Seagrass meadows can recover after major die-offs

Massive seagrass die-offs are currently occurring all over the world due to a variety of stressors. These include high temperatures, hypersalinity, hypoxia, and exposure to sediment-derived hydrogen sulfide, a phytotoxin which accumulates as seagrass meadows become richer in nutrients. 

Although hydrogen sulfide intrusion into seagrass tissue is considered a leading cause of recurrent mortality events, its effects on subsequent recruitment and distribution of new populations, along with the ability of seagrass meadows to “bounce back” and recolonize in open bare patches, have not yet been properly investigated. 

Now, a team of scientists led by Florida Atlantic University (FAU) has examined whether porewater hydrogen sulfide prevents Thalassia testudinum – a tropical Atlantic-Caribbean marine seagrass commonly known as turtlegrass – from recruiting into unvegetated sediment in Florida Bay, one of the largest contiguous seagrass systems in the world.

“Seagrass meadows sustain coastal ecosystems by protecting against erosion, maintaining water quality, and providing habitat and food for many marine species and organisms,” said senior author Marguerite Koch, a professor of Biological Sciences at FAU. “Because of their importance in coastal communities, the current decline of seagrass ecosystems on a global scale across geographic regions is a concern.”

Since the 1980s, seagrass meadows in Florida Bay – an estuary covering 1,100 square miles between Florida Keys and the southern tip of Florida – have experienced recurrent biomass losses and die-offs, usually occurring during high temperature and salinity conditions. 

These particular seagrass meadows are exposed to high levels of porewater hydrogen sulfide and are surrounded by vast unvegetated areas that are usually recolonized by turtle grass recruits after major die-offs, which makes them an excellent case study to investigate seagrass resilience and its relation to hydrogen sulfide exposure.

The experts analyzed the leaf, stems, and roots of turtlegrass to determine tissue exposure to hydrogen sulfide in new recruits and, by using state-of-the-art microsensors and stable isotopic analyses, they measured internal hydrogen sulfide and oxygen dynamics. 

The investigation revealed that, after die-off events, turtlegrass can successfully recruit into open bare sediment due to biomass partitioning during early development (a process by which they efficiently divide their energy among roots, leaves, stems, and reproductive parts), young root structure, and a capacity to efficiently oxidize internally that helps them lower hydrogen sulfide exposure.

Although these findings suggest that turtlegrass can be highly resilient and able to recover after mortality events, their recovery takes quite a long period of time. The research is published in the journal Aquatic Botany. 

“Long-term monitoring programs in Florida Bay indicate that the time frame for full recovery of turtlegrass meadows after major die-off events is at least a decade. Therefore, preventing large-scale seagrass mortality events should be the management goal, particularly as global warming and associated stressors are likely to get more extreme in the future,” Koch concluded.

Seagrass meadows and the environment 

Seagrass meadows are underwater plant communities comprised of flowering plants known as seagrasses. They are found in shallow coastal waters and estuaries, and are essential for maintaining healthy marine ecosystems. Seagrass meadows provide various environmental benefits, including:

Carbon sequestration

Seagrasses are highly efficient at capturing and storing carbon dioxide from the atmosphere. This process, known as “blue carbon” sequestration, helps mitigate climate change by reducing the amount of greenhouse gases in the atmosphere.

Water quality improvement

Seagrasses act as natural water filters, trapping sediment and absorbing excess nutrients such as nitrogen and phosphorus. This helps maintain water clarity and prevents harmful algal blooms, which can lead to dead zones in the ocean.

Habitat provision

Seagrass meadows serve as essential habitats for a wide variety of marine species, including fish, crustaceans, and marine mammals. These habitats support high levels of biodiversity and act as nurseries for juvenile fish, which in turn support commercial and recreational fisheries.

Shoreline protection

Seagrass meadows help to stabilize the seafloor by holding sediments together with their roots and rhizomes. They also dampen wave energy, reducing coastal erosion and protecting shorelines from the impacts of storms and sea level rise.

Food source

Seagrasses provide a direct food source for herbivores, such as green sea turtles and manatees, as well as indirectly supporting other marine life through the food chain.

Despite their importance, seagrass meadows are under threat from various human activities, such as coastal development, pollution, and climate change. Loss of seagrass meadows can lead to reduced biodiversity, increased carbon emissions, and weakened coastal resilience. Efforts to conserve and restore seagrass meadows are essential for maintaining the health of our marine ecosystems and the valuable services they provide.

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By Andrei Ionescu, Earth.com Staff Writer

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