Not all lakes are alike: spatial differences key to successful restoration

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Not all lakes are alike: spatial differences key to successful restoration

Press release

Degradation of lakes is a worldwide problem, with symptoms including toxic algae blooms. Restoration is possible, but takes a lot of time and effort. To determine what approach is most effective, spatial differences between lakes must be considered, argues a NIOO-led team of researchers in Science of The Total Environment. It's a factor that had been largely neglected until now.

Lake Loosdrecht in the Netherlands is a popular recreation area that's on its way to recovery after decades of looking like thick green soup. What hasn't recovered yet is a healthy growth of water plants, and despite all the restoration efforts, many parts of the area remain off-limits to swimmers.

Four lake types

It's a good example of the impact on aquatic ecosystems of human activity. Intensive farming, in particular, has caused eutrophication in many places: an oversupply of nutrients such as nitrogen or phosphorus in the water. Eutrophication leads to the overgrowth of some plants and algae - including blue-green algae which are notorious for their harmful blooms - at the expense of lake biodiversity.

The success of measures against eutrophication has been mixed. Some measures work, while others have proved ineffective or have even backfired. None of this is surprising, says lake researcher Annette Janssen: "After all, not all lakes are alike."

Janssen and her colleagues have been focusing on spatial heterogeneity: differences in where and how nutrients enter a lake, and where and how the water comes in. Based on these differences, they distinguish four types of lakes, all of which require different approaches for restoration measures to be successful.

(Text continues below pictures)

Images
  • Researcher Annette Jansen
  • Diet Bos
    Lake Loosdrecht
  • Annette Janssen
    Lake Taihu
  • No machine-readable author provided. Gengiskanhg assumed (based on copyright claims)., CC BY-SA 3.0, via Wikimedia Commons
    Pátzcuaro
  • Everett Carrico
    Lake Tahoe

Lake Loosdrecht

Returning to Lake Loosdrecht, its natural situation is for nutrients and phytoplankton to be relatively homogeneously distributed. It's a seepage lake, which means the water, too, enters in a relatively diffuse way: from groundwater and precipitation, rather than from a river.

That's also how nitrogen and phosphorus enter the lake; traffic and farming can cause the precipitation to become seriously polluted. In this type of lake, says Janssen, flushing with clean water is often successful. Biomanipulation - the removal of certain species of fish - may likewise prove effective.

"Biomanipulation gives the lake a jump start, which helps its ecosystem recover", says Janssen. Experiments from 2009 with the removal of bream appear to confirm this. The water became clearer, although that did not result in the immediate restoration of the ecosystem's disrupted balance.

Lake Taihu

A lake that belongs to an entirely different category is Lake Taihu near the Chinese city of Shanghai, where Janssen did much of her PhD research for NIOO. Lake Taihu is fed by a river, and close to its mouth the water is covered by a thick layer of smelly algae.

The water at the outflow, on the other hand, is relatively clean. "Plants still grow there", says Janssen. "That's a good sign in terms of the water quality."

The presence of toxic cyanobacteria so close to the intake of a vital water plant prompted the authorities to try flushing the lake, using water from the Yangtze river. But this expensive measure failed to have the desired result. "Flushing Lake Taihu only spread pollution across a larger area. Like an oil spill."

Removal of fish wouldn't work either, as this aims to restore plants and animals that belong to clean water for a longer period. As Janssen has demonstrated, this doesn't happen in lakes such as Taihu, which will eventually return to a turbid state.

So what measures do work? Radically reducing waste is the only solution, believes Janssen. The rapid growth of the Chinese economy has made the discharging of waste into the environment a huge problem. Janssen used models to calculate that a reduction of current waste levels by 90% could restore Lake Taihu to a healthier state.

Lake Pátzcuaro and Lake Tahoe

Lake Pátzcuaro in Mexico and Lake Tahoe in the United States are polar opposites when you compare them in terms of spatial patterns. In the case of Lake Pátzcuaro, there's no river but there is a pipeline that dumps concentrated wastewater in the lake, while the water entering from the groundwater and through precipitation is relatively clean.

As long as the pipeline is far enough away from the lake's outflow, the pollution stays mainly in one place. Compared to Lake Taihu, for instance, its concentration dilutes very rapidly towards the outflow. But according to Janssen, lakes such as Pátzcuaro are relatively rare.

Lake Tahoe does have a river that feeds it. But the highest concentration of nutrients is found at the outflow. That’s because pollution enters the lake not from the river but from the groundwater or the atmosphere. "Just compare it to someone walking from their house to their car in the rain. The closer to the car they come, the wetter they are going to be."

Unlike Lake Taihu and Lake Pátzcuaro, Lake Tahoe might respond well to flushing as the pollution would leave the lake quite rapidly via the outflow. Fish removal and other forms of biomanipulation, on the other hand, only seem to work for lakes belonging to the same type as Lake Loosdrecht. Both Lake Pátzcuaro and Lake Tahoe would soon become turbid again.

(Text continues below pictures)

Images
  • Annette Janssen
  • Manqi-Chang
  • Manqi-Chang
  • NIOO-KNAW
    Click the image to enlarge it.

Prevention is better

Having insight into where nutrients and water enter a lake makes it easier to anticipate which control measures will work: an important conclusion if you consider how much time and money is usually needed for such measures. In the case of Lake Taihu, taking on board the spatial differences compared to other lake types at an earlier stage could have prevented a lot of money going down the drain.

To further boost the restoration of Lake Loosdrecht, on the other hand, there are many choices. Both flushing the lake and biomanipulation may be worth considering. Although even then, recovery tends to be a slow process according to Annette Janssen:

"Nutrients build up in the sediment over time until it's fully charged, like a rechargeable battery. When the influx of nutrients slows down, that 'charge' is first released. Signs of restoration will only become visible once the nutrient battery is empty."

Janssen also warns that in lakes such as Lake Loosdrecht, the influx of nutrients needs to be reduced by a much larger factor than you might expect, as some species that thrive under turbid conditions may put up substantial resistance. "Prevention is always better than cure."