Seagrasses evolved from freshwater plants and use sunlight and carbon dioxide (CO2) for photosynthesis and are able to thrive in depths down to 50 meters. In contrast to algae, they possess roots and rhizomes that grow in sandy to muddy sediments. The grass-like, leaf-shoots produce flowers and complete their life
For approximately 3,000 years, generations of green sea turtles have returned to the same seagrass meadows to eat. This was discovered by Willemien de Kock, a historical ecologist at the University of Groningen, by combining modern data with archaeological findings. Sea turtles migrate between specific breeding places and eating places
Discussions of valuable but threatened ocean ecosystems often focus on coral reefs or coastal mangrove forests. Seagrass meadows get a lot less attention, even though they provide wide-ranging services to society and store lots of climate-warming carbon. But the findings of a new University of Michigan-led study show that seagrass
Mangroves are growing in areas historically dominated by salt marshes and oyster reefs. Invasive pacific oysters are replacing native blue mussels in the Wadden Sea. Macroalgae are exhibiting dominance over hard corals in the Caribbean and Indo-Pacific. Climate change-driven shifts in dominant, habitat-forming species such as these can have significant
Seagrass, a group of aquatic angiosperms, grows in shallow waters in the coastal sea and contributes most of the primary production while participating in many important ecological processes. Heat stress threatens the survival of seagrass, but its damage mechanisms are unclear. Recently, a research team led by Prof. Liu Jianguo
A study of eelgrass meadows planted by researchers from the University of Gothenburg shows that fauna return rapidly once the eelgrass has started to grow. Already after the second summer, the biodiversity in the planted meadow was almost the same as in old established eelgrass meadows. Eelgrass meadows have declined
In Florida alone, thousands of acres of marine seagrass beds have died. Major seagrass die-offs also are occurring around the world. Stressors such as high temperature, hypersalinity and hypoxia or lack of oxygen affect seagrasses’ ability to resist and recover from these stressor-related mortality events or when disturbances lead to
The sea devours large tracts of land when storms wash sand out to sea from the coast. A new study involving a researcher from the University of Gothenburg has shown that seagrass can reduce cliff erosion by up to 70% thanks to its root mats binding the sand. Coastal erosion
“The downward trajectory of the world’s seagrass meadows must be reversed if we are to fight the planetary crisis” say leading seagrass scientists. The United Nations Sustainable Development Goals have recently been described as “the most important to-do list in the history of humankind”. Scientists from Project Seagrass and Swansea
Deep evolution casts a longer shadow than previously thought, scientists report in a new paper published the week of Aug. 1 in the Proceedings of the National Academy of Sciences. Smithsonian scientists and colleagues looked at eelgrass communities—the foundation of many coastal marine food webs along the north Atlantic and
Seagrasses evolved from freshwater plants and use sunlight and carbon dioxide (CO2) for photosynthesis and are able to thrive in depths down to 50 meters. In contrast to algae, they possess roots and rhizomes that grow in sandy to muddy sediments. The grass-like, leaf-shoots produce flowers and complete their life cycle entirely underwater. Seeds are negatively buoyant but seed-bearing shoots can raft, thus greatly enhancing dispersal distances at oceanic scale. As a foundational species, eelgrass (Zostera marina) provides critical shallow-water habitats for diverse biotas and also provides numerous ecosystem services including carbon uptake. Seagrasses have recently been recognized as one of the important nature-based contributions to store carbon in the ocean. The sediment below seagrass meadows can sequester between 30 and 50 times more carbon annually that the roots of forests on land. Unfortunately, the continuing loss of seagrass beds worldwide—including eelgrass—is of acute concern. An international group of researchers, including Richard Unsworth and coordinated by Professor Thorsten Reusch, Head of the Research Division Marine Ecology at GEOMAR Helmholtz Centre for Ocean Research Kiel, used complete nuclear and chloroplast genomes from 200 individuals and 16 locations to reconstruct and date the colonisation history of the eelgrass Zostera marina from its origin in the Northwest Pacific Ocean to the Pacific, Atlantic and the Mediterranean. The findings described in an article and a Research Briefing published in Nature Plants beg the question, “How well will eelgrass adapt to our new, rapidly changing climate?” Using a phylogenomic approach the scientists were able to determine that Z. marina first arose in the Japanese Archipelago region and then crossed the Pacific from west to east in at least two colonisation events, probably supported by the North Pacific Current. The scientists then applied two DNA “molecular clocks”—one based on the nuclear genome and one based on the chloroplast genome—to deduce the time when eelgrass populations diverged into new ones. The DNA mutation rate was calculated and calibrated against an ancient, whole genome duplication that occurred in eelgrass. Both nuclear and chloroplast genomes revealed that eelgrass dispersed to the Atlantic through the Canadian Arctic about 243 thousand years ago. This arrival is far more recent than expected—thousands of years versus millions of years, as is the case with most Atlantic immigrant species during the Great Arctic Exchange some 3.5 million years ago. Reusch explains, “We thus have to assume that there were no eelgrass-based ecosystems—hotspots of biodiversity and carbon storage—in the Atlantic before that time. Recency was also mirrored in an analysis of the associated faunal community, which features many fewer specialized animals in the Atlantic as compared to the Pacific eelgrass meadows. This suggests that there was less time for animal-plant co-evolution to occur.” Mediterranean populations were founded from the Atlantic about 44 thousand years ago and survived the Last Glacial Maximum. By contrast, today’s populations found along the western and eastern Atlantic shores only (re)expanded from refugia after the Last Glacial Maximum, about 19 thousand years ago—and mainly from the American east coast with help from the Gulf Stream. In addition, the researchers further confirmed the huge difference in genomic diversity between the Pacific and Atlantic, including latitudinal gradients of reduced genetic diversity in northern populations. “Both Atlantic compared to Pacific populations, and northern versus southern ones are less diverse on a genetic level than their ancestors by a factor of 35 among the most and least diverse one,” said postdoctoral scientist Dr. Lei Yu, first author of the publication, which was a chapter in his doctoral thesis. “This is due to bottlenecks arising from past ice ages, which raises concerns as to how well Atlantic eelgrass, will be able to adapt to climate change and other environmental stressors based on its genetic capacity.” “Warming oceans have already caused losses of seagrass meadows at the southern range limits, in particular North Carolina and southern Portugal. In addition, heat waves have also caused losses in shallow waters in some the northern parts of the distribution,” noted Reusch. “This is not good news because seagrass meadows form diverse and productive ecosystems, and no other species is able to take on the role of eelgrass if meadows cannot persist under future conditions.” “One possibility for restoration might be to borrow some genetic diversity from Pacific eelgrass to fortify diversity in the Atlantic. Our next step is to interrogate the eelgrass pangenome. A new reference genome from Pacific eelgrass is currently under development and should tell us more about the adaptive ecotypic capacity across its global range of habitats,” said Prof. Jeanine Olsen, emeritus professor from the University of Groningen who initiated the study and coordinated the work between the Joint Genome Institute (JGI) and the research team. More information: Yu, L. etal, Ocean current patterns drive the worldwide colonization of eelgrass (Zostera marina), Nature Plants (2023). DOI: 10.1038/s41477-023-01464-3 Story provided by Helmholtz Association of German Research Centres
For approximately 3,000 years, generations of green sea turtles have returned to the same seagrass meadows to eat. This was discovered by Willemien de Kock, a historical ecologist at the University of Groningen, by combining modern data with archaeological findings. Sea turtles migrate between specific breeding places and eating places throughout their lives–this much was known. But the fact that this stretches over many generations highlights the importance of protecting seagrass meadows along the coasts of North Africa. The results were published in PNAS on July 17. When young green sea turtles hatch, their parents have already left for a long journey. The little turtles clumsily make their way off the beach into the ocean and, not yet able to navigate the long migration of their parents, float around for years. During this time, they are not very picky eaters, omnivores even. Then, at about five years of age, they swim to the same area where their parents went, to eat a herbivore’s diet of seagrass. Along the coasts of the eastern Mediterranean Sea, volunteers are active to protect the nests of the endangered green sea turtles. However, as Willemien de Kock explains, “We currently spend a lot of effort protecting the babies but not the place where they spend most of their time: the seagrass meadows.” And crucially, these seagrass meadows are suffering from the effects of the climate crisis. Analyzing sea turtle bones In the attic of the Groningen Institute of Archaeology at the University of Groningen, De Kock had access to boxes full of sea turtle remains from archaeological sites in the Mediterranean Sea area. The excavations were already done by her supervisor, Dr. Canan Çakırlar. “All I had to do was dig in some boxes,” De Kock says. By analyzing the bones, De Kock was able to distinguish two species within the collection of bones: the green sea turtle and the loggerhead turtle. De Kock was also able to identify what the sea turtles had been eating. This relied on a substance called bone collagen. By inspecting the bone collagen with a mass spectrometer, De Kock could detect what kind of plants the sea turtles must have eaten. “For instance,” De Kock explains, “one plant might contain more of the lighter carbon-12 than another plant, which contains more of the heavier carbon-13. Because carbon does not change when it is digested, we can detect what ratio of carbon is present in the bones and infer the diet from that.” Combining old and new Modern satellite tracking data from the University of Exeter then provided De Kock with information on the current traveling routes and destinations of sea turtles. Researchers from Exeter had also been taking tiny samples of sea turtles’ skins, which revealed similar dietary information as De Kock found in bones. De Kock was, therefore, able to draw conclusions, connecting diets of millennia ago to specific locations. She found that for approximately 3,000 years, generations of green sea turtles have been feeding on sea grass meadows along the coasts of Egypt and West Libya. The results for loggerhead turtles were less specific because they had a more varied diet. So, why is it relevant to know the eating habits of a species over many past generations? Because we collectively suffer from the shifting baseline syndrome: slow changes in a larger system, such as an animal population, go unnoticed because each generation of researchers redefines what the natural state was, as they saw it at the start of their careers. “Even long-term data goes back only about 100 years,” says De Kock. “But tracing back further in time using archaeological data allows us to better see human-induced effects on the environment. And it allows us to predict, a bit.” In fact, recent models have shown a high risk of widespread loss of seagrass in precisely these spots where green sea turtles have been going for millennia. This could be detrimental to the green sea turtle, precisely because of its high fidelity to these places. More information: de Kock, Willemien, Threatened North African seagrass meadows have supported green turtle populations for millennia, Proceedings of the National Academy of Sciences(2023). DOI: 10.1073/pnas.2220747120 Story provided by University of Groningen
Discussions of valuable but threatened ocean ecosystems often focus on coral reefs or coastal mangrove forests. Seagrass meadows get a lot less attention, even though they provide wide-ranging services to society and store lots of climate-warming carbon. But the findings of a new University of Michigan-led study show that seagrass ecosystems deserve to be at the forefront of the global conservation agenda, according to the authors. It’s the first study to put a dollar value on the many services—from storm protection to fish habitat to carbon storage—provided by seagrasses across the Caribbean, and the numbers are impressive. Using newly available satellite data, the researchers estimate that the Caribbean holds up to half the world’s seagrass meadows by surface area, and it contains about one-third of the carbon stored in seagrasses worldwide. They calculated that Caribbean seagrasses provide about $255 billion in services to society annually, including $88.3 billion in carbon storage. In the Bahamas alone, the ecosystem services provided by seagrasses are valued at more than 15 times the country’s 2020 gross domestic product, according to the study published online June 21 in the journal Biology Letters. “Our study is the first to show that seagrass beds in the Caribbean are of global importance in their areal extent, in the amount of carbon they store, and in the value of the economic services they provide to society,” said study lead author Bridget Shayka, a doctoral student in the U-M Department of Ecology and Evolutionary Biology. “The findings underscore the importance of conserving and protecting these highly threatened and globally important ecosystems, which are critical allies in the fight against climate change.” One way to prioritize seagrass conservation would be to include those verdant undersea meadows in global carbon markets through projects that minimize loss, increase areal extent or restore degraded beds. The idea of selling “blue carbon” offset credits, which monetize carbon stored in coastal and marine ecosystems, is gaining traction for several reasons. For one, many island nations that have already been impacted by climate change—through increasingly intense hurricanes or rising sea levels, for example—have large areas of valuable coastal ecosystems that store carbon and that provide other services to society. Blue carbon (the name refers specifically to carbon stored in coastal and open-ocean ecosystems while “green carbon” refers more broadly to carbon stored in all natural ecosystems) offset credits could be a way for wealthier countries to compensate for their contribution to human-caused climate change while at the same time benefiting the economies of impacted countries and helping to conserve coastal ecosystems, which are among the most impaired in the world. Threats to seagrass meadows include coastal development, chemical pollution, recreation, shipping and climate change. “Because seagrass ecosystems are both highly important for carbon storage and sequestration, and are highly degraded globally, they represent an important burgeoning market for blue carbon,” said marine ecologist and study senior author Jacob Allgeier, an associate professor in the U-M Department of Ecology and Evolutionary Biology. “Yet, to date, a fundamental impediment to both evaluating seagrass and promoting it in the blue carbon market has been the lack of thorough seagrass distribution data.” For their study, the U-M-led team used newly available seagrass distribution data collected by the PlanetScope constellation of small DOVE satellites. They classified Caribbean seagrass ecosystems as either sparse or dense and estimated the amount of carbon in plants and sediments using data from Thalassia testudinum, the dominant seagrass species in the region. The researchers then calculated a conservative economic value for the total ecosystem services provided by seagrasses in the Caribbean and for the stored carbon, using previously published estimates for the value of services including food production, nursery habitat for fishes and invertebrates, recreation and carbon storage. Grouper, queen conch and lobster are among the commercially harvested animals that rely on Caribbean seagrass. Green sea turtles, tiger sharks and manatees also depend on it. To estimate the dollar value of the carbon stored in Caribbean seagrass beds, the researchers used $18 per metric ton of carbon dioxide equivalents, borrowed from California’s cap and trade program. In addition to Caribbean-wide estimates, the researchers calculated values for individual countries in the region: The Bahamas has the largest share of Caribbean seagrass (61%), providing total ecosystem services valued at $156 billion annually, including $54 billion in carbon storage. Cuba ranks second in areal seagrass coverage (33% of the Caribbean total), with a value of $84.6 billion per year for all ecosystem services, including $29.3 billion for carbon storage. The dollar value of the carbon in seagrasses around Cuba is equivalent to 27% of the country’s 2020 GDP. “Importantly, the degradation of seagrass beds often leads to erosion and sediment resuspension, which can create a positive feedback of increased seagrass loss and the release of C stored in sediments,” the authors wrote. “Blue carbon finance thus represents a potential mechanism by which the global community can invest in conserving and protecting these vital ecosystems.” More than 60 species of seagrasses grow in shallow coastal waters around the world. They evolved from land plants that recolonized the oceans 70 to 100 million years ago. In a separate paper accepted for publication in the journal Proceedings of the Royal Society, Allgeier and colleagues show that the construction of artificial reefs in the Caribbean can help protect seagrass ecosystems from human impacts, including nutrient pollution and overfishing. Seagrasses use photosynthesis to pull carbon dioxide from the atmosphere, then store the carbon in plant tissues. The seagrasses are quickly inundated by sediments, slowing decomposition. As a result, more than 90% of the carbon stored in seagrass beds is in the top meter of sediment. Caribbean seagrasses and associated sediments store an estimated 1.3 billion metric tons of carbon, according to the new study. That’s a big number, but it’s just 1.09% of the carbon contained in above- and below-ground woody biomass in the Amazon, and just 1.12% of the carbon in the biomass and soils of the world’s temperate forests, according to the new
Mangroves are growing in areas historically dominated by salt marshes and oyster reefs. Invasive pacific oysters are replacing native blue mussels in the Wadden Sea. Macroalgae are exhibiting dominance over hard corals in the Caribbean and Indo-Pacific. Climate change-driven shifts in dominant, habitat-forming species such as these can have significant implications for conservation, and this is a phenomenon that seems to be particularly prevalent for seagrass systems around the world. In research published today in the Proceedings of the National Academy of Sciences, VIMS researchers and their collaborators evaluate the causes and consequences of a new dominant seagrass species rising in the Chesapeake Bay, demonstrating both new threats and management opportunities. For this study, the authors combined 38 years of data on nutrient and sediment pollution from runoff, temperature, plankton blooms, and river flow with aerial seagrass surveys to describe the causes and consequences of shifting seagrass foundation species across 26,000 hectares of habitat in Chesapeake Bay. Demonstrated shifts in seagrass create faster recovery but larger die-offs across Chesapeake Bay Marine heatwaves and poor water clarity in Chesapeake Bay over the last few decades have cut the area occupied by the previously dominant seagrass, eelgrass, in half. At the same time, successfully implemented nutrient reductions throughout the bay have encouraged the rapid expansion of another seagrass, the cosmopolitan widgeongrass. Confined to a fringing area of shallow brackish waters until the mid 1990s, widgeongrass has now replaced eelgrass as the most abundant seagrass in Chesapeake Bay by expanding over 150% due to both high temperature-tolerance and long-lasting seeds that allow for rapid recovery after disturbance. “Widgeongrass’ recovery and expansion ability is so strong,” explains lead author Hensel, “that ideal widgeongrass conditions have fueled two record-setting peaks for Chesapeake Bay seagrass cover. In fact, much of the nearly 300% increase in Bay plants since the mid 1990s has been widgeongrass expansion into areas that eelgrass has vacated.” However, some negative consequences from the shift have concerned habitat managers. “We’ve seen periods of rapid widgeongrass expansion and retraction for decades now, far beyond what we’ve documented for eelgrass,” says Landry, co-author and leader of the Chesapeake Bay Program’s Submerged Aquatic Vegetation (SAV) Workgroup. “But now that widgeongrass has become so widespread in the bay, its fluctuating abundance can have a big impact on our overall acreage trends and on our Bay-wide restoration goal attainment. That’s the management concern. The ecological concern is the impact those fluctuations are having on the animals that have grown to depend on widgeongrass for habitat in the absence of eelgrass.” The causes of these fluctuations have been difficult to understand because widgeongrass and eelgrass appear to respond differently to climate change and nutrient pollution stressors. Yet when the authors examined long-term, large-scale data on climate stressors and watershed pollution from agriculture and development in tandem with year-to-year aerial survey imagery of seagrass meadows, they recognized an important shift in the dominant climate stressor for Chesapeake Bay seagrass: while widgeongrass is resistant to heatwaves and high temperatures, it is highly vulnerable to periods in spring when high rains can bring huge influxes of nutrient- and sediment-loaded water into the bay, reducing water clarity. “This study is an important step forward in building our knowledge of human-ecosystem interactions along the coast and how they are changing over time,” says author Lefcheck. “Our past work showed a record-setting resurgence of underwater grasses in response to nutrient management, but now we are seeing that the story is vastly more complex and in fact, is still being written. Understanding, adapting to, and communicating this shifting narrative is a challenge, but not an insurmountable one by any stretch.” Conserving species with different needs simultaneously is necessary, complex The team’s modeling contributes to a greater understanding of both human and climate drivers of annual changes in widgeongrass and highlights a crucial difference in management compared to an eelgrass-dominated bay. “Heatwave stress is uncontrollable on a local and regional level,” explains Patrick, Director of the SAV Monitoring Program at VIMS, “but managing the amount of nutrients that enter the bay from the watershed during a rainy spring is something that we can actually control.” This study underscores that, because species differ in their traits and stressor sensitivities, managing for conservation of living habitats requires more community- or species-focused research, monitoring, and actions under climate change. Climate change is shifting the landscape of species composition, creating new winners in these novel environments, and management needs to shift alongside them. Detailed monitoring data allow agencies to focus on each species and encourages them to manage for community or individual habitat requirements, adapting strategies as species composition changes. This study also demonstrates pitfalls that can occur if habitat forming species are lumped together into single “stocks,” such as “hectares of seagrass” or “acres of marsh.” In fact, differences between the seagrass species in this study explain many of the shifts in Chesapeake Bay seagrass meadow dynamics. “Widgeongrass has shorter, thinner blades than eelgrass,” says Hensel, “which makes it more vulnerable to springtime run-off events because sunlight can’t reach the short blades through the clouded, nutrient-loaded water. Also, widgeongrass’ shallower root system may not sequester carbon as well and its tendency to wildly fluctuate in cover means that it may not provide consistent habitat for key seagrass-dependent species like Blue Crabs and Black Sea Bass.” The authors call for a parallel shift in coastal monitoring and evaluation of management successes and failures, using the Chesapeake Bay as an example. They cite the necessity of long-term monitoring programs with coordinated and standardized on-the-ground and detail-oriented surveys, as well as the importance of community or species-specific recovery goals across coastlines where multiple seagrass species co-occur and respond to climate differently. “This is a compelling example of the how climate change is unfolding across the globe,” said Patrick. “As regional climates shift, the emergence of novel ecosystems is fundamentally challenging everything we think we know from analysis of historical data. The rules governing the dynamics of the world’s ecosystems are changing and
Seagrass, a group of aquatic angiosperms, grows in shallow waters in the coastal sea and contributes most of the primary production while participating in many important ecological processes. Heat stress threatens the survival of seagrass, but its damage mechanisms are unclear. Recently, a research team led by Prof. Liu Jianguo from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) primarily explained the physiological and biochemical mechanisms underlying the decline of tropical seagrass meadows caused by heat stress combined with high light at different functional levels. The study was published in Marine Pollution Bulletin. During the lowest tide, intertidal seagrass is frequently faced with the combined stress of exposure to air, direct sunlight, and high temperature, which may have a strongly negative impact on the survival of seagrass, especially in the context of global warming. The researchers found that the largest tropical seagrass Enhalus acoroides can withstand heat below 39°C in the dark. However, under high light, the tolerance to heat stress is greatly reduced. The combined effect of short-term exposure to heat and high light stress destroyed the photosystem II (PSII) and destroyed key components of the photosynthetic system in seagrass leaves. Moreover, high light combined with heat stress caused severe oxidative stress in the seagrass, which led to irreversible damage to the seagrass. These results clearly suggest that heat stress coupled with high light, may be an important cause for the decline of E. acoroides meadows. “Our study reveals that ocean warming, especially when coupled with high light, exacerbates the decline of seagrass meadows and affects the ecological function of intertidal seagrass meadows,” said Dr. Zhang Mengjie, first author of the study. “This serves as a warning that the effects of global warming on seagrass meadows will be even worse than expected,” said Prof. Liu, corresponding author of the study. More information: Mengjie Zhang et al, Heat stress, especially when coupled with high light, accelerates the decline of tropical seagrass (Enhalus acoroides) meadows, Marine Pollution Bulletin (2023). DOI: 10.1016/j.marpolbul.2023.115043
A study of eelgrass meadows planted by researchers from the University of Gothenburg shows that fauna return rapidly once the eelgrass has started to grow. Already after the second summer, the biodiversity in the planted meadow was almost the same as in old established eelgrass meadows. Eelgrass meadows have declined heavily in southern Bohus county in recent decades and in many places have disappeared altogether. Researchers at the University of Gothenburg have been working on the restoration of eelgrass meadows for twelve years. These meadows are important for biodiversity, as the eelgrass serves as habitat or nursery for young cod, crabs and shrimps for example. In a new study, the researchers have evaluated how rapidly replanted eelgrass gets populated by various invertebrates. The study has been going on for over two years in a bay near Gåsö island just west of Skaftö in Bohus county, and the findings are very positive. The researchers counted the abundance of invertebrates that live or burrow in bottom sediments or on the surface of bottom sediments. Size less important “The recolonization has been very rapid. After the first three-month growing season, up to 80 percent of the invertebrates had returned to the newly planted eelgrass,” says Eduardo Infantes, marine biologist at the University of Gothenburg. During the summer in 2019, the researchers planted the eelgrass shoots in four test plots of different sizes on the seabed, and with different spacing between the shoots. According to the researchers’ observations in autumn 2020, size has played less of a role in the recovery of biodiversity in the eelgrass meadows. In fact, even if the eelgrass has not had time to grow to the same density as in an established eelgrass meadow, the biodiversity is similar after only two growing seasons as in a reference area of preserved eelgrass in the same bay. Even smaller patches embedded within larger restoration plots showed good results. Their findings were reported in the journal Restoration Ecology. Can save money “This is good news for future restorations and new plantings of eelgrass meadows. We can plant new smaller plots with fewer shoots and this saves money because this is an expensive method for restoring biodiversity on the seabed,” says Eduardo Infantes. Eelgrass meadows have multiple functions that make it imperative to protect them. In addition to their important role in the coastal ecosystem, eelgrass roots bind the sediment and prevent erosion and limit resuspension of sediment in the water. More information: Karine Gagnon et al, Rapid faunal colonization and recovery of biodiversity and functional diversity following eelgrass restoration, Restoration Ecology (2023). DOI: 10.1111/rec.13887
In Florida alone, thousands of acres of marine seagrass beds have died. Major seagrass die-offs also are occurring around the world. Stressors such as high temperature, hypersalinity and hypoxia or lack of oxygen affect seagrasses’ ability to resist and recover from these stressor-related mortality events or when disturbances lead to seagrass die-off events. Seagrass die-offs also are linked to exposure to sediment-derived hydrogen sulfide, a well-known phytotoxin that accumulates as seagrass ecosystems become more enriched in nutrients. While hydrogen sulfide intrusion into seagrass tissue is considered a leading cause of recurring mortality events, its effects on subsequent recruitment and distribution of new populations is unclear. Moreover, few studies have examined the ability of seagrass meadows‘ resilience to “bounce back” and recolonize in open bare patches. Researchers from Florida Atlantic University, in collaboration with the South Florida Water Management District, Coastal Ecosystems Division, examined if porewater hydrogen sulfide prevents Thalassia testudinum, a dominant tropical Atlantic-Caribbean marine seagrass known as turtlegrass, from recruiting into unvegetated sediment in Florida Bay. The bay is an estuary that covers about 1,100 square miles between the southern tip of Florida and the Florida Keys and is one of the largest global contiguous seagrass systems. Since the 1980s, seagrass meadows in Florida Bay have experienced repeated biomass losses, including massive die-off events of turtlegrass, which typically occur during high temperature and salinity conditions in the northcentral and western bay. The bay provided an excellent case-study site due to high porewater hydrogen sulfide and expansive unvegetated areas adjacent to intact meadows that are recolonized by turtlegrass recruits following morality events. For the study, researchers examined the leaf, stems and root tissue of turtlegrass in Florida Bay to establish tissue exposure to hydrogen sulfide in new recruits and measured internal hydrogen sulfide and oxygen dynamics using cutting-edge microsensors in the field and stable isotope analyses. Results, published in the journal Aquatic Botany, provide evidence that turtlegrass can successfully recruit into open bare sediment following die-off events due to biomass partitioning—a process by which plants divide their energy among their leaves, stems, roots and reproductive parts—during early development, young root structure, and an ability to efficiently oxidize internally, which lowers hydrogen sulfide exposure. However, recovery of seagrass meadows takes time. “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,” said Marguerite Koch, Ph.D., senior author and a professor of biological sciences in FAU’s Charles E. Schmidt College of Science. “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.” Findings of the study indicate that recruiting shoot resistance to hydrogen sulfide exposure is linked to adequate oxidation of internal tissue during the day through late afternoon via photosynthesis and internal plant oxidation promoted by water column oxygen diffusion into the leaves at night, driven at times by tides. Limited belowground root development in new recruits potentially constrains microbial community development and associated sulfate reduction that decrease hydrogen sulfide intrusion into roots and negatively affecting sensitive growing tissue at the base of the seagrass leaves. “Seagrass meadows sustain coastal ecosystems by protecting against erosion, maintaining water quality and providing habitat and food for many marine species and organisms,” said Koch. “Because of their importance in coastal communities, the current decline of seagrass ecosystems on a global scale across geographic regions is a concern.” More information: K. MacLeod et al, Resilience of recruiting seagrass (Thalassia testudinum) to porewater H2S in Florida Bay, Aquatic Botany (2023). DOI: 10.1016/j.aquabot.2023.103650
The sea devours large tracts of land when storms wash sand out to sea from the coast. A new study involving a researcher from the University of Gothenburg has shown that seagrass can reduce cliff erosion by up to 70% thanks to its root mats binding the sand. Coastal erosion is a global problem that is often combated by replenishing the coast and beaches with new sand in locations where storms wreak the greatest havoc. According to a 2016 survey conducted by the Geological Survey of Sweden, 12% of Skåne’s coastline in southern Sweden is vulnerable to increasing rates of coastal erosion. It is an even bigger issue in other countries. In the Netherlands, the coastline is protected through the construction of dikes made from stone and mud. Another solution is to utilize nature’s own defenses against coastal erosion. In this new study, researchers examined the importance of seagrass for preserving the coastline. “We have seen that seagrass meadows in the coast are valuable assets in mitigating erosion. We already know that their long canopies serve as breakwaters, but now we can show that their root mats also bind together the underwater sand dunes, effectively reinforcing them,” says Eduardo Infantes, a marine biologist at the University of Gothenburg and the lead author of the study which has been published in the journal Marine Ecology Progress Series. More powerful storms in the future Common eelgrass is a seagrass species that grows along Sweden’s coasts, and there are areas with large seagrass meadows growing on the bottom sediments, such as in Skåne. In other places, the seagrass has disappeared altogether. This not only represents an ecological loss, it can also mean that the coast becomes more vulnerable to erosion. As the climate changes, storms risk becoming more powerful, which in turn can lead to an increase in coastal erosion. Approximately 8% of the world’s population live in areas at an elevation of fewer than 10 meters above sea level. Rising sea levels may see many people affected by coastal erosion. “This is why it is even more important to preserve those seagrass meadows that still exist today and to replant seagrass in those places where it has disappeared. In our research, we have made successful attempts to restore common eelgrass meadows on the Swedish west coast, but if such replanting efforts are to succeed, there is a need for detailed studies of the current status seabed environment,” says Eduardo Infantes. In this study, the researchers took samples of sandy sediments with and without common eelgrass from a number of sites and placed them in a large tank capable of simulating waves. The experiments demonstrated that the sand is eroded far less by waves when seagrass is growing in it. The researchers also took samples from muddy seabeds but found that the effect of the seagrass there was less. However, this matters less since muddy seabeds are most commonly found in fjords and other areas that are less exposed to waves. More factors in field tests The next step will be to move out of the laboratory environment and take measurements of sand erosion on an exposed shoreline along the coast. Other factors such as currents, traffic on the water, inflows from rivers etc. can then affect the erosion. “It’s more complicated in the field, but we have created realistic storm waves in our experiments and the seagrass has clearly shown a protective effect against erosion. I think we will be able to demonstrate the similar effects in field tests,” says Eduardo Infantes. More information: E Infantes et al, Seagrass roots strongly reduce cliff erosion rates in sandy sediments, Marine Ecology Progress Series (2022). DOI: 10.3354/meps14196
“The downward trajectory of the world’s seagrass meadows must be reversed if we are to fight the planetary crisis” say leading seagrass scientists. The United Nations Sustainable Development Goals have recently been described as “the most important to-do list in the history of humankind”. Scientists from Project Seagrass and Swansea University have this week published a unique review that demonstrates how this “To-Do List” of Sustainable Development Goals provides a blueprint for achieving the net recovery of seagrass ecosystems. Conserving and restoring seagrass meadows contributes to achieving 16 out of the 17 Sustainable Development Goals. Recognising this wide role of seagrass meadows in helping achieve humanity’s ‘to-do list’ and thinking beyond their value in carbon sequestration and storage is critical to achieving the recovery of these degraded ecosystems. The call for urgent action comes after a review into the status of seagrass ecosystems and the major ecological role that they play in the coastal environment published in the leading academic journal Science and written by experts at the marine conservation charity Project Seagrass and Swansea University. Seagrass meadows are being increasingly looked to as a climate solution. However, seagrass ecosystems are sensitive to stressors and remain threatened across the globe. These degraded seagrass ecosystems are less effective at supporting biodiversity and tackling climate change. The authors state “Society needs to create meaningful pathways to net gain at local to global scales. Bold steps are needed through improved legal instruments to halt damaging factors such as bottom trawling, prevent use of damaging boating activities and to apportion responsibility for poor water quality that is causing the slow death of seagrass globally”. By recognising that seagrass meadows contribute to finding solutions to global problems such as food insecurity, water quality, wellbeing and gender equality, as well as the more well known issue such as biodiversity loss and climate change there becomes a more holistic view as to the benefits of taking large cumulative levels of action at local, regional and global scales. We need local and regional authorities to create a baseline of where seagrasses are now, where they used to be and where in the future they could be allowed to recover and be restored to get seagrass on the path to recovery. This needs to occur within the next decade if we are to fight climate change, to fight the biodiversity crisis, protect our coastlines and maintain global food security. Richard Unsworth (lead author) said “The world needs to rethink the management of our coastal environment that includes realistic compensation and mitigation schemes that not only prevent damage, but also drive the restoration, enhancement and creation of seagrass habitat. We also need a major shift in how we perceive the status of our marine environment by examining historical information, not just recent ecological baselines”. Ben Jones, a fellow author of the study added, “It is vital to work collaboratively as it is only through utilising scientific environmental studies and working as cogs in a global partnership for seagrass that meaningful change can happen”. Seagrass conservation faces substantial ecological, social and regulatory barriers and requires strong cross-sectoral partnerships to be put on the path to recovery. Identifying the solutions to seagrass conservation and restoration has never been more urgent and is critical to fight the planetary emergency. This can be achieved by using the Sustainable Development Goals as a blueprint towards recovery. Read the paper here.
Deep evolution casts a longer shadow than previously thought, scientists report in a new paper published the week of Aug. 1 in the Proceedings of the National Academy of Sciences. Smithsonian scientists and colleagues looked at eelgrass communities—the foundation of many coastal marine food webs along the north Atlantic and Pacific coasts—and discovered their ancient genetic history can play a stronger role than the present-day environment in determining their size, structure and who lives in them. And this could have implications for how well eelgrasses adapt to threats like climate change. About a half-million years ago, when the world was warmer, some eelgrass plants made the difficult journey from their homes in the Pacific to the Atlantic. Not all the plants were hardy enough to make the journey across the Arctic. For those that succeeded, a series of ice ages during the Pleistocene Epoch further affected how far they could spread. Those millennia-old struggles left lasting signatures in their DNA: Even today, eelgrass populations in the Atlantic are far less genetically diverse than those in the Pacific. Still, in the classic “nature versus nurture” debate, scientists were stunned to discover that genetic legacy sometimes does more to shape modern eelgrass communities than the current environment. “We already knew that there was big genetic separation between the oceans, but I don’t think any of us ever dreamed that that would be more important than environmental conditions,” said Emmett Duffy, marine biologist with the Smithsonian Environmental Research Center and lead author of the report. “That was a big surprise to everybody.” Eelgrasses in hot water Eelgrass is among the most widespread shallow-water plants in the world. Its range spans from semi-tropical regions like Baja California all the way to Alaska and the Arctic. Besides providing food and habitat for many undersea animals, eelgrass offers a plethora of services to humans. It protects coastlines from storms, soaks up carbon and can even reduce harmful bacteria in the water. But in most places where it grows, eelgrass is the dominant—or only—seagrass species present. That makes its survival critical to the people and animals that live there. And the lower genetic diversity in the Atlantic could make it hard for some populations to adapt to sudden changes. “Diversity is like having different tools in your tool belt,” said Jay Stachowicz, a co-author and ecologist with the University of California, Davis. “And if all you’ve got is a hammer, you can put in nails, but that’s about it. But if you have a full complement of tools, each tool can be used to do different jobs more efficiently.” Ecologists have already seen eelgrass disappearing from some regions as the waters heat up. In Portugal, its southernmost spot in Europe, eelgrass has begun pulling back and moving farther north, into cooler waters. “I don’t think that we’re going to lose [eelgrass] in the sense of an extinction,” said co-author Jeanine Olsen, an emeritus professor at the University of Groningen in the Netherlands. “It’s not going to be like that. It’s got lots of tricks up its sleeve.” But local extinctions, she pointed out, are going to occur in some places. That could leave regions that depend on their local eelgrass in trouble. Reaching a more ZEN worldview Realizing the urgent need to understand—and conserve—eelgrass worldwide, Duffy and his colleagues banded together to form a global network called ZEN, which Project Seagrass was a partner of. The name stands for Zostera Experimental Network, a nod to eelgrass’s scientific name, Zostera marina. The idea was to unite seagrass scientists all over the world, doing the same experiments and surveys, to get a coordinated global picture of seagrass health. For the new study, the team studied eelgrass communities at 50 sites in the Atlantic and Pacific. With 20 plots sampled per site, the team came away with data from 1,000 eelgrass plots. First they collected basic eelgrass data: size, shape, total biomass and the different animals and algae living on and around them. Then they collected genetic data on all the eelgrass populations. They also measured several environmental variables at each site: temperature, the water’s saltiness and nutrient availability, to name just a few. Ultimately, they hoped to discover what shaped eelgrass communities more: the environment or the genetics? After running a series of models, they discovered a host of differences between the Atlantic and Pacific eelgrass ecosystems—differences that closely aligned with the genetic divergence from the Pleistocene migration and subsequent ice ages. While Pacific eelgrasses often grew in “forests” that regularly surpassed 3 feet tall and sometimes reached more than twice that high, the Atlantic hosted more diminutive “meadows” that rarely came close to that height. The genetic differences also aligned with the total biomass of eelgrass. In the Atlantic, evolutionary genetics and the present-day environment played equally strong roles in eelgrass biomass. In the Pacific, genetics had the upper hand. These impacts flowed up to other parts of the ecosystem as well. When it came to small animals that lived in the eelgrass, like invertebrates, the genetic signature from the Pleistocene again played a stronger role than the environment in the Pacific—while the two played equally strong roles in the Atlantic. “The ancient legacy of this Pleistocene migration and bottleneck of eelgrass into the Atlantic has had consequences for the structure of the ecosystem 10,000 years later,” Duffy said. “Probably more than 10,000.” Conserving the future That ancient genetics can play such a strong role—sometimes stronger than the environment—has some ecologists concerned about whether eelgrass can adapt to more rapid changes. “Climate warming—by itself—is probably not the primary threat for eelgrass,” Olsen said. Pollution from cities and farms, which can cloud the water and lead to harmful algal blooms, also endangers seagrasses. That said, the vast array of environments eelgrass can survive in testifies to its hardiness. “I’m hopeful because our results illustrate long-term resiliency to repeated, major changes in thermal tolerances and the wide range of eelgrass habitats over about half the Northern Hemisphere,” Olsen said.
