Seagrass meadow
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A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses.
Seagrasses form dense underwater meadows which are among the most productive ecosystems in the world. They provide habitats and food for a diversity of marine life comparable to that of coral reefs. This includes invertebrates like shrimp and crabs, cod and flatfish, marine mammals and birds. They provide refuges for endangered species such as seahorses, turtles, and dugongs. They function as nursery habitats for shrimps, scallops and many commercial fish species. Seagrass meadows provide coastal storm protection by the way their leaves absorb energy from waves as they hit the coast. They keep coastal waters healthy by absorbing bacteria and nutrients, and slow the speed of climate change by sequestering carbon dioxide into the sediment of the ocean floor.
Seagrasses evolved from marine algae which colonized land and became land plants, and then returned to the ocean about 100 million years ago. However, today seagrass meadows are being damaged by human activities such as pollution from land runoff, fishing boats that drag dredges or trawls across the meadows uprooting the grass, and overfishing which unbalances the ecosystem. Seagrass meadows are currently being destroyed at a rate of about 3 m2/s.
Background
Seagrasses are
There are four lineages of seagrasses [4] containing relatively few species (all in a single order of monocotyledon). They occupy shallow environments on all continents except Antarctica:[5] their distribution also extends to the High Seas, such as on the Mascarene Plateau.
Seagrasses are formed by a
There are about 60 species of fully marine seagrasses belonging to four
Seagrass meadows are found in depths up to about 50 metres (160 ft), depending on
Seagrass meadows are sometimes called prairies of the sea. They are diverse and productive
Seagrass meadows are rich biodiverse ecosystems that occur all over the globe, in both tropical and temperate seas.[14] They contain complex food webs that provide trophic subsidy to species and habitats way beyond the extent of their distribution.[15] Given the wide variety of food sources provided by this productive habitat, it is no surprise that seagrass meadows support an equally wide array of grazers and predators. However, despite its importance for sustaining biodiversity and many other ecosystem services,[16] the global distribution of seagrass is a fraction of what was historically present.[17][18] Recent estimates from where records exist indicate that at least 20% of the world's seagrass has been lost.[18] Seagrasses also provide other services in the coastal zone such as preventing coastal erosion, storing and trapping carbon [19] and filtering the water column.[20][21]
The true ecosystem-level consequences of such decline and the benefits that can be afforded through habitat restoration are poorly understood. Given the relatively high-per-unit area costs of marine habitat restoration,[22] making the case for such work requires a thorough examination of the ecosystem service benefits of such new habitat creation.[21]
Global distribution
Seagrass meadows are found in the shallow seas of the
Seagrasses can survive to maximum depths of about 60 metres. However, this depends on the availability of light, because, like plants on the land, seagrass meadows need sunlight if photosynthesis is to occur. Tides, wave action, water clarity, and low salinity (low amounts of salt in the water) control where seagrasses can live at their shallow edge nearest the shore;[23] all of these things must be right for seagrass to survive and grow.[10]
The current documented seagrass area is 177,000 km2 (68,000 sq mi), but is thought to underestimate the total area since many areas with large seagrass meadows have not been thoroughly documented.[11] Most common estimates are 300,000 to 600,000 km2, with up to 4,320,000 km2 suitable seagrass habitat worldwide.[24]
Ecosystem services
Seagrass meadows provide
Many epiphytes can grow on the leaf blades of seagrasses, and algae, diatoms and bacterial films can cover the surface. The grass is eaten by turtles, herbivorous parrotfish, surgeonfish, and sea urchins, while the leaf surface films are a food source for many small invertebrates.[25]
Blue carbon
The meadows also account for more than 10% of the ocean's total carbon storage. Per hectare, they hold twice as much carbon dioxide as rain forests and can sequester about 27 million tons of CO2 annually.[31] This ability to store carbon is important as atmospheric carbon levels continue to rise.
Although seagrass meadows occupy only 0.1% of the area of the ocean floor, they account for 10–18% of the total oceanic carbon burial.
Coastal protection
Seagrasses are also ecosystem engineers, which means they alter the ecosystem around them, adjusting their surroundings in both physical and chemical ways.[2][1] The long blades of seagrasses slow the movement of water which reduces wave energy and offers further protection against coastal erosion and storm surge. Many seagrass species produce an extensive underground network of roots and rhizome which stabilizes sediment and reduces coastal erosion.[36] Seagrasses are not only affected by water in motion; they also affect the currents, waves and turbulence environment.[37]
Seagrasses help trap sediment particles transported by sea currents. The leaves, extending toward the sea surface, slow down the water currents. The slower current is not able to carry the particles of sediment, so the particles drop down and become part of the seafloor, eventually building it up. When seagrasses are not present, the sea current has no obstacles and carries the sediment particles away, lifting them and eroding the seafloor.[3]
Seagrasses prevent erosion of the seafloor to the point that their presence can raise the seafloor. They contribute to coast protection by trapping rock debris transported by the sea. Seagrasses reduce erosion of the coast and protect houses and cities from both the force of the sea and from sea-level rise caused by global warming. They do this by softening the force of the waves with their leaves, and helping sediment transported in the seawater to accumulate on the seafloor. Seagrass leaves act as baffles in turbulent water that slow down water movement and encourage particulate matter to settle out. Seagrass meadows are one of the most effective barriers against erosion, because they trap sediment amongst their leaves.[3]
Archaeologists have learned from seagrasses how to protect underwater archaeological sites, like a site in Denmark where dozens of ancient Roman and Viking shipwrecks have been discovered. The archaeologists use seagrass-like covers as sediment traps, to build up sediment so that it buries the ships. Burial creates low-oxygen conditions and keeps the wood from rotting.[39][3]
Links to seabirds
Birds are an often-overlooked part of marine ecosystems, not only are they crucial to the health of marine ecosystems, but their populations are also supported by the productivity and biodiversity of marine and coastal ecosystems.[40][41][42] The links of birds to specific habitat types such as seagrass meadows are largely not considered except in the context of direct herbivorous consumption by wildfowl.[43] This is despite the fact that both bottom-up and top-down processes have been considered as pathways for the population maintenance of some coastal birds.[44][21]
Given the long-term decline in the population of many coastal and seabirds, the known response of many seabird populations to fluctuations in their prey, and the need for compensatory restorative actions to enhance their populations, there is a need for understanding the role of key marine habitats such as seagrass in supporting coastal and seabirds.[21]
Nursery habitats for fisheries
Seagrass meadows provide
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Seagrass meadows support global food security by (1) providing nursery habitat for fish stocks in adjacent and deep water habitats, (2) creating expansive fishery habitat rich in fauna, and (3) by providing trophic support to adjacent fisheries. They also provide support by promoting the health of fisheries associated with connected habitats, such as coral reefs.[45]
In the oceans, gleaning can be defined as fishing with basic gear, including bare hands, in shallow water not
deeper than that one can stand.
Gallery of species habitats
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Ghost pipefish usually swim in pairs
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Leafy seadragon
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Manatee grass
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Manatee (sea cow)
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Turtle grass
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Green sea turtle grazing
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Stingray in seagrass
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Peppered morayin seagrass
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Blackspot emperorin seagrass
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Grazing sea turtle | |
Grazing manatee |
Other services
Historically, seagrasses were collected as
In February 2017, researchers found that seagrass meadows may be able to remove various pathogens from seawater. On small islands without wastewater treatment facilities in central Indonesia, levels of pathogenic
Movement ecology
Understanding the movement ecology of seagrasses provides a way to assess the capacity of populations to recover from impacts associated with existing and future pressures. These include the (re)-colonization of altered or fragmented landscapes, and movement associated with climate change.[51]
The marine environment acts as an abiotic dispersal vector and its physical properties significantly influence movement, presenting both challenges and opportunities that differ from terrestrial environments. Typical flow speeds in the ocean are around 0.1 m s−1, generally one to two orders of magnitude weaker than typical atmospheric flows (1–10 m s−1), that can limit dispersal.[52] However, as seawater density is approximately 1000 times greater than air, momentum of a moving mass of water at the same speed is three orders of magnitude greater than in air. Therefore, drag forces acting on individuals (proportional to density) are also three orders of magnitude higher, enabling relatively larger-sized propagules to be mobilized. But most importantly, buoyancy forces (proportional to the density difference between seawater and the propagule) significantly reduce the effective weight of submerged propagules.[53] Within seagrasses, propagules can weakly settle (negatively buoyant), remain effectively suspended in the interior of the water column (neutrally buoyant), or float at the surface (positively buoyant).[54][51]
With positive buoyancy (e.g. floating fruit), ocean surface currents freely move propagules, and dispersal distances are only limited by the viability time of the fruit,[55][56] leading to exceptionally long single dispersal events (more than 100 km),[57] which is rare for passive abiotic movement of terrestrial fruit and seeds.[58][51]
There are a variety of
For example, if a waterbird feeds on a seagrass containing fruit with seeds that are viable after defecation, then the bird has the potential to transport the seeds from one feeding ground to another. Therefore, the movement path of the bird determines the potential movement path of the seed. Particular traits of the animal, such as its digestive passage time, directly influence the plant's movement path.[51]
Biogeochemistry
The primary nutrients determining seagrass growth are carbon (C), nitrogen (N), phosphorus (P), and light for photosynthesis. Nitrogen and phosphorus can be acquired from sediment pore water or from the water column, and sea grasses can uptake N in both ammonium (NH4+) and nitrate (NO3−) form.[66]
A number of studies from around the world have found that there is a wide range in the concentrations of C, N, and P in seagrasses depending on their species and environmental factors. For instance, plants collected from high-nutrient environments had lower C:N and C:P ratios than plants collected from low-nutrient environments. Seagrass stoichiometry does not follow the Redfield ratio commonly used as an indicator of nutrient availability for phytoplankton growth. In fact, a number of studies from around the world have found that the proportion of C:N:P in seagrasses can vary significantly depending on their species, nutrient availability, or other environmental factors. Depending on environmental conditions, seagrasses can be either P-limited or N-limited.[67]
An early study of seagrass stoichiometry suggested that the Redfield balanced ratio between N and P for seagrasses is approximately 30:1.[68] However, N and P concentrations are strictly not correlated, suggesting that seagrasses can adapt their nutrient uptake based on what is available in the environment. For example, seagrasses from meadows fertilized with bird excrement have shown a higher proportion of phosphate than unfertilized meadows. Alternately, seagrasses in environments with higher loading rates and organic matter diagenesis supply more P, leading to N-limitation. P availability in Thalassia testudinum is the limiting nutrient. The nutrient distribution in Thalassia testudinum ranges from 29.4 to 43.3% C, 0.88-3.96% N, and 0.048-0.243% P. This equates to a mean ratio of 24.6 C:N, 937.4 C:P, and 40.2 N:P. This information can also be used to characterize the nutrient availability of a bay or other water body (which is difficult to measure directly) by sampling the seagrasses living there.[69]
Light availability is another factor that can affect the nutrient stoichiometry of seagrasses. Nutrient limitation can only occur when photosynthetic energy causes grasses to grow faster than the influx of new nutrients. For example, low light environments tend to have a lower C:N ratio.[69] Alternately, high-N environments can have an indirect negative effect to seagrass growth by promoting growth of algae that reduce the total amount of available light.[70]
Nutrient variability in seagrasses can have potential implications for
A study of annual deposition of C, N, and P from Posidonia oceanica seagrass meadows in northeast Spain found that the meadow sequestered 198 g C m−2 yr−1, 13.4 g N m−2 yr−1, and 2.01 g P m−2 yr−1 into the sediment. Subsequent remineralization of carbon from the sediments due to respiration returned approximately 8% of the sequestered carbon, or 15.6 g C m−2 yr −1.[71]
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Seagrass lagoon, Chek Jawa, Singapore
Threats
Seagrasses are in global decline, with some 30,000 km2 (12,000 sq mi) lost during recent decades. Seagrass loss has accelerated over the past few decades, from 0.9% per year prior to 1940 to 7% per year in 1990.[72]
Natural disturbances, such as grazing, storms, ice-scouring and desiccation, are an inherent part of seagrass ecosystem dynamics. Seagrasses display a high degree of phenotypic plasticity, adapting rapidly to changing environmental conditions. Human activities, on the other hand, have caused significant disturbance and are accountable for the majority of the losses.
The seagrass can be damaged from direct mechanical destruction of habitat through fishing methods that rely on heavy nets that are dragged across the sea floor, putting this important ecosystem at serious risk.[3] When humans drive motor boats over shallow seagrass areas, the propeller blade can also damage the seagrass.
Seagrass habitats are threatened by coastal
Accumulating evidence also suggests that
Increased seawater temperatures,[11] increased sedimentation, and coastal development have also had a significant impact in the decline of seagrasses.[26]
The most-used methods to protect and restore seagrass meadows include nutrient and
Ocean deoxygenation
Globally, seagrass has been declining rapidly. Hypoxia that leads to eutrophication caused from ocean deoxygenation is one of the main underlying factors of these die-offs. Eutrophication causes enhanced nutrient enrichment which can result in seagrass productivity, but with continual nutrient enrichment in seagrass meadows, it can cause excessive growth of microalgae, epiphytes and phytoplankton resulting in hypoxic conditions.[76]
Seagrass is both a source and a sink for oxygen in the surrounding water column and sediments. At night, the inner part of seagrass oxygen pressure is linearly related to the oxygen concentration in the water column, so low water column oxygen concentrations often result in hypoxic seagrass tissues, which can eventually kill off the seagrass. Normally, seagrass sediments must supply oxygen to the below-ground tissue through either photosynthesis or by diffusing oxygen from the water column through leaves to
Because hypoxia increases the invasion of sulfides in seagrass, this negatively affects seagrass through photosynthesis,
Diminishing meadows
The storage of carbon is an essential ecosystem service as we move into a period of elevated atmospheric carbon levels. However, some climate change models suggest that some seagrasses will go extinct – Posidonia oceanica is expected to go extinct, or nearly so, by 2050.[77]
The
Restoration
Using propagules
Seagrass propagules are materials that help propagate seagrass. Seagrasses pollinate by hydrophily, that is, by dispersing in the water. Sexually and asexually produced propagules are important for this dispersal.[79]
Species from the genera
Seagrass restoration has primarily involved using asexual material (e.g., cuttings, rhizome fragments or cores) collected from donor meadows. Relatively few seagrass restoration efforts have used sexually derived propagules.[88][89] The infrequent use of sexually derived propagules is probably in part due to the temporal and spatial variability of seed availability,[90] as well as the perception that survival rates of seeds and seedlings are poor.[91][92] Although survival rates are often low, recent reviews of seed-based research highlight that this is probably because of limited knowledge about availability and collection of quality seed, skills in seed handling and delivery, and suitability of restoration sites.[88][89][83][79]
Methods for collecting and preparing propagules vary according to their characteristics and typically harness their natural dispersal mechanisms. For example, for viviparous taxa such as Amphibolis, recently detached seedlings can be collected by placing fibrous and weighted material, such as sand-filled hessian bags, which the seedlings' grappling structures attach to as they drift past. In this way thousands of seedlings can be captured in less than a square meter.[93] Typically, sandbags are deployed in locations where restoration is required, and are not collected and re-deployed elsewhere.[79]
For species which have seeds contained within
For species that release seeds from fruits that float (Posidonia spp., Halophila spp.), fruits can be detached from the parent plant by shaking; they then float to the surface where they are collected in nets.[96][97] Seeds are then extracted from the fruit via vigorous aeration and water movement from pumps at stable temperatures (25 °C) within tanks. The negatively buoyant seeds are then collected from the tank bottom and scattered by-hand over recipient habitats. Other methods have been trialed with limited success, including direct planting of seeds by hand, injecting seeds using machinery, or planting and deploying within hessian sandbags.[79]
Restoration using seagrass propagules has so far demonstrated low and variable outcomes, with more than 90% of propagules failing to survive.[98][99][93] For propagules to be successfully incorporated within seagrass restoration programs, there will need to be a reduction in propagule wastage (which includes mortality, but also failure to germinate or dispersal away from the restoration site), to facilitate higher rates of germination and survival. A major barrier to effective use of seeds in seagrass restoration is knowledge about seed quality. Seed quality includes aspects such as viability, size (which can confer energy reserves available for initial growth and establishment), damage to the seed coat or seedling, bacterial infection, genetic diversity and ecotype (which may influence a seeds ability to respond to the restoration environment).[79] Nevertheless, the diversity of propagules and species used in restoration is increasing and understanding of seagrass seed biology and ecology is advancing.[93][97][100] To improve chances of propagule establishment, better understanding is needed about the steps that precede seed delivery to restoration sites, including seed quality,[87] as well as the environmental and social barriers that influence survival and growth.[79]
Other efforts
In various locations, communities are attempting to restore seagrass beds that were lost to human action, including in the US states of Virginia,[101] Florida[102] and Hawaii,[103] as well as the United Kingdom.[104] Such reintroductions have been shown to improve ecosystem services.[105]
Dr. Fred Short of the University of New Hampshire developed a specialized transplant methodology known as "Transplanting Eelgrass Remotely with Frames" (TERF). This method involves using clusters of plants which are temporarily tied with degradable crepe paper unto a weighted frame of wire mesh. The method has already been tried out by Save The Bay.[106]
In 2001, Steve Granger, from the University of Rhode Island Graduate School of Oceanography used a boat-pulled sled that is able to deposit seeds below the sediment surface. Together with colleague Mike Traber (who developed a Knox gelatin matrix to encase the seeds in), they conducted a test planting at Narragansett Bay. They were able to plant a 400 m2 (480 sq yd) area in less than two hours.[106]
As of 2019[update] the Coastal Marine Ecosystems Research Centre of Central Queensland University has been growing seagrass for six years and has been producing seagrass seeds. They have been running trials in germination and sowing techniques.[107]
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One flower can produce 15 seeds, and one seed planted in the right conditions can create a hectare of seagrass.