Worldwide, seagrass habitats are declining as runoff from farmland and other nutrient-rich water makes its way to the ocean. And where our wastewater reaches the sea, algae thrive. If all that growth isn’t kept in check, healthy seagrass habitats are rapidly transformed into relatively barren wasteland, unable to support seahorses, sea turtles, fish nurseries and the myriad of other animals that depend on seagrass for shelter and sustenance.

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Green turtles can chomp their way through an impressive amount of algae in a day. Image: Thomas Hubauer, Flickr

To tackle the problem, Michael Heithaus of Florida International University and his team suggest turning to turtles. Until recently, the importance of grazing in keeping seagrass habitats healthy was largely overlooked – simply because scientists had very little information about grazers. But we're now learning that green turtles (Chelonia mydas) – serious herbivores that can can chomp their way through an impressive amount of algae in a day – simply need to dine on the excess algae to keep things in check.

While this enigmatic species is in global decline, the munching trend has motivated great efforts to conserve it, primarily by tackling fishing pressures and protecting nesting beaches. These moves have not gone unrewarded: in some areas, turtle numbers are bouncing back. And that's great news for seagrass meadows since these important habitats would become inhospitable without turtles to keep algal growth down.

There is a caveat though: too many turtles munch down on so much seagrass that the habitat’s health actually declines, becoming barren and unable to support the population. Within Indonesian Marine Protected Areas (MPAs), for example, sea turtle numbers have soared to such heights that their grazing threatens the habitat on which they depend. Too many turtles decimate the habitat – and with it the local turtle population.

The lesson? Not only do we need to keep the algae in check, but we also need something to keep the turtles in check. The answer, Heithaus's study shows, lies in keeping turtle predators in top form. Unfortunately, the turtles' main predators, tiger sharks, are also under threat and fishing pressure has put the species (and many other sharks) on the IUCN Red List.

It's a lesson we've heard before and it's an important one: to keep an ecosystem in good nick, populations that form all parts of the food chain need to be maintained. And to get the best out of the headway we've made in green sea turtle conservation so far, more must be done to protect other species that swim amongst the seagrass. Only then can turtle populations reach healthy, sustainable levels.

Top header image: Andrea Westmoreland, Flickr