Displays

Is lake ice disappearing from the Earth system?

Luke Grant², Zeli Tan⁷, Marjorie Perroud⁸, Victor Stepanenko⁹, Bram Droppers³, Annette B.G. Janssen³, R. Iestyn Woolway⁴, Martin Schmid¹¹, Jacob Schewe⁶, Fang Zhao⁶, Gosia Golub⁵, Rafael Macré¹⁰, Don Pierson⁵, Wim Thiery^1,2, Inne Vanderkelen², Sonia Seneviratne¹

¹ ETH Zurich, Institute for Atmospheric and Climate Science, 8092 Zurich, Switzerland.

² Vrije Universiteit Brussel, Department of Hydrology and Hydraulic Engineering, Brussels, Belgium.

³ Wageningen University & Research, Water systems and Global Change, Wageningen, the Netherlands.

­­⁴ Dundalk Institute of Technology, Dundalk, Ireland

⁵ Uppsala University, Dept of Ecology and Genetics / Limnology, Uppsala, Sweden.

⁶ Potsdam Institute for Climate Impact Research, 14473 Potsdam, Germany

⁷ Pacific Northwest National Laboratory, Richland, WA, USA

⁸ University of Geneva, Institute for Environmental Sciences, Carouge, Switzerland

⁹ Lomonosov Moscow State University, Moscow, Russia

¹⁰ Catalan Institute for Water Research, Girona, Spain

¹¹ Eawag: Swiss Federal Institute of Aquatic Science and Technology, Surface Waters - Research and Management - Kastanienbaum, Switzerland

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Proposed session: HS10.9 - Lakes and Inland Seas in a Changing Environment

Corresponding author’s e-mail address: luke.grant@vub.be

Lakes offer manifold ecosystem services and their ice cover stabilizes lake physics, biogeochemistry and ecological processes. The impacts of climate change on lake ice and lake temperatures are well documented for individual lakes and regions through site specific studies. Yet, future global projections of these variables are mostly limited to empirical approaches and singular, coarse-resolution lake model simulations forced by individual climate model outputs. Through the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) we offer an improved global analysis of the lake temperature (2m depth), lake ice cover duration and lake ice thickness projections of six uncalibrated, global-scale lake models to assess the fate of future lake ice and lake temperatures. Lake model simulations were performed on a generic lake within each lake-containing land cell of a global 0.5^o grid. Simulations were at daily resolution and forced by four bias-corrected global climate models from the Coupled Model Intercomparison Project version 5 (CMIP5) for pre-industrial to future periods (1661-2099) and representative concentration pathways 2.6, 6.0 and 8.5. In all scenarios, a nearly unanimous increase in lake temperatures and disappearance of lake ice is projected. The largest reductions in ice thickness and duration will occur in northern latitudes and coastal regions, respectively. Simulations show a greater shift in the timing of ice break-up rather than the onset of ice cover. These geographical and seasonal trends require further statistical analysis to clarify their significance. In general, discrepancies in the projected magnitudes of change for lake ice and temperature under different RCPs underline the benefit of mitigating climate change as preventative to large changes in the biogeochemistry and ecology of lakes.

How to cite: Grant, L.: Is lake ice disappearing from the Earth system?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-395, https://doi.org/10.5194/egusphere-egu2020-395, 2020.

Lakes modify the structure of the atmospheric boundary layer. They can intensify winter snowstorms, increase/decrease surface temperature and amount of precipitation. At European Centre for Medium-Range Weather Forecasts (ECMWF) lake parametrization was introduced in 2015. Inland water bodies (lakes, reservoirs, rivers and coastal waters) are simulated by the Fresh-water Lake model FLake, which was chosen to be included in the
Integrated Forecasting System (IFS) for its intermediate complexity, particularly adapted for numerical weather prediction and climate applications.

In order to eliminate outdated lake location error it was decided to upgrade lake cover field in the IFS model with Global Surface Water Explorer (GSWE) data. Techniques used to adapt GSWE for the use of global NWP modelling and first results that used upgraded lake field will be presented.

How to cite: Choulga, M., Boussetta, S., Kourzeneva, E., and Balsamo, G.: Update of lake cover for NWP modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10668, https://doi.org/10.5194/egusphere-egu2020-10668, 2020.

Saline lakes play a crucial role in regulating the regional climate, supporting unique biodiversity, and providing a diverse range of economic benefits. However, as a result of growing water withdrawals for human use, most of the large saline lakes worldwide are desiccating at a substantial rate. Water level decline and salinity rise affect physico-chemical characteristics of saline lakes including surface albedo. Water surface albedo impacts lake color and evaporation. Here, we investigate spatio-temporal variation of surface albedo over Lake Urmia, in northwest Iran, using the MODerate Resolution Imaging Spectroradiometer (MODIS) albedo product (MCD43D) from 2000 to 2019. Satellite-derived shortwave albedos were validated against in-situ surface albedo data measured at an online net-radiometer station on the lake. We identified two spatial patterns through Lake Urmia: 1) a decreasing trend from the outer shallow zones toward the deep inner parts, and 2) a higher mean albedo of the south arm compared to the north arm in summer. Moreover, the lake albedo varies seasonally with lake level and reaches its peak between September and October. This is mainly due to an increased concentration of total suspended solids (TSS) and phytoplankton (Duanalliea spp.) growth, which accounts for the lake red color between mid-spring and early autumn. Results also revealed that concurrent with the lake level drop since 2000, both lake-averaged surface albedo and its seasonal variation have constantly increased. The increased lake albedo affects net absorbed radiation by the lake and limits lake evaporation. Consequently, we emphasize that for large saline lakes which experience significant areal fluctuations seasonally, the use of a constant albedo to estimate lake evaporation and heat budget is inadequate. Instead, satellite-derived albedo maps encompassing the effect of lake depth, TSS, and phytoplankton growth can be used with confidence. Our findings can contribute to enhanced water, energy, and salt balance models for saline lakes by better estimation of their surface albedo. 

Keywords: Surface albedo, Lake Urmia, MODIS, Water level, Phytoplankton

How to cite: Sima, S. and Darzi, َ.: Two-decade of monitoring spatio-temporal variation of surface albedo over a large saline lake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1172, https://doi.org/10.5194/egusphere-egu2020-1172, 2020.

The Dead Sea is a terminal hypersaline lake with a depth of ~300 m, at a unique location approximately 430 m below sea level. Because of very high salinity of ~300 g/kg of Dead Sea water, the non-linear absorption of solar radiation is of an order of magnitude greater than that in fresh-water lakes. Consequently, by contrast to surface water temperature in fresh-water lakes, Dead Sea surface temperature is influenced by wind speed and water mixing. In the absence of vertical water mixing under weak winds, solar radiation in the summer months leads to significant warming of Dead Sea surface water. Under such conditions, daytime sea surface temperature (SST) could reach land surface temperature (LST) over land areas adjacent to the lake. This could lead to an essential reduction of surface heat flow from land to sea and, consequently, significant surface heating of land areas adjacent to the lake.

Pronounced asymmetry has been obtained in daytime surface temperature between the east and west sides of the Dead Sea. This asymmetry was observed in the summer months, under uniform solar radiation. Our findings are based on MODIS data (2002–2016) on board the Terra and Aqua satellites. MODIS data showed that, on average for the 15-year study period, daytime SST over the eastern part of the lake exceeded that over the western part by 5 °C. This SST asymmetry (observed in the absence of surface heat flow from land to sea at the eastern side) was accompanied by the asymmetry in LST over areas adjacent to the Dead Sea. Specifically, LST over areas adjacent to the east side exceeded that over areas adjacent to the west side by 10 °C. Such LST difference is the characteristic feature of the hypersaline Dead Sea. In addition to MODIS records (on board the two orbital satellites - Terra and Aqua), Meteosat Second Generation records (on board the geostationary satellites) proved the presence of daytime SST/LST asymmetry.

Regional atmospheric warming led to a decrease in the SST asymmetry during the study period. Temperature difference between daytime SST over the east part and that over the west of the Dead Sea steadily decreased at the rate of 0.32 °C decade^-1, based on MODIS/Terra data, and 0.54 °C decade^-1, based on MODIS/Aqua data.

We found that the Weather Forecast and Research (WRF) model distribution of skin temperature over land and sea does not correspond to satellite observations. At midday, over the sea, WRF was incapable of reproducing the observed SST asymmetry. Over land areas adjacent to both the west and east sides of the lake, WRF incorrectly showed that modeled skin temperature increases with its approach to the coastline. The application to modeling of the observed SST/LST asymmetry in existing regional models will improve simulations of atmospheric dynamics over the Dead Sea.

Reference:  Kishcha P., Starobinets B., Pinker R., Kunin P., Alpert P. (2020). Spatial non-uniformity of surface temperature of the Dead Sea and adjacent land areas. Remote Sensing, Special Issue: Lake Remote Sensing, 12(1), 107; doi:10.3390/rs12010107.

How to cite: Kishcha, P., Starobinets, B., Pinker, R., Kunin, P., and Alpert, P.: Asymmetry in surface temperature between the east and west sides of the Dead Sea under uniform solar radiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1847, https://doi.org/10.5194/egusphere-egu2020-1847, 2020.

Recent findings from North America and Europe suggest that anthropogenic salinization of freshwaters is an emergent threat in the Northern Hemisphere. Causes of salinization include irrigation, water abstraction, resource extraction, land clearing and accelerated weathering, road de-icing, and sewage. Our study shows that freshwater salinization is not only restricted to arid and temperate regions, but is also evident in dilute boreal lakes located on the slowly weathering, mainly granitic and gneissic bedrock of the Fennoscandian Shield. We used quantitative paleolimnology to study natural as well as present-day diatom assemblages and diatom-inferred epilimnetic electrical conductivity (a proxy for salinity) of 70 lakes in southern and central-eastern Finland.

According to our results, electrical conductivity increased from natural to present-day conditions in 61 of the 70 lakes, most likely due to agriculture and urbanization. In these 61 lakes, the maximum increase was 11.7 mS m^-1 and average increase was 3.0 mS m^-1, both notable compared to the natural variation between lakes ranging from 2.3 to 7.2 mS m^-1 (average 3.8 mS m^-1). Natural electrical conductivities were higher in lakes located on the clayey coast of Finland than in lakes further inland characterized by catchments rich in sandy till. In conclusion, our study 1) establishes an estimate of natural salinity variation in dilute boreal lakes overlying acidic bedrock, 2) emphasizes that boreal lakes are also threatened by freshwater salinization, and 3) promotes the importance of management actions targeted to prevent freshwater salinization.

How to cite: Tammelin, M., Kauppila, T., and Mäkinen, J.: Anthropogenic salinization of naturally dilute boreal lakes in Finland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1344, https://doi.org/10.5194/egusphere-egu2020-1344, 2020.

River inflows have a major influence on lake and reservoir water quality through their input of momentum, heat, oxygen, sediment, nutrients and contaminants. The fate of these components is controlled by the hydrodynamic processes at the interface between the inflowing river and the receiving lake or reservoir. The inflow can be positively buoyant, leading to a near surface inflow current (overflow), or negatively buoyant, causing it to plunge and form a gravity-driven current near the bed (density current) and/or intermediate current (interflow). In the case of a plunging inflow, the plunging process provides upstream boundary conditions for density currents, which can continue for significant distances along the lakebed. It is therefore important to understand the mixing processes involving entrainment of ambient water into the plunging flow. The hydrodynamics of the plunging process are still poorly understood, especially in laterally unconfined configurations.

Field measurements of a laterally unconfined plunging flow of the Rhône River into Lake Geneva are presented. A vessel-mounted ADCP was used to measure the three-dimensional velocity field of the plunge region. Remote sensing images of the lake surface in the plunge region were captured with a static camera system set up on a nearby mountain overlooking the inflow. Additionally, a mobile camera system attached to a balloon was operated above the inflow to capture high-resolution videos of the inflow. Both camera systems were equipped with RGB and IR cameras. The ADCP measurements and remote sensing images were combined to detect mixing processes in three dimensions.

The remote sensing images show that the incoming river flow forms a distinct plume of sediment-rich water with a triangular shape leading away from the river mouth in downstream direction towards a sharp tip. Horizontal vortical structures visible at the surface, range from larger gyres, over vortex shedding and dipole formation downstream of the plume, to smaller scale structures such as Kelvin-Helmholtz instabilities at the plume edges. The ADCP measurements show the presence of vertical secondary circulation cells in transects perpendicular and parallel to the plume centerline. In addition, intermittent ‘boils’ of sediment rich water up to more than 200 meter downstream of the plume were observed in the images.

How to cite: Thorez, S., Lemmin, U., Barry, A., and Blanckaert, K.: Field characterization of the negatively buoyant inflow of the Rhône River into Lake Geneva, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20465, https://doi.org/10.5194/egusphere-egu2020-20465, 2020.

Measuring primary production (PP) is of major importance to evaluate how lakes are recovering from eutrophication, and better constrain their role in global biogeochemical cycles. The classical sampling method involving radiolabelled carbon incubated in bottles at selected depths is associated with heavy logistics and prevents scientists from sampling the short-term dynamics of PP. Therefore, we investigate alternative methods based on oxygen sensors and optical properties.

To explore the interplay between biogeochemical and physical processes and how they are related to PP in large lakes, we have deployed since October 2018 a sophisticated autonomous profiler in Lake Geneva, Switzerland. The so-called Thetis profiler measures with a centimeter resolution backscattering and fluorescence at discrete wavelengths, hyperspectral absorption and attenuation, hyperspectral reflectance, along with temperature, dissolved oxygen and conductivity. The profiler was deployed over contrasted seasons and recorded these parameters every three hours over the top 50 m of the water column.

Observations revealed large spatiotemporal heterogeneities of optical properties as a result of seasonal and short-term (weekly to sub-daily) physical and biogeochemical processes. Diel cycles in the optical properties representing phytoplankton abundance resulted from night-time convection and grazing and production during the day. Short-lived but strong upwelling events triggered sudden disruptive shifts in the phytoplankton community composition over the fall to winter transition period which, in turn, impacted the oxygen budgets, opening the door to a better understanding of the vertical heterogeneity of PP directly from inherent optical properties. The data also features advected riverine and resuspended sediments during storms and snowmelt, and during upwelling events, respectively.

This in-situ data is now being combined with remotely sensed water quality parameters (OLCI products from Sentinel 3A and 3B) and a three-dimensional hydrodynamic model of Lake Geneva (www.meteolakes.ch) to upscale PP estimates from local to basin scale.

How to cite: Minaudo, C., Odermatt, D., Fernandez-Castro, B., Chmiel, H. E., Lavanchy, S., Bouffard, D., and Wüest, J. A.: High-resolution spatiotemporal heterogeneities of water optical properties in a large lake to infer physical and biogeochemical drivers of primary production, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15063, https://doi.org/10.5194/egusphere-egu2020-15063, 2020.

Lakes and reservoirs are environments with many important uses in social activities, such as hydropower generation, water supply, landscape element, irrigation and flood containment. An ecosystem with so many environmental services needs to have its water quality well preserved, and besides that, inland waters have a key role in climate change studies, because of their faster response to variability in external forces.

Water circulation in a reservoir is the result of a combination of morphometry and energetic driving forces. A lake’s hydrodynamic characteristics vary with morphometric, meteorological and hydrological conditions. The heat balance involves exchanges at the surface, which are actively mixed and energised by the transfers occurring at the air-water interface, and also the exchanges in the water-soil interface near the banks and bottom, which depend on currents and internal waves; while in the main water column, the heat transfer is influenced by light penetration.

In the context of climate changes and water scarcity worldwide, the development of tools to better understand, maintain and improve water quality in lakes and reservoirs becomes an essential ally to environmental research and limnology. This research aims to demonstrate the lake mixing regime by a different approach, testing two different methods to forecast the climate change influence on a lakes’ mixing regime, using data from climate models.

The first tool is a thermal limit curve proposed by the authors which can forecast water column thermal limits for stability or mixing condition in a lake, based on wind speed, radiation and water profile temperature data. The second applied tool is a quasi-3D mathematical model, well known and reputed in the simulation field.

The results obtained for different experimental lakes in temperate and tropical zones showed that both methods have a good performance in representing lakes hydrodynamics accurately. The curve allows a faster response and minor need for data input, on the other hand, the quasi-3D models are capable to produce more detailed results. Possibly in the lakes’ management, it would be more indicated the use of those two methods together, using the curve to analyse faster the period's trend and be able to delimitate the exact period which needs more detailed studies.

The climate change simulations conducted for two experimental lakes considering different scenarios of climate changes showed the driving forces' strong influence on the lake's mixing regime. The number of mixing events is an interesting proxy to analyse this influence. It was greater in the pessimistic scenarios but still less than in the current situation. This means longer periods of stratification, which can cause dissolved oxygen depletion in the deeper layers.

The pessimistic scenarios have mixing events with greater amplitude, which results from a powerful stratification in previous periods. Mixing events with greater amplitudes create higher vertical velocities, resuspending more organic load and dropping dissolved oxygen levels along the water column, impairing the water quality.

How to cite: Scarati Martins, J. R., Ferrer Amorim, L., Ferreira Nogueira, F., and Paiva da Silva, F.: Tools to forecast climate change effects on lakes's water column, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3159, https://doi.org/10.5194/egusphere-egu2020-3159, 2020.

Lakes play a major role on local climate and boundary layer stratification. At global scale, they have been shown to have an impact on the energy budget, (see for example Le Moigne et al., 2016 or Bonan, 1995 ) . To represent the energy budget of lakes at a global scale, the FLake (Mironov et al, 2008) lake model has been coupled to the ORCHIDEE land surface model - the continental part of the IPSL earth system model. By including Flake in ORCHIDEE, we aim to improve the representation of land surface temperature and heat fluxes. Using the standard CMIP6 configuration of ORCHIDEE,  two 40-year simulations were generated (one coupled with FLake and one without) using the CRUJRA meteorological forcing data at a spatial resolution of 0.5°. We compare land surface temperatures and heat fluxes from the two ORCHIDEE simulations and assess the impacts of lakes on surface energy budgets. MODIS satellite land surface temperature products will be used to validate the simulations. We expect a better fit between the simulated land surface temperature and the MODIS data when the FLake configuration is used. The preliminary results of the comparison will be presented.

How to cite: Bernus, A., Ottle, C., and Raoult, N.: Contribution of lakes in the ORCHIDEE land surface model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18204, https://doi.org/10.5194/egusphere-egu2020-18204, 2020.

Lakes have significant hydraulic connections with their surrounding groundwater, thus the coupled simulation of lake and groundwater interactions is an important component of the numerical modelling of groundwater flow. This paper proposes a new approach for simulating lake-groundwater interactions (Sloping Lakebed Method, short for SLM) based on the block-centered finite difference method. In this approach, a discretization of the lakebed elevation in the vertical direction is conducted independently of the spatial discretization of the aquifer system, greatly simplifying the subdivision of the aquifer system. The lakebed is generalized by a slope across each lake cell, and the lake grid cells are classified as fully submerged, partially submerged, and unsubmerged to simulate the interactions between the lake and the aquifer according to the relative elevations of the lake surface and lake bed. Transitions between different states ensure the continuity of the boundary conditions, improving the convergence of the calculation process. The proposed method accounts for all source and sink terms for the lake, establishing a governing equation for lake water balance. The lake stage can then be obtained by solving this equation. A series of tests were run verifying the rationality and superiority of the SLM, suggesting that it provides similar results to those of the well-established MODFLOW LAK3 Package developed by the United States Geological Survey. However, the SLM has significant advantages over LAK3 in its ease of use and calculation stability.

How to cite: Zhang, B., Lu, C., Wang, J., and Jarsjö, J.: A new approach to simulating lake-groundwater interactions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5420, https://doi.org/10.5194/egusphere-egu2020-5420, 2020.

The radiocarbon, together with several isotopes of noble gasses such as ⁴¹Ar, ^85mKr, ⁸⁷Kr, ⁸⁸Kr, ¹³³Xe, ¹³⁵Xe, ^135mXe, is one of the main radionuclides discharged to the environment by nuclear industry. Different materials of neutron moderator, composition of reactor fuel and constructions, and the concentration of target nuclei for activation in these constructions leads to variations in amounts of discharged ¹⁴C. The RBMK (Russian Acronym for ”Channelized Large Power Reactor”) is a graphite-moderated boiling water channel-type reactor with the principle of electricity generation the same as for boiling water reactors (BWRs). ¹⁴C produced in this type of reactor is released mostly in a gaseous carbon dioxide form and in much smaller quantities as liquid effluents [1].

The Ignalina Nuclear Power Plant (INPP) in north-eastern Lithuania, operated two RBMK-1500 Units (design electric power 1500 MW_e): Unit 1 came online in December 1983 and was shut-down on December 31, 2004 whereas Unit 2 started operation in August 1987 and was shut-down on December 31, 2009.

The INPP used Lake Drūkšiai as a cooling pond by the closed cooling loop and for technological water supply, as well as for controlled industrial drainage discharges from the plant.

In this study radiocarbon activity measurements were performed in two organic fractions of the lake sediment core layers: alkali- soluble and alkali insoluble. During the period of 1945-1999, the radiocarbon activity in both fractions exhibited the parallel course. During first 14 years of operation, excess of ¹⁴C activity in both fractions reached 0.34÷0.41 10⁹ Bq. The period from 1999 to 2013 is distinctive by considerable increase of ¹⁴C activity in alkali soluble fraction (by 2.4 10⁹ Bq) but in alkali insoluble fraction this increase did not exceeded 0.21 10⁹ Bq. However, this radiocarbon distribution in both fractions during this period could be related with additional releases of ¹⁴C incorporated alkali soluble organic compounds used in technological process of NPP operation and/or maintenance. However, no information about increased activity levels of aquatic effluents or different chemical agents used could be found in INPP reports.

[1] IAEA, Management of waste containing tritium and carbon-14, Technical Report Series No. 421, International Atomic Energy Agency, Vienna, 2004.

How to cite: Barisevičiūtė, R., Maceika, E., Juodis, L., Ežerinskis, Ž., Šapolaite, J., Butkus, L., Rakauskas, V., Mažeika, J., and Remeikis, V.: Radiocarbon distribution in sediments of the cooling pond of the RBMK type nuclear power plant , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5196, https://doi.org/10.5194/egusphere-egu2020-5196, 2020.

Lake Urmia (LU) has been the second-largest hypersaline lake in the world located in the northwest of Iran that encountered a drastic drawdown in the water level over the past two decades. Accurate estimation of the water balance components, particularly evaporative loss from the water surface as the main component of the LU water budget, is important for the lake water management and restoration programs. In this study, long-term evaporation from the LU surface was estimated between 2000 and 2017 using the Bowen Ratio Energy Balance (BREB), Priestley-Taylor, DeBruin-Keijman, Penman, and Stephans-Stewart methods that leverage meteorological observations and Terra Moderate Resolution Imaging Spectroradiometer (MODIS) satellite earth observation data. The impact of water salinity on evaporation was also considered through dynamic water activity coefficient as well as water density. Given observations from the saline water pan evaporation located in the Lake vicinity, the Debruin-Keijman method yielded the most accurate estimation with the correlation coefficient of 0.93, the root means square error (RMSE) of 121 mm in annual scale. The minimum and maximum annual evaporation were estimated as 783 mm and 1216 mm in 2011 and 2000, respectively, with the annual average evaporation for the entire period, were 1077 mm. By extracting monthly lake surface area from image classification techniques, monthly and annual volumetric evaporation were estimated, indicating that the annual average of the lake volumetric evaporation approximates to 3.6 BCM, which exceeds LU annual environmental requirement water of 3.1 BCM.

How to cite: Abdoli, M., Danesh-Yazdi, M., Arabzadeh, A., Javadian, M., and Tajrishy, M.: Estimation of Lake Urmia Evaporation from the Combined Ground-based and Satellite Imagery Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9392, https://doi.org/10.5194/egusphere-egu2020-9392, 2020.

Lake Urmia, located in the northwest of Iran, had an initial volume of about 19 km³ and a surface area of 5,700 km² (Alipour, 2006). Once one of the largest hypersaline lakes in the world, this UNESCO Biosphere Reserve site currently shows a remarkable water level decline. About 70% of the lake area (Tourian et al., 2015) and more than 90% of its volume were lost between 2000 and 2014 (Schulz et al., 2020). The lack of a precise water balance of the Lake Urmia catchment is one of the challenges authorities are facing in their efforts to restore the lake to its ecological level. Here, key issues are that lake evaporation rates are mostly assumed and that evaporation of shallow groundwater from dried-up areas (up to 3,000 km²) is often ignored. The objective of this study is to obtain evaporation rate estimates for the dried-up parts of the Urmia lake bed. To this end, we set up a laboratory experiment with undisturbed soil columns collected from dried-up areas of the lake. With the help of a custom-made low-cost environmental chamber, the columns were subject to day- and night-time weather conditions typical for the area. Performed measurements comprise water level logging and monitoring of mass losses from the columns due to evaporation. First experimental results will be presented.

References

Alipour, S., 2006. Hydrogeochemistry of seasonal variation of Urmia Salt Lake, Iran. Saline Systems 2, 9. doi:10.1186/1746-1448-2-9

Schulz, S., Darehshouri, S., Hassanzadeh, E., Tajrishy, M., Schüth, C., 2020. Climate change or irrigated agriculture – what drives the water level decline of Lake Urmia. Sci. Rep. 1–10. doi:10.1038/s41598-019-57150-y

Tourian, M.J., Elmi, O., Chen, Q., Devaraju, B., Roohi, S., Sneeuw, N., 2015. A spaceborne multisensor approach to monitor the desiccation of Lake Urmia in Iran. Remote Sens. Environ. 156, 349–360. doi:10.1016/j.rse.2014.10.006

How to cite: Darehshouri, S., Michelsen, N., Schüth, C., and Schulz, S.: Estimation of shallow groundwater evaporation from dried-up areas of Lake Urmia, Iran, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21046, https://doi.org/10.5194/egusphere-egu2020-21046, 2020.

Seismoacoustic survey is a necessary and important method for studying a water area, both from the point of view of studying the structure of bottom sediments bedding, and choosing the optimal point for selecting core columns. In the process of sedimentation, the bottom sediments of modern lakes react very sensitively to climate changes due to changes in the nature and rate of incoming terrigenous material, changes in the lake level and its area, and changes in its bio productivity. Large lakes are investigated using large ships and installations; however, the study of small water areas is rare for a number of reasons. Nevertheless, new detailed results of the regional climate changes can be obtained in such lakes. In 2019, it was conducted detailed seismic investigations of 6 lakes in the eastern part of the Republic of Bashkortostan (Russia) using the “Boomer” seismic complex. For field work we used an inflatable boat and an environmentally friendly electric motor. All lakes are relatively close to each other. However, different structure of bottom sediments bedding on seismic sections was obtained in all lakes. For example, in Lake Sabakty, a layered thickness of bottom sediments of up to 6 m was observed. In some places there are gas-saturated sediments. A slight uplift is observed in these areas, probably related to the “swelling” of bottom sediments as the gas concentration in the sediments increases. In Lake Bannoye, on seismic section observed a bedding structure on a slope of greater thickness (more than 8m) than in the deepest plane part (thickness 2m). 8 cores were selected in these 2 lakes for laboratory investigations. In the remaining 4 lakes, bottom sediments with lower thickness, with the presence of gas-saturation, or sediments were not detected. This work was funded by the Russian Science Foundation under grant № 18-17-00251.

How to cite: Krylov, P. and Nurgaliev, D.: Seismoacoustic research of lakes bottom sediments of the eastern part of the Republic of Bashkortostan (Russia)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8061, https://doi.org/10.5194/egusphere-egu2020-8061, 2020.

To investigate the impacts of fish farm emissions, we coupled the biogeochemical C-N-P-Si-O-S-Mn-Fe transformation model BROM with a 2-Dimensional Benthic-Pelagic transport model (2DBP), considering vertical and horizontal transport in the water and upper 5 cm  sediments along a 10000 m transect centered on a fish farm. The 2DBP model had 25 m horizontal resolution and was forced by hydrophysical model data for the Hardangerfjord in western Norway. The model predicted significant impacts on seafloor biogeochemistry up to 500 meters from the fish farm (e.g., increased organic matter in sediments, oxygen depletion in water and sediments, denitrification, metal and sulfur reduction) as well as detectable decreases in oxygen and increases in ammonia, phosphate and organic matter in the water near to the fish farm cages. The model results are compared with field data from the Hardangerfjord in August 2016 and indicated satisfactory model performance.

How to cite: Yakushev, E. and Berezina, A.: Modeled based analysis of fish farm emissions impact on the fjord biogeochemistry , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19794, https://doi.org/10.5194/egusphere-egu2020-19794, 2020.

Lake Issyk-Kul is the World's second deepest mountain lake (depth 668 m), containing over 1700 km³ of brackish (about 6 g kg^-1) water. It has been demonstrated by analyses of chemical tracers that the lake mixes very intensively, with the bottom water residence time of only about 10 years [Hofer et al., 2002], although the mechanisms resonsible for such a rapid renewal remain unknown. Some previous studies also suggested that the deep layers of the Issyk-Kul were subject to significant warming at decadal scales in response to climate forcing, however, direct measurements of the lake's thermohaline structure are very sparse.

Field measurements carried out in 5 consecutive expeditions (2015-2019) made it possible to establish previously unknown features of thermohaline fields and circulation of Lake Issyk-Kul. The most detailed ever salinity distribution maps were constructed. An area of ​​slightly increased salinity was found in the central part of the lake, the specific “dipole” shape of which indicates the existence of not only a general cyclonic circulation, but also two separate gyres of a smaller, sub-basin scale (which is partly confirmed by direct measurements of the current velocity). It has been established that, generally speaking, salinity fields in Issyk-Kul are extremely conservative - their interannual and seasonal changes, as well as spatial variability throughout the lake (with the exception of estuarine regions), are usually measured only in hundredths of g kg^-1. An important result of the project is the discovery of a subsurface maximum of salinity persisting from year to year at depths from 70 to 130 m. As shown on the basis of balance estimates and then confirmed by analysis of direct measurements of current velocities, the autumn-winter differential cooling leads to the fact that in canyons (i.e., the ancient river channels) in the eastern littoral region, a significant amount (up to 1 km³) of cold coastal waters freshened by river runoff enters the bottom layers of the central part of the lake. These waters are then mixed with the more saline waters lying above, which, in a situation where the upper layer of the lake is also desalinated by river runoff, leads to the appearance of a salinity maximum at intermediate depths. Our measurements do not confirm the manifestations of global warming in the form of an inter-decadal temperature increase in the deeper layers of Lake Issyk-Kul, which was previously reported: the current (in 2018) temperature at a depth of 500 m exactly coincided with that noted in the 2003 measurements, namely about 4.44^oC. However, one can point to a very weak (about 0.03 g kg^-1) increase in the salinity of the bottom layer over the past 40 years. Based on the analysis of water samples taken from the lake and from the 12 main tributary rivers, improved estimates of the nutrient budget were also obtained.

How to cite: Zavialov, P.: New data on vertical structure and variability of Lake Issyk-Kul, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5426, https://doi.org/10.5194/egusphere-egu2020-5426, 2020.

In terms of water mass, salt lakes occupy 104 000 km3 (47 % of the total water volume of all lakes on the Earth), which is only slightly below that of fresh water lakes - 125 000 km3 (53%). Their waters are of certain economic importance, since they are used as raw materials for the extraction of valuable components and for therapeutic purposes. They also have a great influence on climate formation in the region. The drying out of the lakes entails an increase climate continentality. It also leads to an increase of frequency of dust and salt storms and storms that carry toxic substances over long distances from the source. This negatively affects public health. The drying of the Aral and Dead Seas is one of the largest environmental disasters of the 20th century. The reasons for their drying out are both natural such as global warming and aridization and anthropogenic factors such as diversions of river flows for the economic needs of the population of the regions. A sharp decrease in river runoff leads to a rapid drop in sea level, as well as to sharp interannual changes in the thermohaline structure and chemical composition of water. The annual runoff of the rivers has long ceased to fulfill the water resources of these water bodies.
We report the results of water sampling campaigns conducted in the Aral Sea (2014-2019) and the Dead Sea (2017-2019). The main ionic composition, salinity, density and other parameters of the waters of the Aral and the Dead Seas were obtained. We compare the hydrochemical characteristics of these water bodies and their changes during with historical data and with each other. The studied natural water bodies are terminal lakes, characterized by high salinity of water, which is many times higher than the salinity of ocean waters. The ratios of the main ions in the studied sources differ significantly between water bodies, as well as from similar ratios in the oceans. We determined ionic composition of these water samples using potentiometric titrator Titrando 905 (Metrohm). The density of samples was determined by the density meter DMA 5000M (Anton Paar). Currently, the Aral Sea is a complex of separate residual lakes with diverging hydrological and hydrochemical characteristics. The Dead Sea today is divided into two basins, the southern part of which is used for industrial purposes. Significant changes of the ionic composition of water with time were registered in both lakes. However, in the waters of the Aral Sea, these processes are much more intense and rapid than those in than the waters of the Dead Sea.

How to cite: Andrulionis, N., Izhitskiy, A., Gertman, I., Yakushev, E., and Zavialov, P.: Changes in the basic ionic composition and other parameters of the Aral Sea and the Dead Sea waters during their drying, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7496, https://doi.org/10.5194/egusphere-egu2020-7496, 2020.

A thermistor-string-based Snow and Ice Mass Balance Array (SIMBA) has been used for monitoring snow and ice mass balance in Lake Orajärvi in northern Finland since 2009. The environment temperature (ET) and heating temperature (HT) have been measured vertically every 2 cm by temperature sensors that have a minimum resolution of ±0.0625°C. The first- and second-order spatial derivatives (FOD/SOD) of SIMBA-ET, i.e. temperature changes, in 2 cm fixed interval were calculated and used to identify the air/snow, snow/ice and ice/water interfaces. The derivative threshold values were adapted from an existing SIMBA-algorithm. 
We first investigated SIMBA data obtained in winters 2011/2012, where in situ snow and ice thickness were available. The snow depth and ice thickness calculated by our method and from manual analyses were close to each other. Both results were comparable with in situ observations. The snow and ice thickness were, then, derived from SIMBA data observed since 2009. An upward-moving snow/ice interface was detected for all winters. Snow-ice contributed around 30 - 65% of the total average lake ice thickness. The snow depth on lake ice was only one-third of that on land nearby. The SIMBA-HT was capable to identify snow/ice and ice/water interfaces that were visible form SIMBA-ET profiles during cold conditions. 

How to cite: Cheng, Y., Cheng, B., Zheng, F., Vihma, T., Vihma, T., and Kontu, A.: Seasonal snow and ice thickness measured by SIMBA thermistor string based ice mass-balance buoy on an Arctic lake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21460, 2020.

Internal and external sources of CO₂ release in subarctic lakes

Most lakes are supersaturated with CO₂, and are thus net sources of CO₂ to the atmosphere. Lakes can only sustain annual net CO₂ emissions through either 1) mineralization of terrestrial carbon flushed into the system or 2) transfer of inorganic carbon from the terrestrial landscape, but the relative contribution of these sources are unknown. We studied 14 lakes in the Southern Swedish Scandes during the open-water season. We used loggers to infer CO₂-emissions and net internal production of CO₂ during the open water season, and DIC mass balance to determine winter CO₂-buildup and emission at spring ice-off. All lakes emitted CO₂ to the atmosphere on an annual basis, even though some took up CO₂ during the open water season. Emissions during spring ice off dominated (mean 7,5 g C m^-2 yr^-1) the yearly CO₂-balance of the lakes. The CO₂-emissions from the lakes increased with increasing DOC concentrations, similar to findings from many other regions. However, only a small part of the CO₂-emissions originated from the internal production of CO₂ (mean 31%), implying dominance of external CO₂ sources in these lakes. The external source contribution also increased over the DOC gradient, from 30% in clear lakes to 91% in high DOC-lakes. Our results highlight the importance of seasonal and landscape factors in maintaining net CO2 release from lake waters.

How to cite: Verheijen, H., Klaus, M., Seekell, D., and Karlsson, J.: Internal and external sources of CO2 release in subarctic lakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16226, https://doi.org/10.5194/egusphere-egu2020-16226, 2020.

Seasonal ice cover on lakes and polar seas creates seasonally developing boundary layer at the ice base with specific features: fixed temperature at the solid boundary and stable density stratification beneath. Turbulent transport in the boundary layer determines the ice growth and melting conditions at the ice-water interface, especially in large lakes and marginal seas, where large-scale water circulation can produce highly variable mixing conditions. Since the boundary mixing under ice is difficult to measure, existing models of ice cover dynamics usually neglect or parameterize it in a very simplistic form. We propose a model of the turbulent energy budget in the stably stratified boundary layer under ice, based on the length scale incorporating the dissipation rate and the buoyancy frequency (Dougherty-Ozmidov scaling). The model was verified on fine-scale measurements in Lake Baikal and demonstrated a good agreement with data. The measured ice-water heat fluxes in were among the largest reported in lakes (up to 40 W m⁻²) and scaled well against the proposed relationship. The model yields a scaling relationship for the ice-water heat flux as a function of the shear velocity squared that suggests the traditional bulk parameterizations may significantly underestimate the ice-water heat flux, especially at strong under-ice current velocities. The ultimate result consists in a strong dependence of the water-ice heat flux on the shear velocity under ice. 

How to cite: Kirillin, G., Aslamov, I., Granin, N., and Zdorovennov, R.: A new similarity model for the stratified under-ice boundary layer in lakes and its application to ice-covered Lake Baikal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7478, https://doi.org/10.5194/egusphere-egu2020-7478, 2020.

Differential cooling occurs in the littoral region of lakes, during calm and cold nights in summer and continuously in fall/ winter. For uniform heat loss over the lake surface, shallower regions cool faster than deeper regions leading to horizontal density gradients. Nearshore waters become negatively buoyant and start to plunge creating a cold downslope density current that can reach the pelagic zone. This cross-shore flow, also referred to as “thermal siphon”, has the potential to transport biogeochemical constituents offshore and deeper into the water column. However, its significance for the lake ecosystem remains unknown. Here we combine high resolution numerical simulations and field observations to evaluate the global significance of lateral boundaries and differential cooling on the lake dynamic. We focus on a small elongated lake: Lake Rotsee (Switzerland) and investigated the dynamic of the density current as well as the consequences of this flow such as the flushing of the littoral water.

How to cite: Bouffard, D., Ramón, C., Doda, T., and Ulloa, H.: Density currents induced by differential cooling in lake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19149, https://doi.org/10.5194/egusphere-egu2020-19149, 2020.

The surface mixed layer in lakes is where phytoplankton grow and where most of the primary production occurs. Knowledge of the thickness of the mixed layer is essential to estimate for instance primary productivity and to interpret remote sensing measurements, because it determines the mean light supply and indicates how homogeneous the water column is. Modelling studies, primarily in the ocean, have concluded that the mixed layer will shoal as a result of climate warming, but the empirical evidence does not support this. Here we seek to determine how climate change affects the mixed layer thickness and mean underwater irradiance in lakes. We use an ensemble modelling approach to simulate mixed layer depth in 3 warming scenarios (RCP2.6, 6.0, 8.5) in about 50 lakes across the globe using the hydrodynamic model Flake forced by four downscaled global climate models. Results indicate that warming has little direct effect on the mixed layer depth. Mean underwater light in the mixed layer was nevertheless projected to increase as a result of the global radiation increases in the global climate models.

How to cite: Shatwell, T. and Kirillin, G.: Effect of climate warming on the mixed layer depth in lakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22261, https://doi.org/10.5194/egusphere-egu2020-22261, 2020.

Metalimnetic oxygen minima are observed in many lakes and reservoirs, but the mechanisms behind this phenomenon are not well understood. Thus, we simulated the metalimnetic oxygen minimum (MOM) in the Rappbode Reservoir (Germany) with a well-established two-dimensional water quality model (CE-QUAL-W2) to systematically quantify the chain of events leading to its formation. We used high-resolution measured data to calibrate the model, which accurately reproduced the physical (e.g. water level and water temperature), biogeochemical (e.g. nutrient and oxygen dynamics) and ecological (e.g. algal community dynamics) features of the reservoir, particularly the spatial and temporal extent of the MOM. The results indicated that around 60% of the total oxygen consumption rate in the MOM layer originated from benthic processes whereas the remainder originated from pelagic processes. The occurrence of the cyanobacterium Planktothrix rubescens in the metalimnion delayed and slightly weakened the MOM through photosynthesis, although its decaying biomass ultimately induced the MOM. Our research also confirmed the decisive role of water temperature in the formation of the MOM since the water temperatures, and thus benthic and pelagic oxygen consumption rates, were higher in the metalimnion than in the hypolimnion. Our model is not only providing novel conclusions about the drivers of MOM development and their quantitative contributions, it is also a new tool for understanding and predicting ecological and biogeochemical water quality dynamics.

How to cite: Mi, C., Shatwell, T., and Rinke, K.: The formation of a metalimnetic oxygen minimum exemplifies how ecosystem dynamics shape biogeochemical processes: A modelling study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22305, https://doi.org/10.5194/egusphere-egu2020-22305, 2020.

  Cyanobacterial blooms caused by eutrophication in Lake Taihu, China are recognized as highly heterogeneous spatiotemporally. It is assumed that the high spatiotemporal heterogeneity of algal blooms is determined by divergence/ convergence processes in the fluid medium. To address this issue, three episodes of the dominant spatial patterns of hourly simulated divergence fields of current in Lake Taihu in July of 2012 were analyzed using a hydrodynamic numerical model combined with the Empirical Orthogonal Function (EOF) method. The results showed that, on days that blooms occurred, the first two EOF modes explained 89.4% of the variability and the dominant spatial patterns of stronger convergence zones were in agreement with the regions of bloom occurrence and accumulation. When no blooms occurred, the first EOF mode explained 72.5% of the variability and divergence zones were dominant in the lake. Both the simulated hourly average divergence field and the first EOF mode in the time interval in which blooms occurred further confirmed that blooms accumulate in the current convergence zones. These findings explain the dynamic mechanism of occurrence of cyanobacterial blooms and will facilitate forecasting of short-term blooms for protecting drinking water supplies and managing risk.

How to cite: Li, W. and Qin, B.: Dynamics of spatiotemporal heterogeneity of cyanobacterial blooms in large eutrophic Lake Taihu, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9052, https://doi.org/10.5194/egusphere-egu2020-9052, 2020.

Lake Kivu, located on the border of Rwanda and the Democratic Republic of Congo, is a very peculiar lake in several aspects. The meromictic lake shows a vertical stratification dominated by high salt concentrations of up to 6 ‰ resulting in a very thick monimolimnion of 420 m (max depth ~492 m). This extremely large non mixing part of the lake functions as a reservoir for very high concentrations of volcanogenic gases like methane and carbon dioxide (up to 20 and 100 mmol/l respectively) resulting in a growing hazard for millions of local residents. Our aim of this study is to get insights into the hydrological dynamics, solute transport and the lakes mixing behavior utilizing radiometric dating with ³⁹Ar.

The noble gas isotope ³⁹Ar (t_1/2 = 269 a) covers a unique time span for studying the dynamics of aquatic and glacial systems of the last millennium. Although this tracer has been acknowledged for decades, studies so far are limited by its low abundance, little radioactivity and hence huge required sample sizes (~1000 L water). Until today environmental routine measurements are mainly confined to groundwater reservoirs, where nearly unlimited sampling is possible. The application of techniques from atomic physics using a magneto optical atom trap (MOT) solves the problem by reducing sample volume requirements by several orders of magnitude. The problem of the very low isotopic abundance of 10^-16 is resolved by resonant multi-photon scattering of ³⁹Ar in the MOT. This technique named Argon Trap Trace Analysis with its very low minimal sample size of 0.5 cm³STP pure argon enables easy sample handling in the field as well as common sampling procedures like Niskin bottles for aquatic systems, drill core sampling for glacial systems or as in the case of Lake Kivu spray chamber gas sampling in remote places. It is thus a door opener for new geophysical research fields that were excluded from radio-argon dating so far.

Here we present our most recent results of sampling campaigns in 2018 and 2019 using samples of about 25 – 40 L gas-water mixtures corresponding to 0.5 – 10 cm³STP pure argon showing surprisingly high ages for the lake water.

How to cite: Schmidt, M., Wachs, D., Arck, Y., Bärenbold, F., Ringena, L., Robertz, J., Kersting, A., Schmid, M., Oberthaler, M. K., and Aeschbach, W.: Investigating the dynamics of Lake Kivu with quantum technology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20126, https://doi.org/10.5194/egusphere-egu2020-20126, 2020.

Shallow lake was characterized by distinct hydrology, biochemistry and ecology that influence the carbon balance. This study explored methane and carbon emission responses to water level fluctuation in shallow lake, and also addressed its legacy for wetland restoration. This study used the process-based biogeochemical model, denitrification-decomposition (DNDC) model to simulate the alteration of methane and carbon emission with water level fluctuation in the Baiyangdian Lake (BYD Lake). The results showed: (i) compared with the observed carbon flux, the DNDC model can presented a suitable results in capturing the dynamics of methane and carbon dioxide, and the daily rate of carbon dioxide and methane emission showed sensitive to water fluctuation when it ranged from -10 cm to 10 cm; (ii) for the carbon dioxide, the annual flux showed a decline trend when the duration prolonged from 10 days to 40 days, and then an increasing trend while the duration prolonged to 90 days furtherly, with a lowest flux when the duration is 40 days, while for the methane, annual emission increased with inundation lasting time and the flux changing from -2.27 kg C/ha/y to 1.57 kg C/ha/y; and (iii) The flux of carbon dioxide and methane increased when water level fluctuation frequency increased, for a certain water level fluctuation frequency, carbon dioxide flux is lowest in January and February, and methane flux is negative from December to March of the following year. All of these results indicated that water level fluctuation (e.g., magnitude, duration and frequency) affected the carbon dioxide and methane flux, which will help to reduce the emission of carbon dioxide and methane by regulating ecological water transfer.

Keywords: shallow lake, carbon emission; DNDC; water level fluctuation

Acknowledgments

This study was supported by the National Key R&D Program of China (No. 2018ZX07110001, No. 2017YFC0404505) and the National Natural Science Foundation of China (No. 51579008).

How to cite: Yuan, X. and Liu, Q.: Methane and carbon dioxide dynamics affected by water-level fluctuations in a shallow lake: Implications for wetland restoration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6437, https://doi.org/10.5194/egusphere-egu2020-6437, 2020.

Human activities make significant impacts on water environments and can deteriorate environmental situation of water bodies. It’s remarkably important to control discharging pollutant into the environment. Our research team have conducted research on water environment in the Taihu Lake area in China. We carried out field investigation with sampling and measurement followed by analysis of our measurement data and obtained data from Suzhou local government in order to understand actual water quality situation and to prepare monitoring and regulation for better water environment protection.

Wujiang District, located in the southernmost tip of Jiangsu Province and Suzhou City, lies at the junction of Jiangsu, Zhejiang and Shanghai provinces, and is lower leach area of the Taihu Leke Basin. There’re rich forestry resources, natural stream systems, and flood control districts, which conserve water resources and water quality. Wujiang District has a population of 1.6 million, which supplies agricultural production and support industrial and economic development. Large amount of pollution can be potentially released to the lake area, and further deteriorate lake water situation and river environment. River network of Wujiang District is too complicated to understand pollution situation. Water domain covers 22.7% of the total area, small lakes and marshes are connected to each other by canal system. As the Taipu River is utilized as water resources for drinking water, water level is kept higher than connecting rivers and channels to avoid contamination from tributaries by controlling slice gates.

116 monitoring points were set at 66 river crossing points and in 50 marshes larger than 0.5 square kilometers. Monitoring data of water quality indicators include: pH, permanganate index, ammonia nitrogen, total nitrogen, total phosphorus, cyanide, sulfide, fluoride and some heavy metals. Chlorophyll a, COD, BOD, flow rate, water level, features of monitoring sites have also been investigated. Water quality have been monitored continuously by local government and measured once or twice per month by research team. Monitored water quality data since 2015 was presented by government, and sampling data since 2017 were measured for each parameter. Based on data analysis, pollution situation and mechanism have been discussed.

Higher COD was observed compared to BOD. It means that river water probably contains excess amount of non-biodegradable organic matters. Concerning to Total Phosphorus, they have no major differences in concentration and didn’t show distinctive tendency along the river flow. On the other hand, higher concentration was observed in lower leach regarding to Total Nitrogen even though water level was kept being higher than other water bodies. It’s suggested that there should be pollution sources within the area. And it’s also indicated where probable pollution sources are located. The area is flat terrain in the Yangtze Delta, and flow rate is extremely low. Changes in flow direction were observed, and it’s corresponding to tidal current. This makes it more difficult to understand situation, therefore modelling approach is prepared to assess water environment condition. Considering these analyses and discussion, we’re planning to continue further research to detect pollution sources and to understand situation in the area.

How to cite: Kogure, K., Shi, S., Yang, C., and Li, J.: Analysis on situation of water environment in Wujiang District of the Taihu Lake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12958, https://doi.org/10.5194/egusphere-egu2020-12958, 2020.