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More biodiversity equals cleaner water, but why?

New study shows how greater biodiversity more efficiently scrubs pollutants from freshwater.

A new landmark study not only proves that adding more species to a freshwater stream linearly increases the ecosystem’s ability to clean pollutants, but also shows why. The study, published in Nature found that by increasing the biodiversity of a lab controlled mini-stream from one algae species to eight caused the ecosystem to soak up nitrate pollution 4.5 times faster on average.

To conduct the experiment, researchers used plastic to create 150 mini model streams. Molding the plastic, they recreated real stream-like habitats such as pools, runs, and eddies. Different species of algae gravitated toward particular mini-habitats, creating special ecological niches and allowing more of the stream to be utilized by the algae for soaking up the nitrate pollution. Less utilization of the available habitats resulted in a dirtier river and vice-versa.

“As the different habitats in a stream are filled by diverse populations of algae, the stream receives more total biofiltration,” explains lead author Bradley Cardinale with the University of Michigan. “It’s as if the algae work as a better sponge.”

Algae similar to those used in the study. Photo by: Danuta Bennett.

The study provides a breakthrough in uncovering the exact processes by which higher biodiversity results in a more efficient and richer ecosystem. The key, according to Cardinale, is the utilization of various niches across a complicated ecosystem.

“People as far back as Darwin have argued that species should have unique niches and, as a result, we should see a division of labor in the environment. But demonstrating that directly has proven very difficult. And so one of the primary contributions of this study is that I was able to nail the mechanism and show exactly why streams that have more species are better at removing these nutrient pollutants from the water.”

Nitrate is a nutrient-rich pollutant that commonly enters freshwater habitats from fertilizers used in agriculture and sewage.

Practically conservation of freshwater biodiversity could enhance water quality, says Cardinale. For example, preserving streams that feed freshwater areas of concern, such as the Chesapeake Bay watershed and the Great Lakes in the US, would help recover water health.

While the study provides good news as to another reason why biodiversity matters, it also points to a bleaker conclusion. Cardinale says the “loss of biodiversity through species extinctions could be compromising the ability of the planet to clean up after us.” He adds that his study is “part of a growing body of scientific evidence […] that the modern mass extinction of species is going to affect humanity in some big, and important ways.”

Concernedly, freshwater species appear even more at risk than their land or oceanic counterparts. A study in 2009 found that freshwater species are likely the most threatened on earth with extinction rates four to six times higher than the rates for terrestrial and marine species.

“If we were to maintain streams in their naturally diverse state, these streams that we love for their recreation, for their beauty, for fishing, etc. … have the tangential benefit of cleaning up our water for us,” Cardinale told Reuters, adding that, “if we let nature do its thing, we don’t have to run around creating very expensive water treatment plants all over the planet.”

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