Underwater forest in danger

With their root systems and long leaves, seagrass has the characteristics of land plants. Yet tourists on Bonaire and the other islands in the Caribbean often see seagrass as "something green underwater". "It is less colourful than coral," explains marine scientist Fee Smulders, dubbed the Netherlands' seagrass ambassador by NRC Handelsblad. "But if you spend hours in it, you see the diversity."

She not only enjoys the seagrass itself: "During my experiment in the Bahamas, I was visited by a group of dolphins, one of which kept swimming back to me to see what I was doing: counting seagrass."

Smulders feels that seagrass undervalued but crucial: "Many fish and other marine animals grow up in the seagrass and later move on to the sea. It provides shelter and food for animals of all sizes. It also filters junk and nutrients out of the water, so the water going to the coral reef is cleaner."

And it doesn't stop there. Smulders discovered that seagrass effectively retains carbon: "In the wet, oxygen-poor underwater world, that carbon can remain stored for years, as long as the process is not disrupted. It also breaks the waves and traps sediment. This is important to protect the coasts of the Dutch Caribbean in a changing climate where storms are more frequent and sea levels are rising."

Smulders mainly conducts her seagrass research on Aruba, Bonaire and Curaçao, comparing her findings there with her research on the Bahamas, Bermuda and in Mexico. "Bonaire, for example, is tropical: seagrass grows fast there. And thanks to successful conservation programmes, the sea turtle is doing well there. Other areas still have sharks as predators. I wanted to know whether turtles graze there less intensively because they are wary of sharks."

For her research, funded by the Dutch Research Council (NWO), she collaborated with local nature conservation organisations and universities. She crossed tropics and subtropics, sharks and no sharks, and observed how the seagrass and its functions were affected. Underwater, Smulders and her colleagues built cages of reinforcing steel measuring 1.5 by 1.5 metres. "The cages had exactly the mesh size that could prevent turtles from getting through, while fish could pass," Smulders explains. "That way we only excluded turtles and could compare what happened inside and outside the cage."

The cages remained in place for a year. During that period, leaf growth, soil composition and carbon storage were measured. "We dug in bags of rooibos tea and green tea to measure how bacterially active the soil is, the decomposition rate. That sounds simple, but it was quite difficult: you have to dig by feel in murky sand to find them again."

Apart from the teabags, Smulders' team also installed cameras to monitor turtles: "We wanted to see what they eat, when they eat and how nervous they are."

Comparing results provided important insights: "In Bonaire, we saw turtles grazing away up to 70 percent of the daily production of seagrass. In ecosystems with sharks, grazing pressure appears to be distributed differently. Where turtles feel unsafe, they spread out and the seagrass improves. Sharks seem to be good for seagrass.”
The research team also saw that a little grazing could be beneficial for carbon sequestration, as less decomposition would occur. With too much grazing, the balance tips: “It then erodes and little storage remains. For fish too, extensive turtle grazing means less habitat for fish."

Local pressure plays a role on many fronts, says Smulders: “Seagrass is removed at resorts because their guests find it unpleasant between their toes. This is short-sighted, because it will cause the beach to erode faster. Anchors from boats, surfboards and foils also leave their mark: they cut the seagrass and it gets trampled when getting on and off. This allows stored CO2 to be released again."
And then there is the problem of sargassum, the seaweed from the Sargasso Sea that has been washing ashore in record amounts between March and August in recent years: "Where it accumulates, the seagrass dies due to lack of sunlight and high concentrations of sulphide. In some bays, the seagrass can suddenly disappear."

Initiatives to tackle the sargassum range from quick clean ups to systems which are also used to capture oil spills at sea. "We are also exploring the possibilities of turning it into fodder. But Bonaire has little agriculture and besides, there are heavy metals in the sargassum. I am also studying whether heavy metals from the sargassum end up in the food chain via seagrass to turtles."

"It’s easier to protect what’s there than to bring back what has disappeared," Smulders says. "Simple measures often work quite well. For example, cordons around vulnerable patches, information signs and the ban on casting anchors in seagrass. Lac Bay on Bonaire is a hotspot for turtles as well as surfers. With proper zoning and information, you can reduce that pressure. Furthermore, a comprehensive system of water quality monitoring and management would be beneficial in Bonaire and other locations in the Caribbean. In Europe, we take this for granted, but talks on this have only recently started in Bonaire."

Smulders emphasises that the well-protected turtle, with its favourite animal status, is not the main culprit: "If you want to protect the turtle in the long term, you have to look at the whole ecosystem, including food and predators. Sharks keep grazing pressure in check."

And then there is an invasive seagrass species that crossed from the Red Sea and quickly spread across the bays of the Dutch Caribbean. Although fish eat some of it, the opportunist seems to fill gaps where native seagrass species fare poorly: "It is a smaller plant and ecologically less valuable, but it partly takes over functions of the native seagrass."

In short: cordon off vulnerable fields, engage in structural monitoring and protection of water quality, and manage the ecosystem wisely. She concludes: “Seagrass is not a green fringe; it is a coastal backbone. If we intervene smartly now, we can preserve functions and ensure that the islands become proud of this ecosystem.”

Today, the Netherlands has hardly any eelgrass. That used to be different, Smulders says: "The western Wadden Sea and the Zuiderzee had fields of waist-high eelgrass. There was a whole economy around it. For example, eelgrass was used for padding mattresses and to reinforce dykes. After the construction of the Afsluitdijk, it disappeared due to changing sediment flows and a devastating fungal disease. Once it's gone, it's difficult to get it back. Particularly since there are far fewer mussel and oyster beds in the Netherlands, where eelgrass thrives."

• Centre for Ocean Research & Education (Bahamas)
• Government of Bermuda
• National Autonomous University of Mexico
• Dutch Research Council (NWO)
• Royal Netherlands Academy of Arts and Sciences (KNAW)
• STINAPA Bonaire
• Wageningen University