Seagrass as a carbon sponge?
U-M studies suss out the impact of nutrients on coastal seagrasses
University of Michigan
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University of Michigan researchers, including recent doctoral graduate Bridget Shayka, studied the impact of nutrients on seagrass. They studied plots of seagrass growing in a bay in The Bahamas, where Shayka took this photo.
view moreCredit: Bridget Shayka
ANN ARBOR—Seagrass has the potential to be one of the world's most effective sponges at soaking up and storing carbon, but we don't yet know how nutrient pollution affects its ability to sequester carbon.
In a pair of studies, U-M researchers evaluated the impact of nitrogen and phosphorus on seagrass, short, turf-like grasses that live in shallow, coastal areas. Examining data gathered from a plot of seagrass enriched with nutrients over a period of nine years, the scientists found that nutrients can increase seagrass's ability to store carbon. However, in a second study, they also found that an overload of nitrogen could lead to increased phytoplankton growth, which can shade out and kill seagrass.
Both studies, published in Global Change Biology and Conservation Letters, respectively, were supported by the National Science Foundation and the David and Lucille Packard Foundation.
Jacob Allgeier, associate professor of ecology and evolutionary biology, studies fish and seagrass and coral ecology in bays of the Bahamas and Dominican Republic. He noticed that seagrasses growing in bays overloaded with nutrients, mostly from human wastewater, quickly died off. Light couldn't penetrate through the phytoplankton, which also proliferated under the high-nutrient conditions.
However, in other bays that experienced nutrient runoff but didn’t have light-shading phytoplankton, he saw that seagrass grew well. Coastal tropical areas are often nutrient-starved. When the seagrasses growing there are bathed in nutrients, their growth takes off as long as there is not too much phytoplankton blocking out light, Allgeier says.
The researchers, led by recent U-M doctoral graduate Bridget Shayka, found that in relatively nutrient-poor beds of seagrass, phosphorus and nitrogen helped seagrass grow. As the seagrass grew, it first invested in its underground growth, or root systems, storing carbon in its roots. Then, the grass invested in above ground structures—their blades. The end result was that the roots grew quickly but also died quickly, which shunted extra carbon into the sediment surrounding the roots and sequestered it at a higher rate.
"People thought excess nutrients were killing seagrass," Allgeier said. "But we show that as long as there are not too many nutrients, which would also increase phytoplankton, the seagrass will just increase growth with excess nutrients."
To study nutrient impacts, Shayka and Allgeier took samples of the seagrass from test plots in the Bahamas that had been treated with nutrients for nine years. Shayka and a raft of 17 undergraduates painstakingly untangled the seagrass, separating the grass into parts: the blades that grow above ground but under water, the sheath from which the blades emerge, the seagrass's roots and the seagrass's rhizomes—basically an underground stem from which other seagrass plants can grow.
They then freeze-dried each part, pulverized them and tested them for nitrogen, phosphorus and carbon. In addition to finding that increasing nutrients in the system increased carbon turnover in the plants, the researchers also found that nutrients supplied by human sources had a greater impact on the seagrasses than those supplied by fish.
"The systems we study are pretty low-nutrient systems, so adding nutrients can increase seagrass production," said Shayka, now a program officer for the nonprofit Ocean Visions. "But we also know that when you go too far and add too many nutrients, it really destroys these systems. It's one of the leading causes of their destruction around the world and in coastal systems."
In the second study, the researchers tested which nutrient, nitrogen or phosphorus, had the greatest impact on seagrass, as well as whether the ratio of nitrogen to phosphorus or the total amount of each nutrient had the greatest effect on the system. They also examined nutrient impacts on phytoplankton.
To do this, they created 21 different ratios of nitrogen to phosphorus and added nutrients to test plots of seagrass growing in a different part of the same bay as well as to phytoplankton in bottles. They found that phosphorus had a bigger positive effect on seagrass growth than nitrogen in the nutrient-poor conditions.
Longstanding ecological theory suggests that the ratio of nutrients has the greatest impact on a system—but the researchers found that in this particular case, phosphorus had a greater effect on seagrass while nitrogen had a greater effect on phytoplankton growth. In particular, the researchers found that the addition of nitrogen caused the rates of phytoplankton in the bottles to skyrocket, showing that increasing nitrogen in the natural landscape could lead to levels of phytoplankton that would shade out the seagrass.
"When you grow tomatoes, you don't just add nitrogen. You add a perfect ratio of nitrogen and phosphorus. That idea is replete in our society," Allgeier said. "But because we tested the water column and because we tested the seagrass, we're able to say that model doesn't work in our system."
This finding could inform how local communities control for nutrient impacts to seagrass.
"We're not stopping nutrient enrichment. It's just not going to stop," Allgeier said. "But we can manage it. And how do you best manage it? We scrub it for nitrogen."
Study 1: Nutrient Enrichment Increases Blue Carbon Potential of Subtropical Seagrass Beds
Study 2: A New Conceptual Model of Tropical Seagrass Eutrophication: Evidence for Single Nutrient Management
Journal
Global Change Biology
Method of Research
Experimental study
Article Title
Nutrient Enrichment Increases Blue Carbon Potential of Subtropical Seagrass Beds
Seagrass swap could reshape Chesapeake Bay food web
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Seagrasses in the Chesapeake Bay provide foundational structure impacting biodiversity, food webs and ecosystem processes. Photo by Frederick Corey Holbert
view moreCredit: Frederick Corey Holbert
Beneath the surface of the Chesapeake Bay, a subtle but dramatic shift is taking place as eelgrass gives way to its warmer-water relative, widgeon grass. A new study from researchers at William & Mary's Batten School & VIMS shows that this seagrass swap could have ecological impacts across the Bay’s food webs, fisheries and ecosystem functions.
Published in Marine Ecology Progress Series, the study reveals that while both seagrass species offer valuable habitat, they support marine life in very different ways. The researchers estimate that the continued shift eelgrass to widgeon grass could lead to a 63% reduction in the total quantity of invertebrate biomass living in seagrass meadows in the Bay by 2060.
“Several factors including water quality, rising temperatures and human development are threatening eelgrass in the Chesapeake Bay. In its place, particularly in the middle Bay, widgeon grass has expanded due to its ability to tolerate warmer, more variable conditions,” said Associate Professor Chris Patrick, who is also director of the Submerged Aquatic Vegetation (SAV) Monitoring & Restoration Program at the Batten School of Coastal & Marine Sciences & VIMS. “However, the two grasses provide structurally distinct habitats that shape the animals living within.”
All grasses are not created equal
While working with Patrick and earning her master’s degree at the Batten School & VIMS, lead author Lauren Alvaro M.S. ’23 engaged in extensive fieldwork studying seagrass meadows in Mobjack Bay. Her team surveyed and compared habitats consisting of eelgrass, widgeon grass as well as mixed beds. They documented everything from burrowing clams and snails to crabs and fishes to get an idea of life living within the sediment and amongst the grasses.
The findings showed that while widgeon grass supports more individual invertebrates per gram of plant material, eelgrass meadows are home to larger animals and have more plant biomass per square meter. As a result, eelgrass supports a greater total animal biomass per square meter.
“Our findings suggest that we’re likely to see a fundamental shift in the structure of the food web that favors smaller creatures as eelgrass is replaced by widgeon grass,” said Alvaro. “The eelgrass meadows produced fewer animals, but they’re bigger and more valuable to predators like fish and blue crabs.”
Much of the difference is due to the physical characteristics of the two types of seagrasses. Widgeon grass beds have a greater surface-to-biomass ratio due to their narrower leaf structure, which provides more area for small invertebrates to cling to. However, eelgrass’ broader leaves provide a type of canopy favored by animals like pipefish, blue crabs, and larger isopods, which are small shrimp-like crustaceans.
The bigger picture
The researchers extrapolated their findings and estimated that current seagrass habitats in the Chesapeake Bay support approximately 66,139 tons of invertebrate biomass living in the sediment and amongst the grass beds and produce 35,274 tons of new animal biomass each growing season. Termed “secondary production,” this is the biomass the habitat makes available to higher levels of the food chain.
If seagrasses continue to shift as expected, by 2060 secondary production could be reduced by more than 60% under a scenario where no further nutrient reductions occur. Nutrient runoff into the Bay is the largest threat to submerged aquatic vegetation. Even in a best-case nutrient management scenario, the Bay could still lose approximately 15% of secondary production biomass.
“Within the limits of our study, it wasn’t possible to determine whether it was the meadow’s physical structure, the meadow area, or available food sources that contributed to greater numbers of fish in the eelgrass meadows,” said Alvaro. “This makes it difficult to accurately estimate fishery-level impacts of changes in meadow composition, but several lines of reasoning support an expectation of reduction in numerous commercial and recreational species.”
The study adds to a growing body of research documenting the effects of changes in foundational species influenced by a warming planet. The authors cite similar research involving Florida’s mangroves and a worldwide shift from coral to algae-dominated ecosystems.
As states within the Bay’s extensive watershed work to maintain and improve the health of the estuary, the team hopes their findings will help inform management decisions and restoration strategies. Protecting and restoring the remaining eelgrass and better understanding the role of widgeon grass may help preserve ecological resources for future generations and provide a buffer against future shocks.
Visit the website for the SAV Monitoring and Restoration Program for more information about seagrass research at the Batten School & VIMS.
Journal
Marine Ecology Progress Series
Article Title
Changing foundation species in Chesapeake Bay (USA): implications for faunal communities of two dominant seagrass species
Article Publication Date
4-Sep-2025
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