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Fast coastal waters flow with diversity

BROWN U. (US) — The faster coastal waters flow, the greater the number of invertebrate species that live on rocks beneath the tides.

The findings, published this week in the journal Ecology Letters, could help improve management of delicate and complex coastal ecosystems, says lead author James Palardy, a former Brown University doctoral student.

Finding the fastest water could point scientists to areas where diversity is likely greatest—and perhaps especially worthy of protection—and to zones where invasive species could establish their first beachheads.

Study coauthor Jon Witman, professor of ecology and environmental biology, says the results were clear and consistent in Palau, Alaska, and Maine, where they experimentally manipulated water flow speed.

“It totally blew us a way that we got almost identical results in two marine regions of the world separated by 4,000 miles with completely different regional diversities, and no species shared in common,” Witman says. “It’s a wake-up call saying that water flow is a really strong predictor of how many species are present in a particular area of the ocean.”

The reason why faster flow seems to promote diversity, Witman says, is that it allows for the larvae of rock-dwelling invertebrates, such as barnacles, sea squirts, corals and sponges, to spread farther. Although the environments are quite different, it’s somewhat analogous to how trees and flowers can disperse their seeds farther in a stiff wind.

Simple physics
Palardy and Witman are not the first to observe a connection between water flow and diversity, but they are the first researchers to prove it with experiments. The research began five years ago when the pair started brainstorming about how they might make the important scientific transition from being able to notice the phenomenon to being able to produce and test it.

The pair’s goal was to speed up water flow without resorting to expensive and short-lived battery-powered pumps. Instead, the ecologists relied on simple physics that require a volume of water to flow faster when it moves through a narrowed space.

Based on prototypes developed in a giant flume in the basement of the BioMed research building at Brown, they built channels about 7 feet long and about 18 inches high. They lined the walls with plates where organisms could latch on and grow.

The test channels narrowed to about half their width in the middle, taking on a bow tie shape. The control channels remained the same width throughout. The control and test channels were placed about 3 to 6 feet below the lowest tide in each of two sites in Maine and Alaskan coastal waters.

Stem the tide
In every case they found that the number of different species on the plates in the test channels was much higher than on the plates in the control channels. The greater diversity was no flash in the pan, either.

The pattern was visible from early stages and persisted for more than a year of study. Witman also surveyed natural areas in Palau, and Palardy and Witman did the same in Alaska, finding similar effects in areas with faster flow.

Witman says his hope is that the work will not just explain greater biodiversity but will help stem the tide of its loss. “There’s a global biodiversity crisis where we’re losing species,” he says. “Ecology is very much concerned with sustaining natural processes.”

Funding for the research came from the National Oceanographic and Atmospheric Administration, the National Science Foundation, and Abt Associates Inc., a Cambridge, Mass., and Washington, D.C.-based consulting firm where Palardy now works as a senior environmental analyst.

More news from Brown University: http://news.brown.edu/

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