Where Has All the Eelgrass Gone?

A slide showing two different sets of eelgrass plants glowed on the screen. The set on the left seemed stunted and short. The set on the right had willowy, long shoots. If you knew nothing about eelgrass, you might conclude that the plants on the right were the healthier of the two.

In fact, however, the long seagrass blades indicate the plants were struggling to find light and put energy into growing taller, rather than sprouting new shoots every few days. The shorter plants with more shoots were actually hardier, had a more extensive root system, and would be better able to withstand a storm with wave action that could pull up the longer, “top heavy” plants with less-developed root systems. And the health of eelgrass is closely tied to the health of aquatic ecosystems.

“Boaters hate it; it runs into their props, but we love it because it provides a protective habitat for juvenile finfish and food for waterfowl. It … increases the clarity [of water], and … it’s a resource for food and habitat for a variety of different animals and some other plants, too,” said Steve Granger, a researcher at the University of Rhode Island’s Graduate School of Oceanography (GSO), who discussed the state of eelgrass beds in Rhode Island at the Ronald C. Baird Sea Grant Science Symposium.

Also known as seagrass, eelgrass used to thrive in extensive, thick green carpets in the Providence River estuary—giving 33-acre Green Jacket Shoal in the Providence River its name. One of the most productive ecosystems in the world, eelgrass also buffers coastlines from storm surge and waves, filters water, and removes carbon from the atmosphere.

Only a fraction of Narragansett Bay’s eelgrass beds remain, having been compromised by impacts of coastal development, nutrient loading from runoff and wastewater discharge, and climate change.

“[Eelgrass] can grow in a variety of sediments, but it doesn’t like highly organic sediments,” he said, because they can develop sulfides that are toxic to eelgrass, referring to the increase in nitrogen and other nutrients in the bay bottom.

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Many species want to grow on eelgrass leaves. Healthy plants grow new shoots to keep ahead of fouling organisms, which, unlike parasites, do not take nutrients directly from their host. Photo courtesy Cornell Cooperative Extension Marine Program

“The bay is a continuum of water quality,” he added, “The poorest is at its head because 60 percent of the nutrients entering the bay enter through the Providence River … This little portion of the bay is receiving an enormous amount of nutrients, translating it into primary production and pumping it down into the bay.”

Even though the state has achieved a 55 percent reduction in nitrogen from wastewater treatment facilities, nitrogen levels are still three times higher than they have been historically, said Candace Oviatt, a professor at GSO specializing in biological oceanography. On top of that, water temperatures in the bay have risen nearly 2˚C (3.6˚F).

Eelgrass is “a cold-loving plant,” Granger said, explaining that it migrated from the Arctic thousands of years ago to populate U.S. coastal waters as far south as North Carolina in the East and the Gulf of California in the West, where average summer temperatures reach 85˚F. But, as a result of increasing nutrient inputs, the long continuous ribbon of eelgrass that once extended along the East and West coasts broke apart.

“The geographic distance between beds became so great that they can’t exchange pollen between beds and …. They become genetically isolated in that particular area”—preventing the exchange of genetic material that might help plants in Narragansett Bay, for instance, adapt to conditions such as warming waters.

During experiments to better understand the relationship between nutrients and temperature on eelgrass, Granger found that colder temperatures could help offset some of the effects from increased nutrients, but once temperature was also increased it was all over.

“The combination of rising temperatures and nutrients is just lethal,” he said. “It stresses the plant and stops it from producing new leaves, which get long and heavily fouled and die.”

Despite this general downward trend for eelgrass beds in both the bay and the coastal ponds, there is some hopeful news. New eelgrass beds have formed in East Greenwich and in the Narrow River.

Charlestown Pond is also looking good, said Granger, as shoots that were transplanted there have all taken to their new home. The future, however, is a warmer one that is compelling researchers like Granger to look at alternatives.

“Southern populations are more heat tolerant and developed that ability over time,” he said. “Looking for genetically disposed plants that are better suited to heat, I think, is the future.”

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– Meredith Haas | Rhode Island Sea Grant

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More information about the Baird Symposium, including links to videos of the morning and afternoon sessions and the PowerPoint presentations, is available online. Results will be used to inform the upcoming Rhode Island Sea Grant research request for proposals and the Rhode Island Coastal Resources Management Council’s Narragansett Bay Special Area Management Plan.

The symposium was sponsored by GSO, Rhode Island Sea Grant, the URI Coastal Resources Center, and the van Beuren Charitable Foundation. 

 

 

 

 

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