reference to yearly minimum DO at bottom (DOmin). DOmin changed yearly (3.0 ± 1.2 mg l−1) and significant differences in DOmin were not observed between Period I (1980–1993; cooler and worse in water quality) and Period II (1994–2015; warmer and better in water quality). This unclear trend in DOmin was probably due to the offsetting influences between warming induced by global warming and oligotrophication attempted by local governments etc. for the study period. DOmin was positively correlated with disturbance time (timing of last cold water intrusion observed from Mar to Aug), which could be related to the start of DO depletion at bottom. Thus, the linear model using this parameter could predict yearly DOmin fairly well for the entire study period (r2 = 0.60). In addition, DOmin and time of disturbance were correlated negatively with water density at bottom in Jan and positively with water density equilibrated to air temperature (AT) in Mar. Higher lake water density after full depth mixing advances the disturbance time. In contrast, lower AT in Mar and/or higher density of influent water after Mar delays the time likely due to the larger amount of snowfall in the watershed. Further, DOmin was positively correlated with maximum wind velocity in Sep which probably induced the recovery of DO. Multiple-regression models to predict DOmin using these meteorological and water quality parameters were developed (r2 ≥ 0.38, worse performances than the model using disturbance time) to forecast future trends of DOmin through global warming and/or climate change. Significant influences of water or sediment oxygen demands on DOmin were not detected. We also discuss the applicability of the proposed models.

Analysis of long‐term records of temperature profiles in subtropical Lake Kinneret revealed changes in thermal stratification during the period 1969–1991. Thermocline depth and rate of seasonal thermocline deepening have decreased and the period of stable stratification has increased. These changes appear related to a long‐term decline in mean winter air temperatures, which has produced cooler hypolimnetic waters and increased density gradients across the metalimnion. These changes in thermal structure have been accompanied by increases in hypolimnetic phosphorus concentrations and increased phytoplankton abundances in subsequent years. Although mean thermocline depth in the lake is intermediate relative to predictions based on surface area or fetch from published data on temperate and tropical lakes, winter air temperatures in the region have a salient impact on Lake Kinneret thermal dynamics in any given year.

<p>Located in a highly sensitive climate area, according to the IPCC reports, Lake Kinneret is exposed to extreme changes in the next few decades. Lake Kinneret is a warm monomictic lake, located between subtropical and arid climatic belts. It is thermally stratified throughout most of the year and mixes thoroughly each winter when the epilimnion water temperature reaches equilibrium with the hypolimnion water temperature by surface cooling and turbulence. Using high-resolution atmospheric projections obtained from COSMO-CLM simulation to force a 3D hydrodynamical model, we show significant changes in the stratification and circulation of the lake over the next five decades. As air temperature is expected to rise by up to 2.5<sup>o</sup>C in winter and autumn by 2070, the water column full mixing is expected to be suppressed. Unmixed years are expected to appear more often and full mixing will be re-activated just when cold enough winter conditions allow a full water column turnover. The lack of mixing between the epilimnion and the hypolimnion may create an environment that is oxygen depleted in which very few organisms can live, hence, might strongly affect the lake's present ecological system, its sustainability, and its water quality. Stratification Index analysis results show that Lake Kinneret will remain stratified throughout the year 20%-30% of the time in 2030-2070 under the moderate climate change scenario RCP4.5. However, abrupt cooling of the lake surface due to enhanced latent heat loss is identified, around the year 2065, in the model results, and is expected to restrain the dramatic change in the lake stratification.</p>

Turbulent and radiative energy exchanges between lakes and the atmosphere play an important role in determining the process of lake‐mixing and stratification, including how lakes respond to climate and to climate change. Here we used a one‐dimensional hydrodynamic lake model to assess seasonal impacts of climate change on individual surface heat flux components in Lough Feeagh, Ireland, a deep, monomictic lake. We drove the lake model with an ensemble of outputs from four climate models under three future greenhouse gas scenarios from 1976 to 2099. In these experiments, the results showed significant increases in the radiative budget that were largely counteracted by significant increases in the turbulent fluxes. The combined change in the individual surface heat fluxes led to a change in the total surface heat flux that was small, but sufficient to lead to significant changes in the volume‐weighted average lake temperature. The largest projected changes in total surface heat fluxes were in spring and autumn. Both spring heating and autumnal cooling significantly decreased under future climate conditions, while changes to total surface heat fluxes in winter and summer were an order of magnitude lower. This led to counter‐intuitive results that, in a warming world, there would be less heat not more entering Lough Feeagh during the springtime, and little change in net heating over the summer or winter compared to natural climate conditions, projected increases in the volume‐weighted average lake temperature were found to be largely due to reduced heat loss during autumn.

<p>Evaporation of surface water is critical to the basic functioning of lakes. It directly and, in some cases, substantially modifies the hydrologic, chemical, and energy budgets, making evaporation one of the most important physical controls on lake ecosystems. Predicting lake evaporation response to climate change is, therefore, of paramount importance. Most studies that simulate climate change impacts on lake evaporation have utilised only a single mechanistic model. Whilst such studies have merit, the advantage of applying multiple, independently developed models (i.e., an ensemble approach), is that some of the inherent uncertainties in the individual lake models due to, for example, different model structures, can be reduced thus enabling increased robustness of historic and future projections. In this study, we present results from the Inter-Sectoral Impact Model Intercomparison Project phase 2b (ISIMIP) Lake Sector, where lake evaporation responses to 20<sup>th</sup> and 21<sup>st</sup> century (1901-2099) climate change has been simulated with a suite of independently developed lake models under different climate change scenarios (Representative Concentration Pathways, RCP, 2.6, 6.0 and 8.5). Our study focuses on Lake Kinneret (Israel), a sub-tropical monomictic lake of socioeconomic importance. Our simulations are validated during the historic period with bulk evaporation estimates calculated from high frequency meteorological and in-lake observations. Our results demonstrate that the lake models provide an accurate representation of historical variability in lake evaporation, with promising comparisons of the magnitude, timing and seasonality of evaporative water loss. Future evaporation projections at Lake Kinneret show that evaporation anomalies will increase by the end of the century. We show that multi-model projections of lake evaporation can accurately represent the historic period and hence provide reliable future projections that will be vital for water management.</p>

Nutrient availability and climate have substantial effects on the structure and function of lakes. Predicted changes to climate (particularly temperature) over the 21st century are expected to adjust physical lake functions, changing thermal and nutrient use processes. Both increasing anthropogenic nutrient inputs and net reductions following remediation will also drive ecological change. Therefore, there is an increasing necessity to disentangle the effects of nutrient and temperature change on lakes to understand how they might act in additive and antagonistic ways. This study quantified internal and external nutrient loads at Rostherne Mere, U.K., a deep (zmax = 30 m), monomictic eutrophic lake (average annual total phosphorus >100 μg/L) that has a long, stable period of stratification (c. 8.5 months). A lake biophysical model (PROTECH) was used to assess the effect of changes in these loads and climate change on lake productivity in a factorial modelling experiment. During the summer, phosphorus released from the sediment is largely restricted to the hypolimnion and phytoplankton production is supported by the external load. On overturn, phosphorus at depth is distributed throughout the water column with the elevated concentration persisting to support algal productivity in the following spring. Consequently, the model showed that internal nutrient loading was the main driver of current and future changes in the concentration of phosphorus (responsible for up to 86% P reduction), phytoplankton chlorophyll a and cyanobacterial blooms. However, although the external phosphorus load had a relatively small influence on annual mean phosphorus concentration, it had a statistically significant effect on chlorophyll a concentration, because it supported algal production during summer stratification. Climate had minimal direct impact, but a substantial indirect impact by altering the timing, depth and length of lake stratification (c. 14 days longer by 2100), and therefore altered nutrient cycling and phosphorus availability. In summary, the recovery trajectory at Rostherne Mere is limited by the annual internal soluble reactive phosphorus load replenishment that realistically is unlikely to change greatly on a shorter time‐scale. Therefore, the external soluble reactive phosphorus load has the potential to play an important role as it can be managed further, but is complicated by the indirect impact of climate changing stratification and flushing patterns.

This special section of Hydrobiologia includes a series of papers that document the ecological effects of an intensive programme of management designed to combat eutrophication of a small, monomictic lake with a long history of cyanobacteria blooms (Walsby & McAllister, 1987) and bottom-water anoxia (McColl, 1972). In more than four decades of research on, and management of, lake eutrophication across the globe (e.g., Jeppesen et al., 2005; Davis & Kloop 2006; Smith & Schindler, 2009) and despite pressures from lake acidification, bio-invasions and climate change (Smol, 2008), the effect of human activities acting at a catchment scale remains arguably the most persistent threat to the sustainability and biodiversity of natural lake ecosystems. The potential for these stressors to act in a synergistic way, for example, through increased catchment nutrient loads and climate change exacerbating eutrophication (Trolle et al., 2010), necessitates continued emphasis and vigilance on catchment management measures to address lake trophic status and biodiversity via controls on point and diffuse sources of nutrients, sediments and other contaminants. Recent history has shown that with management practices that target removal of point sources of nutrients as well as changes in, or improvement of, land use practices, it is possible to effect re-oligotrophication of lake ecosystems on time scales of 1–2 decades (Anderson et al., 2005). In other cases, in-lake techniques have been used for eutrophication control, including applications of flocculants (Welch & Cooke, 1999; Cooke et al., 2005), artificial mixing (Schladow & Fisher, 1995) and biomanipulation (Robertson et al., 2000), both with and without active catchment management programmes. In the series of papers in this special issue, the main focus is on the effects of a novel material that was applied to attempt to bring about control of internal nutrient loads in a small eutrophic lake, which has also been subject to an intensive catchment management programme. Control of internal loads was deemed necessary to combat the disproportionately high internal phosphorus load occurring with prolonged seasonal anoxia in bottom waters of the lake. The novel material referred to in this issue was initially known as Z2G1 (subsequently renamed to Aqual P) and was sought to bind phosphorus (P) in sediment porewaters and prevent the release of P Guest editors: D.P. Hamilton, M.J. Landman / Lake Restoration: An Experimental Ecosystem Approach for Eutrophication Control

Periodically asymmetric responses of deep chlorophyll maximum to light and thermocline in a clear monomictic lake: Insights from monthly and diel scale observations

Freshwater mussels are large, long‐lived and can be important contributors to benthic biomass and processes. They are currently one of the most endangered groups of organisms and it is urgent to develop tools to predict their distribution and the potential effect of their decline or disappearance on these ecosystems. Cyr () showed that the distribution of Elliptio complanata (Unionidae) in Canadian Shield lakes is constrained by physical forces. Here we test Cyr's model in a very different group of mussels (Hyriidae), in different types of lakes (warm monomictic lakes) that cover a wider range of sizes and morphologies. We use data on the depth distribution of Echyridella menziesii along 38 depth transects in 11 warm monomictic lakes located in New Zealand to test three hypotheses: (1) wave‐mixed depth and bottom slope are good predictors of the depth of maximum mussel density, (2) thermocline depth does not limit the distribution of mussels in warm monomictic lakes, and (3) the lower boundary of mussel distribution is determined by the mud deposition boundary. Mussels in New Zealand lakes reach their maximum density within the epilimnion, at increasing depths with increasing lake size and increasing site exposure. The only exceptions were found in large lakes or parts of large lakes with shallow bathymetric slopes, where high mussel densities were found in relatively shallow waters. Mussel density peaks are found much deeper in the volcanic New Zealand lakes than in (glacial) Canadian Shield lakes, a discrepancy we hypothesise could be due to lower sediment stability along the steep slopes of volcanic lakes. Mussels appear to have a very broad range of depth distribution in these warm monomictic lakes. The deepest samples collected in deep oligotrophic lakes (down to 12–30 m) all contained mussels, often in substantial numbers (up to 2–186 mussels/m2), and can only offer a minimal estimate of the lower boundary of their distribution. However, the distribution of mussels in highly productive lakes is limited by hypolimnetic anoxia, and therefore by position of the thermocline. Echyridella menziesii are found in a wide range of substrates. Mussels were found below the mud deposition boundary in many lakes, suggesting that the presence of fine flocculent organic sediments does not prevent them from living in deep areas. The distribution of E. menziesii in warm monomictic lakes appears to be governed by relatively simple physical processes. This supports findings for a very different group of mussels in Canadian Shield lakes and suggests that general models could be developed to predict the distribution of mussels in lakes.

Seasonal stratification plays a significant role in contaminant spreading in a warm monomictic lake. If any accident related to pollutant spills occurs in a warm monomictic lake, different risk levels will result depending on the type of lake stratification, location of pollutant release, and hydrodynamic conditions. In this study, seasonal stratification effects on risk of contaminant spreading in the warm monomictic lake was investigated through simulating the distribution of tracers (contaminants) in Lake Constance under different hydrodynamic conditions when wind speed and wind direction were varied. All tracers were released at the lake bed around the drinking water intake of Bodensee-Wasserversorgung near Sipplingen, Germany. The modelling results illustrated that the arrival time of tracers at the drinking water intake was dependent on seasonal stratification, wind direction, wind speed, distance of tracer away from the water intake and the depth where tracer was released. During winter stratification, tracers arrived at the intake earlier than that during summer stratification due to lake mixing in winter. The arrival concentration level of tracer also varied in a complex manner depending on seasonal stratification and hydrodynamic condition.

Abstract. Warming trends in the Laurentian Great Lakes and surrounding areas have been observed in recent decades, and concerns continue to rise about the pace and pattern of future climate change over the world's largest freshwater system. To date, most regional climate models used for Great Lakes projections either neglected the lake-atmosphere interactions or are only coupled with a 1-D column lake model to represent the lake hydrodynamics. This study presents a Great Lakes climate change projection that has employed the two-way coupling of a regional climate model with a 3-D lake model (GLARM) to resolve 3-D hydrodynamics essential for large lakes. Using the three carefully selected Coupled Model Intercomparison Project Phase 5 (CMIP5) general circulation models (GCMs), we show that the GLARM ensemble average substantially reduces surface air temperature and precipitation biases of the driving GCM ensemble average in present-day climate simulations. The improvements are not only displayed from an atmospheric perspective but are also evident in the accurate simulations of lake temperature and ice coverage. We further present the GLARM projected climate change for the mid-21st century (2030–2049) and the late 21st century (2080–2099) in the Representative Concentration Pathway (RCP) 4.5 and RCP 8.5 scenarios. Under RCP 8.5, the Great Lakes basin is projected to warm by 1.3–2.1 ∘C by the mid-21st century and 4.1–5.0 ∘C by the end of the century relative to the early century (2000–2019). Moderate mitigation (RCP 4.5) reduces the mid-century warming to 0.8–1.8 ∘C and late-century warming to 1.8–2.7 ∘C. Annual precipitation in GLARM is projected to increase for the entire basin, varying from 0 % to 13 % during the mid-century and from 9 % to 32 % during the late century in different scenarios and simulations. The most significant increases are projected in spring and fall when current precipitation is highest and a minimal increase in winter when it is lowest. Lake surface temperatures (LSTs) are also projected to increase across the five lakes in all of the simulations, but with strong seasonal and spatial variability. The most significant LST increases occur in Lakes Superior and Ontario. The strongest warming is projected in spring that persists into the summer, resulting from earlier and more intense stratification in the future. In addition, diminishing winter stratification in the future suggests the transition from dimictic lakes to monomictic lakes by the end of the century. In contrast, a relatively smaller increase in LSTs during fall and winter is projected with heat transfer to the deep water due to the strong mixing and energy required for ice melting. Correspondingly, the highest monthly mean ice cover is projected to reduce to 3 %–15 % and 10 %–40 % across the lakes by the end of the century in RCP 8.5 and RCP 4.5, respectively. In the coastal regions, ice duration is projected to decrease by up to 60 d.

To determine how climate-induced changes in hydrology and water level may affect the trophic state (productivity) of stratified lakes, two relatively pristine dimictic temperate lakes in Wisconsin, USA, were examined. Both are closed-basin lakes that experience changes in water level and degradation in water quality during periods of high water. One, a seepage lake with no inlets or outlets, has a small drainage basin and hydrology dominated by precipitation and groundwater exchange causing small changes in water and phosphorus (P) loading, which resulted in small changes in water level, P concentrations, and productivity. The other, a terminal lake with inlets but no outlets, has a large drainage basin and hydrology dominated by runoff causing large changes in water and P loading, which resulted in large changes in water level, P concentrations, and productivity. Eutrophication models accurately predicted the effects of changes in hydrology, P loading, and water level on their trophic state. If climate changes, larger changes in hydrology and water levels than previously observed could occur. If this causes increased water and P loading, stratified (dimictic and monomictic) lakes are expected to experience higher water levels and become more eutrophic, especially those with large developed drainage basins.

In warm monomictic lakes, the hypolimnion is important for accumulating and decomposing organic matter derived from surface production, and the regenerated nutrients will be supplied to the epilimnion through winter vertical mixing. So far, we know little about microbial community composition and function in the hypolimnion when the significant thermal stratification disappears. In this study, we investigated microbial community compositions and functional gene contents by means of metagenomics along a depth profile in the warm monomictic alpine Lake Fuxian during holomictic period. Overall, bacteria were the dominant microbial group at different water depths, while phages had their high relative abundance in the epilimnion. We observed slight thermal but strong chemical stratification even during this typical winter overturn. The anaerobic respiration with nitrate and sulfate as the terminal electron acceptors was accumulated at bottom of hypolimnionin as indicated through metabolic pathway reconstruction. We were able to get 440 metagenome‐assembled genomes (MAGs) and unraveled a high genomic diversity of freshwater pelagic microbiomes along this depth profile. We furthermore defined a new class of “Plancto_FXH1” of Planctomycetes from these MAGs, of which a distinct nitrate reduction operon was identified. Representatives of this phylum mainly thrive in the hypolimnion as previously suspected, but few lineages were detected in the epilimnion. In summary, metagenomics enabled us to find a new group of Planctomycetes, probably involved in denitrification in the hypolimnion in Lake Fuxian, which expand our knowledge on denitrifying bacterial diversity and their denitrification potential in deep freshwater lakes.