Decreasing levels of oxygen in deep lake water linked to longer warm seasons

The level of dissolved oxygen is a determining factor in various aspects of aquatic ecosystems, influencing its quality as biodiversity habitat – low oxygen levels make water unsuitable for aerobic organisms (which need oxygen to breathe). In temperate lakes across the world, data indicating that average summer oxygen levels are declining are, therefore, of concern. Warmer temperatures lead to less oxygen solubility in lake water closer to the surface, and this has been well studied in relation to climate change. However, much remains unknown about the factors contributing to deoxygenation in deep water, where temperatures have not consistently warmed in recent decades.

Despite the importance of this part of lake ecology, there has been no large-scale investigation into the amount of deep lake habitat suffering critically low oxygen levels. To address this knowledge gap, researchers looked at data on more than 400 temperate lakes, in widespread locations, to identify drivers of this phenomenon and quantify the volume of lake water reaching levels of depletion that could be damaging for ecology.

They note that deoxygenation could be due to seasonal dynamics, such as increased consumption of oxygen associated with eutrophication. Eutrophication causes high rates of microbial respiration, leading to rapid oxygen depletion in some lakes in summer. Increased organic matter in lake water also blocks out light, reducing oxygen generated by plant photosynthesis at lower depths.

Another possibility is less frequent mixing and transport of oxygenated water in the water column – in other words, altered patterns of stratification, whereby water in lakes comprises different temperature layers rather than being well mixed. In many lakes, upper waters begin to warm in spring and this less dense water sits on top of colder, denser water, separating it from atmospheric oxygen. If the stratified season is lengthening, this could increase deep-water deoxygenation. In addition, metabolic activity of organisms also causes a decline in oxygen in deep water during stratification.

Ecological consequences of deep-water oxygen depletion are also insufficiently quantified. Not all deoxygenation has adverse effects, especially if minimal, but, if oxygen declines to below 5 mg per litre (mg/L ) of water, habitat for sensitive cold-water fish species – such as salmon – is reduced. Some highly sensitive fish and invertebrate species experience detrimental effects at 8 mg/L, while a threshold of 2 mg/L is often used to define hypoxic habitat, unsuitable for most fish. Waters where oxygen is completely absent (anoxia) may undergo substantial changes that increase nutrient loading and impact greenhouse gas emissions.

The researchers analysed data on temperature and oxygen in 429 lakes, (collected by academic institutions and government agencies primarily in North America and Europe¹). On average, these time series data reflected the past 25 years. They assessed changes and trends in seasonal deep-water depletion rates, stratification characteristics, and changes in the proportion of lake water below critical oxygen thresholds.

Findings showed that over three-quarters of analysed lakes exhibited an increase in the duration of stratification (the occurrence of layers of warmer and colder water). This would provide more time for seasonal deep-water oxygen depletion to occur and appeared to be one of the primary drivers of increasing deep-water deoxygenation over time. The data indicated that the onset of stratification is moving earlier, by about 1.2 days per decade, and is shifting to end 2.5 days later, increasing the overall length by 3.7 days per decade.

Over two-thirds of the lakes studied have experienced increasing amounts of water that exceed thresholds for oxygen depletion. The volume of water in these lakes with less than 5 mg/L oxygen grew by 61% in the timeframe studied (25 years on average). The total volume of this additional oxygen-depleted water among all 400 lakes exceeded 700 million cubic metres, corresponding to 0.71% of the total volume of these lakes. The volume of anoxic water grew by 52% over the same time period. The proportion of lake water columns below the 5 mg threshold grew between 0.9% and 1.7% on average each decade, depending on the threshold examined.

Such oxygen depletion threatens the habitability of deep-water environments. Ongoing, climate-induced lengthening of stratification has already contributed to reduced habitat for many species, say the researchers. Anoxia, meanwhile, is linked to internal loading of phosphorous due to chemical processes involving lake sediment – this likely increases eutrophication and fuels harmful algal blooms.

Increased anoxia also has implications for greenhouse gas emissions, since it enhances the release of methane from lakes, especially when lake water mixes as stratification ends, in autumn. The researchers note that there are large uncertainties surrounding the contribution of inland waters to atmospheric methane. They also highlight that changes in stratification length do not fully explain oxygen declines in deep water. Increases in dissolved organic matter, which decrease light penetration, may strengthen stratification and fuel microbial growth and respiration, they posit.

Future warming will probably exacerbate the trends found, warn the researchers. Under a high GHG emissions scenario, by 2100, the period of stratification is predicted to lengthen by over a month for some lakes. Active management to maintain deep-water oxygen levels above critical thresholds may be required to avoid substantial ecological effects.

Footnotes:

1. Stetler, J. T., Jane, S. F., Mincer, J. L., Sanders, M. N., & Rose, K. C. (2021). Long-term lake dissolved oxygen and temperature data, 1941–2018 ver 2. Environmental Data Initiative. 

Source:

Jane, S.F., Mincer, J.L., Lau, M.P., Lewis, A.S., Stetler, J.T. and Rose, K.C. (2023) Longer duration of seasonal stratification contributes to widespread increases in lake hypoxia and anoxia. Global Change Biology, 29(4): 1009–1023.

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“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

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The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.