USA - 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.

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