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Filtration capacity of seagrass meadows discovered

In addition to filtering pathogens, seagrass provides a habitat for young fish and food for marine life. Photo courtesy Wikimedia Commons.

In addition to filtering pathogens, seagrass provides a habitat for young fish and food for marine life. Photo courtesy Wikimedia Commons.

In addition to filtering pathogens, seagrass provides a habitat for young fish and food for marine life. Photo courtesy Wikimedia Commons.

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A recent study out of Cornell University focused on seagrass meadows in the Indo-Pacific  has given rise to a new environmental battle cry: “Save the seagrass!”

Marine biologist Dr. C. Drew Harvell and post-doctoral researcher Joleah B. Lamb investigated the bacterial populations in the waters of coral reefs with and without adjacent seagrass meadows. Their study revealed the pathogen filtration capacity of seagrass beds.

While many people travel the globe to swim with dolphins, watch whales, photograph marine birds, dive in coral reefs or snorkel in kelp forests, most people do not know how valuable the seagrass meadows are to human health and economy.

“Killer whales are a big tourist attraction, but people probably don’t realize that seagrass beds are really important as nurseries for the fish that the salmon require, which the killer whales require,” Dr. Harvell told NY Times. Many commercially caught fishes begin life in seagrass beds, migrating to deeper waters later in life.

Besides providing nurseries for commercial fishes, seagrass meadows also provide other services that benefit the planet. Seagrass sequesters carbon dioxide from the atmosphere and sinks agricultural fertilizer runoff into the sand. Seagrass meadows also stabilize sediment in the shallow water where water movement churns sand on the seafloor. Now, according to their study, Harvell and Lamb believe that seagrass meadows also filter out bacteria that are pathogenic to humans and to other reef species.

Harvell and his team investigated the presence of human pathogens and disease-causing bacteria in waters above coral reefs. They selected four different reefs near islands in Indonesia: two with and two without an adjacent seagrass meadow. The presence of the pathogenic bacteria Enterococcus is correlated with the presence of other human pathogens. Enterococcus is also a “source of multiple infections in humans and marine organisms,” according to Harvell and co-authors.

Coral reefs growing next to a seagrass bed exhibited lower levels of the indicator bacteria, Enterococcus, according to their article published last week in Science Magazine. The research team observed that “seawater collected above the coral reef adjacent to seagrass meadows had twofold lower levels of Enterococcus than paired sites,” according to their article.

Water above seagrass meadows also contained less bacteria pathogenic to marine species, stated their article. Similarly, coral reefs were 50% healthier when growing adjacent to a seagrass meadow. “Two of five globally occurring coral diseases, white syndrome and black band disease, as well as signs of coral tissue mortality associated with bleaching and sediment deposition, were significantly less on reefs adjacent to seagrass meadows compared with paired reefs,” stated the authors. Black band disease and white syndrome (white band disease) are easily identified by a ring of dead coral tissue, banded by black or white, respectively.

Using DNA samples taken from the seawater above reefs without an adjacent seagrass meadow, Harvell and the research team successfully identified 18 separate species of bacteria that are known human pathogens; roughly half that many species were found in water from reefs growing near a seagrass meadow.

“Regardless of the exact mechanisms involved, alleviating coral disease is vital for the well-being and livelihoods of 275 million people living within 30 km of a coral reef, as well as being of direct benefit to reef-dwelling species,” concluded Harvell and his co-authors in Science Magazine.

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