Getting the edge on seagrass meadows.

By Natalie Ritter (The School for Field Studies), Violet Apodaca (Bowdoin College), Kira Kaplan (Smith College), Lily Marynik (Wellesley College), Sophie Moniz (Connecticut College), Rory O’Connor (Lehigh University), Paul Stanley (University of Massachusetts Amherst) and Aliya Swearngin (Grinnell College)

Edited by Dr. C.E. O’Brien, The School for Fields Studies, Center for Marine Resource Studies 

Over the past year, students and staff from the School for Field Studies (SFS) Center for Marine Resource Studies (CMRS) conducted a study on the relationship between seagrass beds—a critical marine ecosystem—and a small and unassuming group of clams called lucinids, in the shallow waters surrounding South Caicos. In collaboration with Dr. Robert Johnson from the University of Wisconsin, this team was interested in learning how these organisms support and strengthen each other’s ability to survive in our ever-changing world. 

Why study seagrass meadows?

A graceful southern stingray glides over a seagrass meadow off the coast of Providenciales.

A seagrass meadow is much like a terrestrial grass meadow, with lush expanses of green grass covering the seafloor. While they may not look like much, seagrass meadows play a crucial role in cycling essential nutrients in the oceans, making them available to other organisms such as fish, turtles, and various invertebrate species like conch and lobster, thus sustaining a healthy and diverse ecosystem and promoting the success of local fisheries and tourism.

Seagrass meadows in the tropical western Atlantic, including the Turks & Caicos Islands, are composed of three main species: turtle grass, manatee grass, and shoal grass. Combined, these species cover over 88,170 km² (about 54,786.3 mi²) and are estimated to be able to store up to 1,337.8 Tg of carbon annually. This is equal to the yearly carbon emissions of 74,000 US households! Thus, despite occupying only a small fraction of the ocean floor, they are responsible for 15% of the ocean’s total organic carbon storage. This makes them one of the most productive ecosystems on our planet and key to reducing planet-warming greenhouse gas emissions.

Seagrass meadows in the tropical western Atlantic are also critical for regional biodiversity as they are home to at least 313 species of fish. These meadows act as habitat, allowing small organisms to hide from their predators and reproduce. The extensive root and rhizome systems of seagrasses also help prevent coastal erosion by trapping sediment and pathogens, keeping the water clean and clear.

Unfortunately, the Caribbean and wider tropical western Atlantic are experiencing an increase in human-caused problems such as pollution, industrial development, and the effects of climate change. In the face of these challenges, it is vital to understand the effects of disturbance to seagrass ecosystems in order to predict potential changes in ecosystem services and productivity of seagrass meadows in this region. 

An important relationship

This tiger lucine (Codakia orbicularis) is lying amid sand and seagrass off the shores of South Caicos.

Key to seagrasses’ ability to provide ecosystem services is their mutualistic relationship with lucinids. The high productivity of seagrass results in the rapid accumulation of dead plant tissue (seagrass blades and roots) and other organic matter (waste from animals inhabiting the meadow) in the sediment. Decomposing bacteria works quickly to break this down, and this process produces a toxic compound called hydrogen sulfide as a byproduct. Over time, hydrogen sulfide accumulates, making it difficult for seagrass to grow and thrive.

Lucinids live in the sediment beneath seagrass meadows and contain a special type of “sulfide-oxidizing” bacteria. This mutualistic bacteria breaks down the toxic hydrogen sulfide into a much safer substance called sulfate, which is used by seagrasses to grow. Measurements of sulfides in seagrass meadow sediments have found that lower levels of hydrogen sulfide are correlated with the number of lucinids present, demonstrating that these small bivalves and their symbiotic bacteria are vital to reducing the concentration of hydrogen sulfide in seagrass meadows. Our research team set out to investigate the relationship between seagrass and lucinids of South Caicos by conducting snorkel and SCUBA surveys during Fall 2023 and Spring 2024.  

Getting edgy

Here is a seagrass erosion edge approximately 0.3 meters in height.

To study the relationship between seagrass and lucinids, our team took advantage of a natural feature of seagrass meadows called “erosion edges.” These are an abrupt break in a seagrass meadow, with a cliff-like ledge that is composed of several centimeters of sediment, exposed seagrass roots, and rhizomes. These edges form as a result of natural disturbances like storms and animal grazing, as well as human-caused disturbances like dredging or damage from boating activities. Above the edge lies undisturbed seagrass and below is typically a sandy plain that is devoid of seagrass as a result of the disturbance.

These erosion edges allow us to observe the relative abundance of lucinids in their preferred habitat (meadow above the edge) versus a less ideal habitat (sandy plain below the edge). Since seagrass meadows are fragmenting more rapidly as a result of increasing storm and human activity, it is critical to understand how fragmentation could potentially impact seagrass meadows, lucinids and the ecosystem services (habitat, food, clean water) they provide.

In November 2023 and April 2024, our CMRS research team, known affectionately as the “Core-lation Corps,” conducted surveys and collected sediment samples from eight different sites around the island of South Caicos. Snorkeling or SCUBA diving, we performed a transect (essentially a long tape measure) survey measuring the percentage of seagrass cover across ten meters both above and below eight erosion edges. This was accomplished by placing a PVC frame (a “quadrat”) every two meters along the transect and estimating the proportion of it covered by each of the three species, as well as measuring a random subset of the seagrass blades. By doing this five times, we were able to get a good estimation of the species composition of the meadow and the density of seagrass coverage.

We also used a cylindrical “drill” to collect sediment cores from both above and below the erosion edge. This was the most difficult part of the work, since pushing the drill into the sea floor took a great deal of effort and air, especially in the water with no point of leverage. The process was not without its rewards, however, as stirring up the sediment with this drill attracted many curious wrasses and pufferfish that wanted to see what we would dig up!

SFS students assess seagrass coverage along the transect line.

Back in the lab, we sorted through the sediment cores by hand (resulting in lots of mud under our fingernails!) to identify, count and measure every live and dead lucinid so that we could compare their abundance above and below the edge. As predicted, we found that live lucinids were more abundant in seagrass meadows above the erosion edge than in the mostly sandy plain below.

This finding supports the idea that the less stable, lower-sulfide sediments below erosion edges are less suitable habitats for lucinids, and thus that disturbances to seagrass meadows limit the lucinids’ access to suitable habitat. More difficult to explain was the fact that we found no direct correlation between seagrass abundance and lucinid abundance. This suggests that the amount of plant material produced by the seagrass in a meadow and the hydrogen sulfide produced by its subsequent breakdown is not the only determinant of lucinid success. Other factors, such as high levels of predation on lucinids in the meadow or differences in the ease with which lucinids can burrow into the sediment may also play a role. Future research will hopefully uncover some of these factors.

As human activities and habitat disturbances increase, seagrass beds become more fragmented, leading to more erosion edges. With lower sulfide levels in the sediment below the edge, fewer lucinids populate the disturbed sediment, which may hamper the ability of seagrass to recolonize that area, slowing any potential recovery of the meadow. If the pace of fragmentation exceeds that of recovery, the beautiful seagrass meadows of the Turks & Caicos Islands will disappear over time, leading to a loss of fish, conch, and lobster, more eroded coastlines and dirtier water. 

The Core-lation Corps’ research has emphasized the importance of protecting these vital habitats. To help protect our seagrass, it is important to set anchors carefully to avoid damaging seagrass beds and to support sustainable tourism practices by avoiding resorts that dredge to remove seagrass for aesthetic purposes. These simple practices can help ensure the survival of seagrass meadows and the myriad of species that depend on them, preserving the natural beauty and abundance of the Turks & Caicos Islands for generations to come.

The School for Field Studies is a US-based academic institution that provides multidisciplinary, field-based environmental study-abroad opportunities to undergraduate university students. Long term data collection by staff and students is a crucial element of observing and understanding climate impacts, beneficial to stakeholders, and essential to policy makers.

To learn more about the SFS TCI program, visit www.fieldstudies.org/tci. For detailed article references, contact Director Heidi Hertler on South Caicos at hhertler@fieldstudies.org.