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Marine habitats
Seagrass meadows are major carbon sinks and highly productive nurseries for many marine species
Coastal habitats
Littoral zone Intertidal zone Estuaries Mangrove forests Seagrass meadows Kelp forests Coral reefs Continental shelf Neritic zone
Ocean surface
Surface microlayer Epipelagic zone
Open ocean
Pelagic zone Oceanic zone
Sea floor
Seamounts Hydrothermal vents Cold seeps Demersal zone Benthic zone Marine sediment
v t e

A seagrass meadow, also known as a seagrass bed, is an underwater ecosystem formed by seagrasses. Seagrasses are marine (saltwater) 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 and 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 dulongs. 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 two football fields every hour.

Overview

Seagrasses are flowering plants that evolved from the land back to the sea, and now occupy the sea bottom in shallow waters along the coast all over the world. Seagrass meadows are hidden underwater grass fields that protect the coast and offer shelter to many sea creatures.

Global distribution

Seagrasses are found all over the world, in both hot and cold locations. Seagrasses live in shallow seas on the continental shelf of all continents except Antarctica. The continental shelf is the underwater area of land surrounding each continent, creating an area of relatively shallow water known as a shelf sea. It is believed that seagrasses cover 125,000 km around the world, but other estimates suggest that this number might be a lot bigger—seagrasses may cover up to 600,000 km of the shallow ocean.

Seagrass meadows are found in areas with soft sediment that are either intertidal (uncovered daily by seawater, as the tide goes in and out) or subtidal (always under the water). Seagrasses prefer sheltered places, such as shallow bays, lagoons, and estuaries (sheltered areas where rivers flow in to the sea), where waves are limited and light and nutrient levels are high. Seagrasses can be found up to around 60 metres deep, but this depends on the availability of light because, like plants on the land, seagrass meadows need sunlight for photosynthesis 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, all of these things must be just right for seagrass to survive and grow.

Ecosystem services

Seagrass meadows are one of the more important ecosystems. Seagrasses also cleanse the water of excess nutrients and toxic pollutants.

Habitats for other species

• Stingray in seagrass
• Peppered moray in seagrass
• Blackspot emperor in seagrass

Nursery habitats

They provide nursery habitats for many commercially important fish species, and it's estimated that about half of the global fisheries get their start because they are supported by seagrass habitats. If these seagrass habitats are lost, then the fisheries are lost as well.

Coastal protection

"Seagrasses are not only affected by water in motion, they also affect the currents, waves and turbulence environment is referred to as ecosystem engineering."(Jones et al., 1994, 1997; Thomas et al., 2000).

Seagrasses prevent erosion of the seafloor to the point that their presence can raise the seafloor. Seagrasses 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. Seagrasses 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.

Human impacts

Human activities, such as fishing methods that rely on heavy nets that are dragged across the sea floor, put this important ecosystem at serious risk.

References

1. ^ Fusi M and Daffonchio D (2019) "How Seagrasses Secure Our Coastlines". Frontiers for Young Minds. 7: 114. doi:10.3389/frym.2019.00114. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
2. Reynolds PL (2018) "Seagrass and Seagrass Beds" Smithsonian Ocean Portal.
3. ^ Cullen-Unsworth, L.C., Jones, B.L., Lilley, R. and Unsworth, R.K. (2018) "Secret gardens under the sea: What are seagrass meadows and why are they important?" Frontiers for Young Minds, 6(2): 1–10. doi:10.3389/frym.2018.00002. Material was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
4. Hemminga, M. A., and Duarte, C. M. (2000) Seagrass Ecology, first edition, Cambridge University Press. ISBN 9780521661843.
5. Koch, E.W., Ackerman, J.D., Verduin, J. and van Keulen, M. (2007) "Fluid dynamics in seagrass ecology—from molecules to ecosystems". In" Seagrasses: biology, ecology and conservation, pages 193–225, Springer, Dordrecht. doi:10.1007/978-1-4020-2983-7_8.
6. Gregory, D., Jensen, P. and Strætkvern, K. (2012) "Conservation and in situ preservation of wooden shipwrecks from marine environments". Journal of Cultural Heritage, 13(3): S139–S148. doi:10.1016/j.culher.2012.03.005.

• Seagrass