the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jan 2025
Consequences of the Aral Sea restoration for its present physical state: temperature, mixing, and oxygen regime
Abstract. The restoration of the North Aral was an unprecedented effort to save a large water basin by construction of a dam that separates it from the rest of the desiccating Aral Sea area. As a result, the lake volume has stabilized at 27.5 km3, the area has increased from 2800 km2 (2006) to 3400 km2 (2020), and the salinity has dropped from 18 to 10 g kg-1. The consequences of this unique experiment include highly dynamic changes of the thermal conditions, seasonal stratification, ice regime, and dissolved oxygen content and remain not fully quantified to date. We analyze the current state of the North Aral Sea with regard to stabilization of its long term dynamics, as well as consider the possible future projections in view of the global change effects on the regional hydrological regime and potential water management measures. Using data from a series of expeditions to the North Aral Sea in 2016–2019 and year-long continuous monitoring of the annual thermal and oxygen regime by an autonomous mooring station, we present the first comprehensive analysis of the North Aral system behavior on seasonal to interannual scales after its "cold restart". We demonstrate that the new seasonal mixing regime is intermediate between dimictic and polymictic, with relatively weak summer thermal stratification occupying only a small deep part of the lake. Salinity does not contribute to the summer density stratification but a stable salinity stratification can develop during ice melt in late winter. On the background of weak thermal stratification, highly energetic internal waves with periods of ˜4.5 days dominate the near-bottom dynamics and facilitate mixing at the lake bottom. As a result, the bulk of the water column remains well saturated with oxygen throughout the year. However, low-oxygen conditions may develop in the deepest part of the lake in mid-summer. In summary, the mixing regime of the restarted lake favors vertical transport of dissolved matter and water-sediment mass exchange ensuring oxygenation of deep waters and supply of nutrients to the upper water column. While the North Aral Sea is restored to the well-mixed state similar to that before its desiccation started, its seasonal mixing regime is currently in unstable equilibrium, wobbling between polymictic and dimictic conditions. The fragility of this seasonal pattern is demonstrated by modeling results: slight changes of the water level or transparency may turn the Aral Sea to steadily dimictic or polymictic state.
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Status: final response (author comments only)
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RC1: 'Review of the manuscript', Anonymous Referee #1, 28 Feb 2025
Please find the detailed review in the attached pdf.
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AC3: 'Reply on RC1', Georgiy Kirillin, 01 Apr 2025
We thank the Reviewer for the positive evaluation of our study and appreciate the recommendation for publication of our findings in HESS. While the physical processes considered in our study are generally known, their interplay and the resulting mixing pattern has not been described before: In large lakes with the mixed layer thickness comparable to the mean lake depth, the basin-scale internal waves produce the effect of wandering hypolimnion with important consequences for near-bottom mixing and littoral-pelagic exchange. We are unaware of any previous studies discussing this mixing regime. We particularly appreciate the detailed comments and suggestions of the Reviewer, which helped improve our results presentation. We address them in our point-to-point replies and have incorporated the majority of the suggestions in the revised paper. Here, we briefly address the general comments of the Reviewer.
- On the measurement setup: We fully agree with the Reviewer: an additional mooring in the center of the Shevchenko Bay and a couple of moorings in other parts of the North Aral would have allowed obtaining a more comprehensive quantitative picture on the lateral variability. Unfortunately, field investigations are inevitably constrained by available instrumentation and logistic issues. The fact that our mooring was positioned away from the lake center allowed us to capture the wave-driven lateral motions of the thermocline. The latter could have been missed if we chose the ``typical'' location at the deepest point of the lake, which is close the nodal point of the basin-scale waves in nearly-round Shevchenko Bay (cf. mid-panel of Fig. 10 in the ms). The lateral water exchange between the Shevchenko Bay and the central part of the North Aral Sea is not expected to significantly affect the temperature, stratification, and waves pattern, because the upper 7 m of the water column remain well-mixed vertically (Fig. 3A), i.e. the water characteristics on both sides of the 7-m deep strait are nearly identical. We mentioned potential differences between different parts of the lake in the revised text. These differences do not affect the core results of the study: The potential effect of the river inflow on the east-west salinity gradient is expected to be limited to the eastern shallow part of the lake (Fig. 1) and does not influence thermal stratification and vertical mixing in our observations. Taking into account the similar morphometries of the Shevchenko Bay and of the central part of the Aral Sea, the same wave pattern should dominate the mixing in the bulk of the Sea except shallow bays in the north and at the river estuary.
- On the description of the transitional mixing regime: We have added definitions of dimictic and polymictic regimes and referred the reader to more details in (Kirillin and Shatwell 2016). We prefer to avoid calling any mixing regime ``natural'' or to use it as a reference point. We know that the Aral Sea was dimictic as early as 1904 (Berg 1908: The Aral Sea). There are no earlier observations on the vertical thermal structure, but paleolimnological studies suggest that the Aral Sea underwent strong lake level variations on centennial time scales, which would have inevitably caused changes in the seasonal mixing. Therefore, we focus on the specific features of the present mixing regime, which is different from the commonly considered in conventional lake classification. There is no preferred seasonal mixing regime for the North Aral. The present regime appears to be advantageous compared to both di- and polymictic ones. Dimictic conditions are typically characterized by extensive long-lasting deep hypoxia. In turn, polymictic conditions may provoke massive algal blooms due to enhanced nutrients supply from the sediment. We discussed these scenarios in the revised ms. Regarding the hypothesis on existence of the summer stratification in the deep parts of the North Aral: we report the evidence of it, based on the analysis in terms of basin-scale waves and supported by 1D modeling results. The existence of the deep stratification is the only possible source of the observed temperature and oxygen variations. We have explicitly mentioned and quantified the potential effects of water level and light attenuation on stratification by means of modeling (see Section 3.4). The consequences for biogeochemistry can be discussed only qualitatively, what we did in the last paragraph of Discussion. The words ``changes'' and ``shifts'' are used in context, because both increase and decrease of corresponding factors affect seasonal stratification. More details on the specific potential effects of an increase or decrease in water level and light attenuation are provided in the revised Discussion. The Southern Aral Sea is an interesting research object with extreme environmental conditions, but is out of scope of this study and is mentioned only as an asymptotic example of what could have happened to the North Aral Sea without restoration measures.
- On the role of salinity: We agree with the Reviewer: the role of salinity on vertical stability of the water column cannot be completely ignored: short-term and/or localized events of salt stratification are not to be excluded. The three short-term (< 1 day) events of inverse stratification in November were rather driven by the combined effect of differential cooling between near-shore and central areas and wind surges. The inversions were immediately followed by strong convective mixing, which quickly eliminated the stratification and returned the water column to the perfectly mixed state. The free convection, reflected by high-frequency oscillations in the temperature records, would not develop if salinity contributed to the vertical stability. Herewith, the role of salinity gradients in the autumn overturn and the summer stratification is apparently negligible, as acknowledged by the Reviewer. The slight increase of the electric conductivity near the bottom in the June CTD profile makes only a modest contribution to stability: the mean density ratio within the layer is around 15, i.e. temperature contribution to stability is 15 times higher than that of salinity. The vertical conductivity gradient in this layer is therefore a consequence of the temperature-driven stratification and subsequent accumulation of dissolved matter near the sediment surface. Vertical mixing under ice and near-bottom stratification during the spring overturn are indeed prone to be affected by salt stratification, and we discussed it throughout the ms. We have refined this discussion: “Our modeling projections, while reliably simulated the seasonal thermal stratification pattern, did not take these processes into account. Incorporating of the salinity effects of vertical salt gradients may improve short-term model predictions of the vertical stratification in brackish lakes, especially in the ice-covered period and during the spring overturn, when the density dependence on temperature vanishes and the near-bottom salinity gradients may prevent convective mixing at the water-sediment interface”’. Decrease of salinity to zero would reduce the thermal expansion coefficient by about 11 %; no remarkable effects on stratification on seasonal time scales should be expected.
- Methods description, document structure, nomenclature, and figures: the comments have been addressed in the point-to-point response and incorporated in the revised version.
Citation: https://doi.org/10.5194/egusphere-2025-113-AC3
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AC3: 'Reply on RC1', Georgiy Kirillin, 01 Apr 2025
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RC2: 'Comment on egusphere-2025-113', Anonymous Referee #2, 04 Mar 2025
The manuscript analyses the thermal and oxygen dynamics in the North Aral Sea after its artificial isolation from the desiccating old Aral Sea. A combination of intensive field campaigns and prolonged monitoring in the Shevchenko Bay allows the authors to obtain a good understanding of the current dynamics. The use of a one-dimensional model supports the analysis of some scenarios.
I enjoyed reading the manuscript and I believe that it is a substantial piece of work that will make an important contribution to assessing the impact of the engineering works aimed at preserving the quality and of water and ecosystems of what remains of the old Aral Sea.
I had only a few minor suggestions for improving the clarity of some of the text and figures, but after reading the very detailed comments of the Anonymous Referee #1, I feel that they do not need to be listed here. I convincingly support the publication of this manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-113-RC2 -
AC1: 'Reply on RC2', Georgiy Kirillin, 01 Apr 2025
We thank the Reviewer for the positive evaluation of our study and appreciate the recommendation for publication of our findings in HESS. While the physical processes considered in our study are generally known, their interplay and the resulting mixing pattern has not been described before: In large lakes with the mixed layer thickness comparable to the mean lake depth, the basin-scale internal waves produce the effect of wandering hypolimnion with important consequences for near-bottom mixing and littoral-pelagic exchange. We are unaware of any previous studies discussing this mixing regime. We particularly appreciate the detailed comments and suggestions of the Reviewer, which helped improve our results presentation. We address them in our point-to-point replies and have incorporated the majority of the suggestions in the revised paper. Here, we briefly address the general comments of the Reviewer.
- On the measurement setup: We fully agree with the Reviewer: an additional mooring in the center of the Shevchenko Bay and a couple of moorings in other parts of the North Aral would have allowed obtaining a more comprehensive quantitative picture on the lateral variability. Unfortunately, field investigations are inevitably constrained by available instrumentation and logistic issues. The fact that our mooring was positioned away from the lake center allowed us to capture the wave-driven lateral motions of the thermocline. The latter could have been missed if we chose the ``typical'' location at the deepest point of the lake, which is close the nodal point of the basin-scale waves in nearly-round Shevchenko Bay (cf. mid-panel of Fig. 10 in the ms). The lateral water exchange between the Shevchenko Bay and the central part of the North Aral Sea is not expected to significantly affect the temperature, stratification, and waves pattern, because the upper 7 m of the water column remain well-mixed vertically (Fig. 3A), i.e. the water characteristics on both sides of the 7-m deep strait are nearly identical. We mentioned potential differences between different parts of the lake in the revised text. These differences do not affect the core results of the study: The potential effect of the river inflow on the east-west salinity gradient is expected to be limited to the eastern shallow part of the lake (Fig. 1) and does not influence thermal stratification and vertical mixing in our observations. Taking into account the similar morphometries of the Shevchenko Bay and of the central part of the Aral Sea, the same wave pattern should dominate the mixing in the bulk of the Sea except shallow bays in the north and at the river estuary.
- On the description of the transitional mixing regime: We have added definitions of dimictic and polymictic regimes and referred the reader to more details in (Kirillin and Shatwell 2016). We prefer to avoid calling any mixing regime ``natural'' or to use it as a reference point. We know that the Aral Sea was dimictic as early as 1904 (Berg 1908: The Aral Sea). There are no earlier observations on the vertical thermal structure, but paleolimnological studies suggest that the Aral Sea underwent strong lake level variations on centennial time scales, which would have inevitably caused changes in the seasonal mixing. Therefore, we focus on the specific features of the present mixing regime, which is different from the commonly considered in conventional lake classification. There is no preferred seasonal mixing regime for the North Aral. The present regime appears to be advantageous compared to both di- and polymictic ones. Dimictic conditions are typically characterized by extensive long-lasting deep hypoxia. In turn, polymictic conditions may provoke massive algal blooms due to enhanced nutrients supply from the sediment. We discussed these scenarios in the revised ms. Regarding the hypothesis on existence of the summer stratification in the deep parts of the North Aral: we report the evidence of it, based on the analysis in terms of basin-scale waves and supported by 1D modeling results. The existence of the deep stratification is the only possible source of the observed temperature and oxygen variations. We have explicitly mentioned and quantified the potential effects of water level and light attenuation on stratification by means of modeling (see Section 3.4). The consequences for biogeochemistry can be discussed only qualitatively, what we did in the last paragraph of Discussion. The words ``changes'' and ``shifts'' are used in context, because both increase and decrease of corresponding factors affect seasonal stratification. More details on the specific potential effects of an increase or decrease in water level and light attenuation are provided in the revised Discussion. The Southern Aral Sea is an interesting research object with extreme environmental conditions, but is out of scope of this study and is mentioned only as an asymptotic example of what could have happened to the North Aral Sea without restoration measures.
- On the role of salinity: We agree with the Reviewer: the role of salinity on vertical stability of the water column cannot be completely ignored: short-term and/or localized events of salt stratification are not to be excluded. The three short-term (< 1 day) events of inverse stratification in November were rather driven by the combined effect of differential cooling between near-shore and central areas and wind surges. The inversions were immediately followed by strong convective mixing, which quickly eliminated the stratification and returned the water column to the perfectly mixed state. The free convection, reflected by high-frequency oscillations in the temperature records, would not develop if salinity contributed to the vertical stability. Herewith, the role of salinity gradients in the autumn overturn and the summer stratification is apparently negligible, as acknowledged by the Reviewer. The slight increase of the electric conductivity near the bottom in the June CTD profile makes only a modest contribution to stability: the mean density ratio within the layer is around 15, i.e. temperature contribution to stability is 15 times higher than that of salinity. The vertical conductivity gradient in this layer is therefore a consequence of the temperature-driven stratification and subsequent accumulation of dissolved matter near the sediment surface. Vertical mixing under ice and near-bottom stratification during the spring overturn are indeed prone to be affected by salt stratification, and we discussed it throughout the ms. We have refined this discussion: “Our modeling projections, while reliably simulated the seasonal thermal stratification pattern, did not take these processes into account. Incorporating of the salinity effects of vertical salt gradients may improve short-term model predictions of the vertical stratification in brackish lakes, especially in the ice-covered period and during the spring overturn, when the density dependence on temperature vanishes and the near-bottom salinity gradients may prevent convective mixing at the water-sediment interface”’. Decrease of salinity to zero would reduce the thermal expansion coefficient by about 11 %; no remarkable effects on stratification on seasonal time scales should be expected.
- Methods description, document structure, nomenclature, and figures: the comments have been addressed in the point-to-point response and incorporated in the revised version.
Citation: https://doi.org/10.5194/egusphere-2025-113-AC1 -
AC2: 'Reply on RC2', Georgiy Kirillin, 01 Apr 2025
We thank the Reviewer for the positive evaluation of our study and highly appreciate the explicit recommendation for publication. We did our best to improve the presentation clarity in the revised version.
Citation: https://doi.org/10.5194/egusphere-2025-113-AC2 -
AC5: 'Sorry for cross-posting', Georgiy Kirillin, 01 Apr 2025
The same comment have been posted to both Reviewers. I apologize for this unintentional cross-posting and hope that it does not cause any inconvenience. Georgiy Kirillin
Citation: https://doi.org/10.5194/egusphere-2025-113-AC5
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AC1: 'Reply on RC2', Georgiy Kirillin, 01 Apr 2025
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RC3: 'Additional comment on the correlation between temperature and DO oscillations', Anonymous Referee #1, 05 Mar 2025
Dear authors,
I have an additional comment about the DO-temperature relationship.
As mentioned in my previous comments about Figure 7, it would be very helpful to have temperature and DO time series at the same depth shown on the same figure and to discuss the correlation between the two time series in Sect. 3.3. I have tried to compare the two time series by combining Fig. 7 with Fig. 5A for the summer period (see the figure below). I do not understand why the DO concentration and saturation are both increasing during stratified periods (blue area on the figure) and decreasing during mixing periods (yellow area). I thought it was the opposite: stratified periods should be associated with the presence of a bottom low-oxygen layer as explained in l. 365-367 and mixing periods should bring more oxygen. Is it because I have incorrectly linked the two figures (e.g., the x-axis ticks in Fig. 7 do not correspond to the first day of the month as in Fig. 5A) or is there a physical explanation?
Citation: https://doi.org/10.5194/egusphere-2025-113-RC3 -
AC4: 'Reply on RC3', Georgiy Kirillin, 01 Apr 2025
DO concentrations generally follow the wave-driven temperature pattern (see the attached Figure): The concentrations drop at the beginning of stratification events indicating upwelling of oxygen-poor hypolimnetic waters. The DO dynamic in the internal wave shoaling zone is apparently more complex than that of temperature and affected by other processes such as mixing due to wave breaking, shear-induced water-sediment gas exchange, primary production and respiration. These processes are worthy of a dedicated study on fine-scale DO dynamics in the littoral and their quantification requires detailed field observations.
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AC4: 'Reply on RC3', Georgiy Kirillin, 01 Apr 2025
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