An Ambassador for
Embattled Seagrasses Seeks
Global Monitoring System
By Steve Grant
A scientist who has spent decades researching seagrasses is refining a system that could be the key to the long-term preservation of these often-overlooked but invaluable marine plants.
Anitra Thorhaug, a research professor since 1979, is working on a sophisticated process to better assess the condition of seagrasses and seaweeds worldwide, relying upon satellite surveillance data that are more comprehensive, less expensive and safer than the labor- and cost-intensive alternative—on-the-spot observation by crews of technicians.
Working in cooperation with Graeme Berlyn, E. H. Harriman Professor of Forest Management and Physiology of Trees, Thorhaug uses a technology called spectral reflectance to monitor seagrasses and seaweeds.
“Nobody has ever tried these spectral techniques on seagrasses and macroalgae,” as seaweeds are more formally known, Thorhaug said. “We are being very ambitious and bold here.”
A seagrass meadow is like a thriving city, a dynamic concentration of life whose influence radiates far beyond its confines. Seagrass leaves, or blades, as they are known, are food for conch and parrotfish, as well as a safe place for hundreds of species of fish to lay their eggs. Seagrasses also are shelter for many species of juvenile fish, including gray snapper, and prevent erosion of near-shore waters.
“They also take a lot of carbon dioxide out of the air and the sea,” Thorhaug said. “They are a big part of the carbon cycle” and therefore can be valuable in containing global warming.
They are, in fact, a bigger part of the carbon puzzle than most people would suspect. “They are not equal to terrestrial forests, but they are responsible for a substantial percentage of the productivity and carbon production of a terrestrial forest,” Thorhaug said. Algae and seagrasses add about 6 percent to globally sequestered carbon. If these plants were to be restored to habitats where they once flourished, that figure could be considerably higher, she said.
Seagrasses, similar in appearance to dune grasses, differ from seaweeds in that they are a higher order of plant, the only flowering plants in the sea. One common seagrass found in waters from North Carolina through New England and Canada is marine eelgrass, with long, bright-green ribbon-like leaves.
Unfortunately, many seagrass meadows throughout the world have been destroyed over the centuries. For example, seagrasses once were found along the entire Connecticut coast but now are essentially confined to the eastern end of Long Island Sound. Oil spills; pollution, including sewage discharges; dredging and fill; and excessively high water temperatures from electricity generating station discharges all have taken their toll. Alterations of freshwater flows through canals, channels and dams throughout the world sometimes concentrate those flows into coastal waters, reducing salinity and wiping out seagrasses, which cannot tolerate prolonged exposure to fresh water.
Spectral reflectance, which Berlyn helped pioneer and refine over the past four decades, identifies both healthy and stressed seagrasses. It takes advantage of the fact that stressed leaves develop physiological constraints that decrease the amount of light-absorbing pigment they produce. Moreover, the amount of light-absorbing pigment varies depending on the exact stress. Subject a blade of seagrass to one kind of stress, like sewage pollution, and its pigments react one way. Subject a blade of seagrass to a different stress, like a drop in salinity, and the pigments react slightly differently.
Those different pigments reflect differing amounts of light, which can be measured by a leaf reflectometer, a highly sensitive and compact device about the size of a shoebox.
“Light is either absorbed, transmitted or reflected. This instrument measures how much light is reflected after light enters the leaf,” said Nancy Marek ’09, who is studying spectral reflectance as a possible noninvasive measure of carbon sequestration. This reflected light “can give us a window into the health of the plant.”
In a paper published in 1968, Berlyn reported that by measuring the light passing through sections of wood from a red pine tree, many of the structural and chemical properties of its cell walls were revealed. By the 1980s, Berlyn used spectral reflectance on individual leaves, perhaps the first scientist to do so. That research led to a paper in 1993 about the use of the technology to document damage to spruce needles because of acid rain.
Working in Greeley Memorial Laboratory, Thorhaug pinpoints precisely which pigment signatures are associated with specific stresses, such as sewage pollution or excessively warm water temperatures.
Starting with healthy seagrass plants, Thorhaug exposes them in the lab to different stresses, measuring the resulting pigment signatures. That science is fundamental to the monitoring system she envisions. She and her colleagues applied for a $250,000 grant from the United Nations Food and Agriculture Organization to help support the research.
The idea is to use remote sensing data already gathered each day by NASA satellites orbiting the Earth. The remote sensing imagery from a satellite itself is a form of spectral reflectance, though on a grand scale. Berlyn and Thorhaug use spectral reflectance at the individual plant scale. But whether done at the macro or micro level, both rely upon the same band of light waves and can be correlated, Thorhaug said.
Remote sensing has many advantages, including the ability to monitor seagrasses in extreme cold conditions or in hard-to-reach places. For example, if an oil spill blanketed a seagrass meadow in a remote part of the world, a satellite might quickly spot the signature of oil-stressed seagrass, potentially hastening a cleanup. More gradual impacts from other stresses could be measured over a period of months or years. Drawing upon satellite data from previous years, a nation also could determine where its seagrass meadows once flourished and restore them.
“It is a technology that works for all plant species. This is just perhaps the most dramatic application of it today,” said Philip Marshall ’02, an F&ES doctoral student and lecturer.
Thorhaug is president of the United States Association for the Club of Rome, which supports efforts to ensure a sustainable world, and is also president of the Greater Caribbean Energy and Environment Foundation. For decades she’s worked on seagrass restoration and protection projects for the United Nations. Noel Brown, former North American director of and now a special advisor to the United Nations Environment Programme, said Thorhaug is not only a highly respected scientist, but also a skilled communicator who works easily with people in other cultures around the world.
“I would call her equally a scientist and diplomat,” Brown said. In fact, if seagrasses have an ambassador, it is Thorhaug, who is knowledgeable about the condition of seagrass meadows in much of the world.”
The need for an aerial monitoring system is greatest among “poor developing countries, such as those in the Caribbean, that don’t have the skill sets and technologies to monitor their own resources,” Thorhaug said. “Along the shores of nations such as Nicaragua, Papua New Guinea, Indonesia and the Philippines are vast seagrass beds. But those nations generally have little money for environmental protection, especially for underwater plant species, regardless of their importance.” Unfortunately, “seagrasses are the last thing you preserve,” she said.
Signatures for seagrasses stressed by heated waters discharged from power plants and by low salinity already are documented and can be recognized in remote sensing imagery for areas off Florida, Texas and Jamaica.
While the technology is promising, it’s probably a decade away from worldwide implementation, Thorhaug said. There are about 50 seagrass species, and signatures for specific stresses vary among them, so the process of identifying signatures is time-consuming.
Even when all the signatures are documented, in places like the mouths of the Amazon, Mississippi and Nile rivers, where turbidity is a constant condition, the technology is unlikely to work, she said. Elsewhere, however, turbidity is usually temporary. Likewise, cloud cover is usually temporary. Thorhaug said aerial surveillance is worthwhile because it can precisely quantify degradation and pinpoint its causes.
At Yale, Thorhaug, Berlyn and Andrew Richardson ’98, Ph.D. ’03, determined just how sensitive seagrasses are to reduced salinity. They used spectral reflectance to study the effects of reduced salinity on an important seagrass species, turtle grass, Thalassia testudinum, usually the dominant vegetation in Atlantic subtropical and tropical underwater ecosystems. They found that Thalassia is unable to endure salinities that drop to 16 or 20 parts salt per thousand parts water for periods of 48 hours or more—levels that seagrasses are often subjected to when freshwater flows are concentrated in one area. “They get sick and die, in short,” Berlyn said.
Thorhaug remains positive.
“You have to be an optimist if you are going to find good solutions,” she said. “You have to keep working. You have to believe there is a better way. People call me a technological optimist. That is what I am.”