Study found niche partitioning resulted in better nitrate uptake by algae
Biologically diverse streams are better at cleaning up pollutants than less rich waterways, and a University of Michigan ecologist said he has uncovered the long-sought mechanism that explains why this is so.
Bradley Cardinale used 150 miniature model streams, which use 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, a nitrogen compound that is a nutrient pollutant of global concern.
He found that nitrate uptake increased linearly with species richness. On average, the eight-species mix removed nitrate 4.5 times faster than a single species of algae grown alone. Cardinale reported his findings in the April 7 edition of the journal Nature.
The more diverse streams are better pollutant filters because of niche partitioning, Cardinale said.
In the stream experiments, each algae species was best adapted to a particular habitat in the stream and gravitated to that location—its unique ecological niche. As more algae species were added, more of the available habitats were used, and the stream became a bigger, more absorbent sponge for nitrate uptake and storage.
In the experiments, the channels inside each flume were lined with a continuous slab of molded plastic. The plastic 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.
Evidence supporting the finding that niche partitioning was responsible for the results includes the fact that different morphological forms of algae dominated unique and complementary habitats in the streams, as predicted by ecological theory. 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. 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.