Art Courtesy of Luna Aguilar.
Due to climate change, sea levels could rise twelve inches in the next three decades, equivalent to the measured rise seen over the last century. This rise stresses vegetated coastal ecosystems, which include mangroves, salt marshes, and seagrasses. Coastal ecosystems provide habitats for a wide variety of wildlife and protect both humans and animals from storms. They also contribute to stabilizing the shoreline, filtering nutrients, and storing carbon dioxide from the atmosphere in the ground. “Coastal ecosystems are very near and dear to my heart,” Sinéad Crotty, associate director of science at the Yale Carbon Containment Lab, said.
In a recent paper published in Nature Communications, Crotty examined the direct and indirect effects of one unassuming invertebrae—mussels—on the persistence of vegetated coastal ecosystems.
Even small changes in sea levels can substantially alter these ecosystems through coastal flooding, resulting in higher storm surges—the rise of seawater during a storm—and an influx of saltwater into freshwater habitats. Sea level rise has prompted more effort and resources to be directed to vertical and horizontal accretion, a natural process that results in a change in the elevation of salt marshes. Accretion can indicate how well a salt marsh is persisting in spite of changes in sea level, and, according to Crotty, refers to how the marsh moves upwards to compensate for sea level rise. It can increase vertically, up and down in direction, or horizontally, in the landward direction of the boundary of the marsh.
Studying accretion is particularly important because it helps prevent a phenomenon known as drowning, in which the vegetated area of a salt marsh is converted into an open water area. This originally looks like a small pond that eventually grows larger. As the salt marshes drown, the vital services that they provide, including habitation, storm buffering, and carbon storage, also disappear, destabilizing the wildlife around it.“We care deeply about accretion because for marshes to not drown, we need the rate of accretion to be greater than the rate of sea level rise,” said Hallie Fischman, a Ph.D. student at the University of Florida and an author of the paper.
Historically, studies on accretion focused on environmental factors, such as tidal range and sediment supply. The tidal range consists of the difference between the highest and lowest point of the tide, and sediment supply refers to the availability and transport of sediment. However, organisms can also shape their environment, which includes modifying accretion processes. This concept is known as faunal engineering. It refers to animals that provide habitats, nutrients, or some other alterations that allow other organisms, as Crotty put it, to ‘persist and thrive.’
Mussels: The Underdogs of Coastal Ecosystems
One of the most abundant faunal engineers present in salt marshes in the United States are Atlantic ribbed mussels. Mussels can be easy to miss in these vast ecosystems. Yet even though these organisms are tiny, their functions are vital–they can, either directly or indirectly, alter plant growth and improve water quality by removing organic matter and unwanted particles. These particles and organic matter are filtered by mussels, processed, and subsequently excreted. Therefore, mussels help with sediment deposition, the deposition of the organic material that remained after digestion. Mussels are also an important food source for many terrestrial and aquatic organisms.
Researchers at the Yale Carbon Containment Lab and the University of Florida were interested in quantifying the effects of the Atlantic ribbed mussel on accretion in southeastern US salt marshes. They performed three field experiments to fulfill this objective.
The first field experiment was to investigate whether the sediments deposited by mussels could supply marshes beyond the areas where the mussels were gathered. Where exactly was the sediment spreading? To answer this question, the researchers tagged biodeposits that were already on the mound. Then, the fluorescent chalk was mixed with already deposited biodeposits, and the researchers came back to trace the movement at night. “It was a crazy idea that we could feed mussels orange chalk, they would poop it out, and we could follow it across the marsh,” Fischman said.
The researchers returned at night and used black light detection to trace the distribution of the fluorescently-tagged biodeposits. Blacklight, or ultraviolet (UV) light, is a tool commonly used to detect fluorescently-tagged materials. The maximum distance that fluorescent biodeposits traveled was measured in every direction.
Interestingly, the researchers discovered that the biodeposits were quickly redistributed beyond the areas where mussels were present. To confirm these results, the researchers collected approximately ten mussels from each of their six mounds, brought them to the University of Georgia’s laboratory, and fed them a mixture of seawater and chalk, which resulted in the excretion of fluorescently-tagged biodeposits. After the previous fluorescent material was washed away, the researchers planted the mussels in the respective mounds and performed the study again, subsequently confirming their previous findings. One limitation of this experiment that the researchers hope will be addressed in future studies is that the fluorescent dye in the biodeposits functioned for only the first twenty-four hours, so the study was performed on a limited time scale.
In the second field experiment, the researchers wanted to investigate the effects of cordgrass, also known as marsh grass, and mussels on sediment deposition at different marsh elevations. Seven different scenarios were tested in two zones in the state of Georgia for a month: the creekhead, or where a narrow inlet called a tidal creek enters onto the marsh, and the marsh platform, or the main flat surface that extends landward. These scenarios included alterations to the presence of cordgrass and mussels, as well as the density of mussels.
Cordgrass was found to have no significant effect on sediment deposition in both zones, which further highlights the importance of mussels. As for mussels, after performing statistical analysis, the researchers discovered a positive correlation between the number of mussels and increased sediment deposition. The implications of this study are limited by the size of the areas that were observed.
In the third field experiment, to assess the potential effects of mussels on marsh accretion at the level of a creekshed, a smaller version of a watershed, researchers manipulated the presence and population size of mussels. Approximately two hundred thousand mussels were manually moved from one creekhead to another. A control creekhead side in the same marsh was also established. With the help of a tool called the Digital Elevation Model (DEM), the researchers were able to assess the elevation of the creekheads after three years. DEM was created using drone imagery, which was then filtered and edited to remove vegetation to focus on the sediment. The team discovered that the creek from which the mussels were removed decreased in elevation, but the creek to which mussels were added increased in elevation. Mussels were putting in the work.
To further explore and validate the conclusions from their field experiments, the researchers used the Delft-3D-BIVALVES model, a digital tool to create simulations, generating different scenarios that mimicked salt marshes in the region. This model determined that the highest sediment accretion was found on mussel aggregations using simulations based on the assumption that mussels filter the water column—which refers to the space between the surface and floor of a body of water. Mussels expel the filtered content and their feces on mounds, which facilitates a build-up of sediments that increases the elevation of the salt marsh.
“While the manual labor and methodology development were at times challenging, I think ultimately this has been one of the most collaborative and gratifying scientific efforts that I have been a part of,” Crotty said. Each of these field experiments in addition to the simulation models contributed to the overall claim that mussels increase salt marsh accretion. The results suggest that mussels contribute significantly to sediment deposition and vertical accretion in salt marshes.
New Ideas for Mitigating Sea Level Rise
“The coolest takeaway of this study is that mussels increase salt marsh accretion, which means that mussels are important in helping marshes keep up with sea level rise,” Fischman said. Other researchers can use the insights provided by this study to direct their research projects on animals similar to mussels. Mussels may not, however, be the only animals that are contributing to the health of their ecosystems.
Due to climate change, scientists are observing a phenomenon in which small animals are adapting their roles to their changing environment. Understanding the behavior of these animals can help guide future research on other types of terrestrial and marine ecosystems. “Relatively small animals are increasing in their relative importance and the roles that they play in response to the changes that climate change is implementing,” Crotty said.
As for next steps, Fischman is interested in the effect of mussels on nitrogen cycling, which is a cycle of various processes in which nitrogen moves through living and non-living things in the environment.
As climate change continues to worsen, an additional rise in sea levels is inevitable. Among its dangers are more frequent and intense floods, higher storm surges, and loss and alteration of coastal habitats. Future studies, including those on overlooked parts of an ecosystem, can hopefully help mitigate its harmful effects.