Global Lake Deoxygenation and Heat Waves

Recent studies highlight the alarming trend of deoxygenation in lakes worldwide. A 2025 study published in *Science Advances* reveals that short-term heat waves and long-term global warming are reducing dissolved oxygen (DO) levels in lakes. This phenomenon poses severe risks to aquatic ecosystems and human livelihoods.

About Deoxygenation in Lakes

Deoxygenation refers to the reduction of dissolved oxygen levels in water bodies. It is critical for aquatic life. The study shows that 83% of the examined lakes are experiencing continuous deoxygenation. The average rate of deoxygenation in lakes is outpacing that of oceans and rivers.

Causes of Deoxygenation

The study identifies two main contributors to surface oxygen loss. Global warming accounts for 55% of this loss by decreasing oxygen solubility. Eutrophication, which results from nutrient over-enrichment, is responsible for an additional 10%.

Projected Changes by 2100

Future projections indicate decline in surface DO levels. Under the SSP2-4.5 and SSP5-8.5 scenarios, DO levels may drop by 0.34 and 0.76 mg/litre, respectively. By 2100, average global DO could decrease to 9.11 mg/litre (SSP2-4.5) and 8.70 mg/litre (SSP5-8.5).

Impact of Heat Waves

Heat waves are becoming more frequent and intense, exacerbating the deoxygenation issue. The average duration of heat waves over lakes is now 15 days annually. Over the past two decades, the number of heat wave days has increased across all continents. This trend is particularly pronounced in Africa and Oceania.

Geographical Variations in Deoxygenation

The study predicts that deoxygenation will occur more rapidly in Europe and North America compared to other regions. Future projections indicate that 238 lakes (under SSP2-4.5) and 279 lakes (under SSP5-8.5) will face stress conditions, particularly in tropical areas.

Consequences of Deoxygenation

The decline in DO levels has dire implications. It leads to reduced nitrogen fixation and increased emissions of nitrous oxide, a potent greenhouse gas. Aquatic species face habitat loss, affecting food security and livelihoods. The potential for species extinction and the collapse of commercial fishing industries is .

Research

The research led by Yibo Zhan utilised model-derived DO estimates for 15,535 lakes. This comprehensive analysis marks the critical need for monitoring and addressing the factors contributing to lake deoxygenation. The findings tell the importance of climate action to mitigate these effects.

Questions for UPSC:

1. Analyse the factors contributing to the deoxygenation of aquatic ecosystems in the context of climate change.
2. Critically discuss the implications of declining dissolved oxygen levels on biodiversity and food security.
3. Examine the role of eutrophication in freshwater ecosystems and its impact on dissolved oxygen levels.
4. Estimate the potential effects of increased heat wave frequency on freshwater resources and aquatic life.

Answer Hints:

1. Analyse the factors contributing to the deoxygenation of aquatic ecosystems in the context of climate change.

1. Global warming decreases oxygen solubility, accounting for 55% of surface oxygen loss.
2. Eutrophication, caused by nutrient over-enrichment, contributes an additional 10% to oxygen depletion.
3. Heat waves increase the frequency and intensity of deoxygenation events, affecting oxygen dynamics.
4. 83% of studied lakes are experiencing continuous deoxygenation, indicating a widespread issue.
5. Projected declines in dissolved oxygen levels by 2100 highlight the urgency of addressing climate change impacts.

2. Critically discuss the implications of declining dissolved oxygen levels on biodiversity and food security.

1. Reduced dissolved oxygen (DO) levels threaten aquatic life, leading to species extinction and habitat loss.
2. Lower DO levels impair nitrogen fixation, affecting nutrient cycling in aquatic ecosystems.
3. Increased emissions of nitrous oxide (N2O) contribute to climate change, further disrupting ecosystems.
4. Declining fish populations impact commercial fishing industries and food security for communities relying on these resources.
5. Changes in aquatic biodiversity may lead to less resilient ecosystems, affecting overall ecological balance.

3. Examine the role of eutrophication in freshwater ecosystems and its impact on dissolved oxygen levels.

1. Eutrophication results from nutrient over-enrichment, leading to algal blooms that consume oxygen when decomposed.
2. It is responsible for 10% of surface oxygen loss in lakes, exacerbating deoxygenation issues.
3. Decomposing algal blooms create hypoxic conditions, further stressing aquatic organisms.
4. Increased nutrient runoff from agriculture and urban areas enhances eutrophication, worsening the situation.
5. Management strategies are vital to mitigate eutrophication effects and protect aquatic ecosystems.

4. Estimate the potential effects of increased heat wave frequency on freshwater resources and aquatic life.

1. Heat waves reduce DO solubility in water, leading to rapid fluctuations in oxygen levels.
2. Increased frequency of heat waves is projected to intensify deoxygenation, particularly in North America and Europe.
3. Average heat wave duration over lakes is 15 days per year, increasing stress on aquatic ecosystems.
4. Projected future conditions show that 238 to 279 lakes will face stress due to climate change scenarios.
5. Impacts include habitat loss, reduced biodiversity, and threats to food security and livelihoods dependent on freshwater resources.