Shaken, not stirred: How climate change can upset the chemistry of freshwater lakes

Climate change could shake up the ecology of freshwater lakes, such as Lake Tahoe

Much like James Bond's favorite cocktails, freshwater lakes need to be shaken up in order to make sure vital ingredients are evenly distributed within. Without a giant cocktail shaker at its disposal, nature carries out this task by way of big storms in the colder months that turn over the bodies of water and preserve the health of the ecosystem. But scientists are now warning that rising surface temperatures may bring an end to this, which would give algae new rein over these lakes and seriously threaten fish populations and vital freshwater resources.

Recent data has shown that the average temperature of the Earth's lakes is rising by 0.61° F (0.34° C) each decade. This warming is expected to bring about an increase in algal blooms by as much as 20 percent, which would spell big trouble for fisheries, tourism, water supplies and the environment as a whole.

The buildup of algae is problematic and not just because it can poison water supplies. When it dies, its decomposition soaks up dissolved oxygen in the water, leaving low-oxygen dead zones for suffocating animal life to endure.

One way we are already contributing to the growth of algae in lakes and streams is through something known as nutrient pollution, or eutrophication. Fertilizer runoff, improperly monitored sewage systems and other human activity is loading up lakes and reservoirs with more nutrients than they can handle. This is in turn fertilizing the algae in the water and resulting in more frequent and severe algal blooms.

Scientists at the University of California, Davis (UC Davis) have now uncovered yet another source of nutrient pollution, which they have dubbed climate eutrophication. For almost 50 years, UC Davis researchers have tracked surface temperatures at Lake Tahoe, finding that they have risen by almost 0.9° F (0.5° C) since 1968.

This has had the effect of stabilizing the lake's thermal layers and lengthening of the so-called stratification season, in which water rests in separate layers as a result of varying temperature, by 24 days. This makes it more difficult for the storms that sweep through in fall and late winter to have their usual effect of shaking up the lake water, leaving dissolved oxygen in the surface layers and the starved lower layers left wanting.

Over time, if these stratification seasons continue to grow longer and the levels of dissolved oxygen in the lower levels keep diminishing, the livelihood of marine life operating at these depths won't be the only concern. The researchers say this will trigger a chemical reaction particular to low-oxygen environments that draws nutrients out of the lakebed and into the water, offering another avenue for nutrient pollution to impact the planet's freshwater reserves.

To get an idea of how this may affect Lake Tahoe in the future, the scientists came up with projections of future conditions based on a couple of carbon emission scenarios published by the Intergovernmental Panel on Climate Change, paired with a model of how water moves around the lake. One scenario had carbon emissions slow down and halted by the year 2100, while the other had them increasing rapidly. This saw the stratification season grow by 12 and 38 days, respectively.

This would be brought about by increases in air temperature between 4.5 and 7° F (2.5 and 4° C) and a drop in wind speeds by seven to 10 percent. While the study and these projections pertain only to Lake Tahoe, the researchers note that the process could also play out in other deep lakes around the world.

The research was published in the journal Limnology and Oceanography.

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