Seagrass meadow

A seagrass meadow or seagrass bed is an underwater ecosystem formed by seagrasses. Seagrasses are marine plants found in shallow coastal waters and in the brackish waters of estuaries. Seagrasses are flowering plants with stems and long green, grass-like leaves. They produce seeds and pollen and have roots and rhizomes which anchor them in seafloor sand.
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.

Background

Seagrasses are flowering plants which grow in marine environments. They evolved from terrestrial plants which migrated back into the ocean about 75 to 100 million years ago. In the present day they occupy the sea bottom in shallow and sheltered coastal waters anchored in sand or mud bottoms.
There are four lineages of seagrasses containing relatively few species. They occupy shallow environments on all continents except Antarctica: their distribution also extends to the High Seas, such as on the Mascarene Plateau.
Seagrasses are formed by a polyphyletic group of monocotyledons, which recolonised marine environments about 80 million years ago. Seagrasses are habitat-forming species because they are a source of food and shelter for a wide variety of fish and invertebrates, and they perform relevant ecosystem services.
There are about 60 species of fully marine seagrasses belonging to four families, all in the order Alismatales. Seagrasses beds or meadows can be made up of either a single species or mixed. In temperate areas one or a few species usually dominate, whereas tropical beds are usually more diverse, with up to thirteen species recorded in the Philippines. Like all autotrophic plants, seagrasses photosynthesize, in the submerged photic zone. Most species undergo submarine pollination and complete their life cycle underwater.
File:Carbon uptake and photosynthesis in a seagrass meadow.png|thumb|upright=1.3|left| Carbon uptake and photosynthesis in a seagrass meadow. Special cells within the seagrass, called chloroplasts, use energy from the sun to convert carbon dioxide and water into carbohydrates and oxygen through photosynthesis. Seagrass roots and rhizomes absorb and store nutrients and help to anchor the seagrass plants in place.
Seagrass meadows are found in depths up to about, depending on water quality and light availability. These seagrass meadows are highly productive habitats that provide many ecosystem services, including protecting the coast from storms and big waves, stabilising sediment, providing safe habitats for other species and encouraging biodiversity, enhancing water quality, and sequestering carbon and nutrients.
Seagrass meadows are sometimes called prairies of the sea. They are diverse and productive ecosystems sheltering to and harbouring species from all phyla, such as juvenile and adult fish, epiphytic and free-living macroalgae and microalgae, mollusks, bristle worms, and nematodes. Few species were originally considered to feed directly on seagrass leaves, but scientific reviews and improved working methods have shown that seagrass herbivory is an important link in the food chain, feeding hundreds of species, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs. Some fish species that visit or feed on seagrasses raise their young in adjacent mangroves or coral reefs.
File:Labelled drawing of seagrass plants.png|thumb|upright=1.3|Seagrasses differ from seaweeds. Where seaweeds use holdfasts to secure them to the seafloor and internally transport nutrients by diffusion, seagrasses are flowering plants with a rhizome and root system connecting them to the seafloor and a vascular system for internal transport.
Seagrass meadows are rich biodiverse ecosystems that occur all over the globe, in both tropical and temperate seas. They contain complex food webs that provide trophic subsidy to species and habitats way beyond the extent of their distribution. 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, the global distribution of seagrass is a fraction of what was historically present. Recent estimates from where records exist indicate that at least 20% of the world's seagrass has been lost. Seagrasses also provide other services in the coastal zone such as preventing coastal erosion, storing and trapping carbon and filtering the water column.
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, making the case for such work requires a thorough examination of the ecosystem service benefits of such new habitat creation.

Global distribution

Seagrass meadows are found in the shallow seas of the continental shelves of all continents except Antarctica. Continental shelves are underwater areas of land surrounding each continent, creating areas of relatively shallow water known as shelf seas. The grasses live in areas with soft sediment that are either intertidal or subtidal. They prefer sheltered places, such as shallow bays, lagoons, and estuaries, where waves are limited and light and nutrient levels are high.
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 control where seagrasses can live at their shallow edge nearest the shore; all of these things must be right for seagrass to survive and grow.
The current documented seagrass area is, but is thought to underestimate the total area since many areas with large seagrass meadows have not been thoroughly documented. Most common estimates are 300,000 to 600,000 km², with up to 4,320,000 km² suitable seagrass habitat worldwide.

Ecosystem services

Seagrass meadows provide coastal zones with significant ecosystem goods and services. They enhance water quality by stabilizing heavy metals and other toxic pollutants, as well as cleansing the water of excess nutrients, and lowering acidity levels in coastal waters. Further, because seagrasses are underwater plants, they produce significant amounts of oxygen which oxygenate the water column. Their root systems also assist in oxygenating the sediment, providing hospitable environments for sediment-dwelling organisms. Additionally, the conservation of seagrass meadows contributes to 16 of the 17 UN Sustainable Development Goals.
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.

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. This ability to store carbon is important as atmospheric carbon levels continue to rise.
Blue carbon refers to carbon dioxide removed from the atmosphere by the world's coastal marine ecosystems, mostly mangroves, salt marshes, seagrasses and potentially macroalgae, through plant growth and the accumulation and burial of organic matter in the sediment.
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. Currently global seagrass meadows are estimated to store as much as 19.9 Pg of organic carbon. Carbon primarily accumulates in marine sediments, which are anoxic and thus continually preserve organic carbon from decadal-millennial time scales. High accumulation rates, low oxygen, low sediment conductivity and slower microbial decomposition rates all encourage carbon burial and carbon accumulation in these coastal sediments. Compared to terrestrial habitats that lose carbon stocks as CO₂ during decomposition or by disturbances like fires or deforestation, marine carbon sinks can retain C for much longer time periods. Carbon sequestration rates in seagrass meadows vary depending on the species, characteristics of the sediment, and depth of the habitats, but on average the carbon burial rate is about 140 g C m⁻² yr⁻¹.

Coastal protection

Seagrasses are also ecosystem engineers, which means they alter the ecosystem around them, adjusting their surroundings in both physical and chemical ways. 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. Seagrasses are not only affected by water in motion; they also affect the currents, waves and turbulence environment.
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.
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.
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.