U. MICHIGAN (US) — In an environmental division of labor, streams that are biologically diverse do a better job cleaning up pollution than purer waterways.
Bradley Cardinale, assistant professor of ecology and evolutionary biology at the University of Michigan, created 150 miniature model streams, each using recirculating water in flumes to mimic the variety of flow conditions found in natural streams. He grew between one and eight species of algae in each of the mini-streams, then measured each algae community’s ability to soak up nitrate.
As species richness increased, so did nitrate uptake. On average, the eight-species mix removed nitrate 4.5 times faster than a single species of algae grown alone.
Details of the research are reported in the journal Nature.
“The primary implication of this paper is that naturally diverse habitats are pretty good at cleaning up the pollutants we dump into the environment, and loss of biodiversity through species extinctions could be compromising the ability of the planet to clean up after us,” Cardinale says.
“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.”
For the study, channels inside each flume were lined with a continuous slab of molded plastic that provided a growth surface for the algae, and variations in the shape of the slab’s surface created a variety of water features—riffles, pools, and eddies, for example—found in real streams.
That different morphological forms of algae dominate unique and complementary habitats in the streams, as predicted by ecological theory, suggests niche partitioning is at work.
Also, high-velocity habitats were dominated by small, single-celled diatoms that latch onto the streambed in a way that is resistant to displacement by shear and low-velocity habitats were dominated by large, filamentous algae that are susceptible to shear.
When the streams were simplified so that they contained just one habitat type, the effects of diversity on nitrate uptake disappeared, confirming that niche differences among species were responsible for the results.
The algal species used in the study included eight forms of diatom and green algae that are among the most widespread and abundant species in North American streams. The experiment was performed in the stream flume facility at the University of California, Santa Barbara.
Nitrate, a common ingredient in fertilizer, is found in surface runoff from agricultural land that makes its way into streams, lakes, and coastal zones and is a leading cause of degraded water quality worldwide, Cardinale says.
“One of the obvious implications of this study is that if we want to enhance water quality in places like the Chesapeake Bay watershed or around the Great Lakes, then conserving natural biodiversity in our streams will have the added benefit of helping to clean up these larger bodies of water.”
The research was funded by grants from the National Science Foundation.
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