Seagrass meadow

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Seagrass meadow

Seagrass meadows are a community of plants on sandy soils in the sea or mud flats . The benthic ecosystems formed in this way exist primarily in coastal areas ( estuaries ), predominantly under water, where they form extensive meadows. They line the flat coastal regions almost all over the world and occur from the water surface to a depth of 40 m. The seagrass species are their character plants.

Seagrass plants are rooted in the sandy soil. Its leaves strive upwards through Archimedean buoyancy in the sea water, swaying back and forth under the influence of currents from overflowing sea ​​waves and at the same time dampening the flow velocity at the bottom so strongly that sand particles remain and the roots find a better hold. Seagrass meadows therefore have sharp boundaries. Dead leaf parts of the species Posidonia oceanica ( Neptune grass ) that have fallen off in fiber form sink to the bottom in seawater. Grooves and ripples in the sandy bottom collect these fibers, the back and forth of the current of the waves causes these fibers to tangle in the boundary surface of the current on the bottom and form seaballs from them, which are washed out of the water by stronger waves and at higher areas of the Strands are deposited where they can be easily collected.

Ecosystem functions

The sea ​​grasses live in these meadows in communities with other plants and animals. Their leaves offer stable, hard surfaces. They often carry large populations of epiphytic or growth algae , but also of small animals that could not live in the soft sediments . With this growth, they represent an important source of food for herbivorous fish and other swimming organisms. The seagrass meadows are used by many species of fish as “nurseries”. For example garfish and herrings attach their eggs to the stalks of the sea grass. The dense leaf mats of the "meadow" protect the young fish from predators. Other animals create their tunnels near the root systems of the seagrass. This protects them from predatory enemies, and makes it easier for them to dig here. In autumn, brent geese and wigeons visit the seagrass meadows on their bird migration to feed on the leaves and roots of the seaweed. Waders feed on snails and other small animals in the meadow.

Seagrass meadows are among the oceanic habitats with the highest primary production . Around half of the production comes from the seagrasses themselves, plus that of the epiphytes and macroalgae native to them. A significant part of the biomass is produced in the seabed in the rhizomes and roots of the seagrass. As a result, they permanently fix carbon dioxide as carbohydrates , such as cellulose . The roots also protect the seabed from erosion and delay remineralization of the carbon they contain. In addition, the reduced flow velocity in dense seagrass meadows leads to higher sedimentation rates. Seagrass meadows thus form an important carbon sink ; their complete loss could lead to a release of CO 2 in the order of 10 Gt.

Seagrass meadows stabilize the sandy tidal areas of the river mouths ( estuaries ) and thus also contribute to coastal protection .

Ecological dangers

The seagrass stocks are declining worldwide. The rate of loss has increased from 0.09% per year before 1940 to 7% per year since 1980. About a quarter of the species are listed as threatened on the red list of endangered species . In the course of the last century, for example, in the area of ​​the North Sea - Wadden Sea , overfishing and diseases wiped out not only the oyster beds but also permanently flooded seagrass meadows.

When large parts of the meadows with common seagrass ( Zostera marina ) were largely destroyed by a fungal disease on both the European and American Atlantic coasts in the 1930s, this also had an impact on the Brent geese grazing the seagrass meadows Percent of the previous year's population densities decreased.

A displacement by the neophyte Caulerpa taxifolia takes place especially in the Mediterranean and influences the existing habitats .

Seagrasses, which get their nutrients from the sandy, silty seabed through roots, thrive in relatively nutrient-poor water. Increased input of fertilizer leads to increased algae growth and thus to shading of seagrass meadows and reduced growth.

The current climate change is causing sea temperatures to rise, which is accompanied by an increasing frequency and intensity of extreme events. This also contributes to the endangerment and decline of seagrass meadows. For example, a marine heat wave in Shark Bay on the west coast of Australia damaged more than a third of the significant seaweed stocks there, which are a UNESCO World Heritage site. In the western Mediterranean, Neptune grass meadows could functionally become extinct by the middle of the century. On the other hand, most seagrasses as C3 plants could benefit from the increasing carbon dioxide content of the sea.

Today, shipping close to the coast represents one of the main threats to the seagrass population. Inorganic nitrogen concentrations above 112 µg / L and 10 a of water damage the habitat.

literature

  • AJ Underwood, MG Chapman: Our Wonderful Planet . P. 191
  • Harald Asmus, Ragnhild Asmus: Neptune's swaying gardens - seagrass meadows are oases on sandy coasts . In: Gotthilf Hempel, Irmtraut Hempel, Siegrid Schiel (eds.): Fascination Marine Research . HM Hauschild, Bremen 2006, ISBN 3-89757-310-5 , p. 216-221 .

Web links

Individual evidence

  1. ↑ a b Daniel M. Alongini: Blue Carbon - Coastal sequestration for Climate Change Mitigation (=  Springer Briefs in Climate Studies ). 2018, ISBN 978-3-319-91697-2 , 4 Seagrass Meadows.
  2. ↑ Harald Asmus, Ragnhild Asmus: Neptune's swaying gardens - seagrass meadows are oases on sandy coasts . In: Gotthilf Hempel, Irmtraut Hempel, Siegrid Schiel (eds.): Fascination Marine Research . HM Hauschild, Bremen 2006, ISBN 3-89757-310-5 , p. 216-221 .
  3. ↑ A. Arias-Ortiz et al. a .: A marine heatwave drives massive losses from the world's largest seagrass carbon stocks . In: Nature Climate Change . 2018, doi : 10.1038 / s41558-018-0096-y .
  4. ↑ Gabriel Jordà, Núria Marbà, Carlos M. Duarte: Mediterranean seagrass vulnerable to regional climate warming . In: Nature Climate Change . May 2012, doi : 10.1038 / nclimate1533 .
  5. ↑ Gema Hernán u. a .: Seagrass (Posidonia oceanica) seedlings in a high-CO 2 world: from physiology to herbivory . In: Scientific Reports . tape 6 , no. 38017 , January 2016, doi : 10.1038 / srep38017 .
  6. ↑ Volker Mrasek: Marine Ecology - Seagrass Death on the Coasts. In: deutschlandfunk.de. February 27, 2019, accessed February 27, 2019 .
  7. ↑ China: Wastewater destroys seagrass meadows. Retrieved July 6, 2020 .