Underwater grasslands can cut concentrations of harmful bacteria in half

Here’s some advice to make your swimming sessions safer: Stick to seagrass meadows. That’s because these underwater grasslands, widespread in coastal areas all over the world, can filter out much of the seaborne bacteria that is harmful to humans, according to a new study. To find out just how effectively seagrasses can block bacteria from their environments, scientists went to four islands in the Spermonde Archipelago off the western coast of Indonesia. There, they found that a common intestinal bug, Enterococcus, was 10 times above the recommended exposure level set by the U.S. Environmental Protection Agency in shore waters around the islands. But in water where dense mats of seagrass covered the sea floor, Enterococcus levels were three times lower. Looking deeper, the scientists found that the chance of encountering dozens of different bacterial pathogens harmful to both human and marine life was cut in half near seagrass meadows, they report today in Science. Corals appear to benefit from these meadows, too. Surveying more than 8000 reef-building corals, researchers found that several deadly coral diseases were 50% less prevalent in regions with seagrass meadows. The scientists aren’t entirely sure how the seagrass combats bacteria so effectively, but one possibility could be that it anchors the nutrient-rich sediment to the sea floor, effectively preventing harmful microbes from feeding on the nutrients.