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Flutender Wasserhahnenfuß

Common eelgrass (Zostera marina L.)

Common eelgraass (Zostera marina L.)
(Photo: P. Jonas)

There are about 60 species of eelgrass throughout the world. They all belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae und Cymodoceaceae). Although most species are submerged, some species can survive a certain period of time exposed to air, for example during low tide. Usually they require a lot of light and therefore do not exist in great depths.

Common eelgrass (Zostera marina L.) is a rhizomatous, herbacious aquatic plant, which occurs submerged a long the coasts of the Northsea, the Baltic Sea, as well as the Atlantic, the Mediterreanean Sea, the Black Sea, and the Pacific. It i a flowering plan with the greatest occurrence in the northern hemisphere.



The common eelgrass (Zostera marina L.) and the non-indegenous Japanese eelgrass (Zostera japonica) can survive exposure during low tide
(Photo: G. Fritz)

The leaves are elongated with lengths between 30 and 150 cm and a width between 0,3 and 0,9 cm. Their color varies between green and brownish-green. The leaves have three to seven parallel lef veins. The shape is linear with rounded tips. The plant has an extensive rhizomal growth, anchoring in the ground with clusters of roots.

The plant is monoecious: male and female flowers are on the same individual, growing in alternating clusters. Seeds are produced in great numbers, sometime several thousand seeds per square meter. They can be dispersed over large distances.

Eelgrass can also undergo vegetative reproduction, as new leaves are sprouting from the rhizoms, allowing them to spread into meadowlike colonies. Some of the meadows have been determined to be several thousand years old.



Some seagrass species grow in the lower marshes of estuaries, like the Coos Bay National Estuarine Research Reserve, OR, USA.
(Photo: G. Fritz)

Commn eelgrass grows a long the coastlines of the northern hemisphere. It prefers calm, cold to warm water, but not tropical or subtropical. In Germany it is found along the coasts of the Northsea and the Baltic Sea.

As it requires enough light, it only can be found in depths of 10 to 17 m maximum on muddy substrates. It also grows in brackish waters, thus, it can be found in the lower marshes of estuarines, which can be exposed during low tides.



Dreispitzige Meerassel



baltic isopod

broadnosed pipefish

Seagrass is habitat for many species: e.g. the lumsucker (Cyclopterus lumpus), the baltic isopod (Idotea baltica) and the broadnosed pipefish (Syngnathus typhle) (Photo: P. Jonas)

Eelgrass beds belong to the most important ecological aquatic habitats. They provide nursery and feeding grounds for many fish species, including the commercially important Pacific herring (Clupea pallasii), the provide protection for juvenile fish such as Atlantic cod (Gadus morhua). In addition it provides substrate for species that can not dwell on soft bottoms such as the blue mussel (Mytilus edulis).

The plant itself is provides an ecosystem as, espcially older individuals, are coverd with epibionts such as numerous algal species, bryophytes and many more.

Only a few animals actually eat eelgrass, some of them are isopods, sea urchins, a few fish species and the Atlantic brant (Branta bernicula hrota).

When dying eelgrass is washed up on shore, a whole new ecosystem is created as many insects and other invertebrates live in and of the decaying mater.



Eelgrass on the mudflats of Humboldt Bay, CA, are measrued to determine the extent of the wasting disease (Photo: G. Fritz).


Healthy eelgrass (Photo. P. Jonas).

Eelgrass is protected worldwide as the populations are endangered. Human activities, such as dredging and trawling, have destroyed many areas as the plants were derooted and the bottom turned. In addition, stirring soft bottom sediments increases turbidity and reduced the available light. Furthermore, aquaculture and coastal developments endanger existing meadows. The slime mold Labyrinthula zosterae is a threat to eelgrass colonies all over the world causing the so called wasting disease. Since 1932 large colonies were destroyed along the Atlantic coastlines of Northamerica and Europe due to the outbreak of the wasting disease. Increasing brown areas on the leaves indicate the disease, which grow into black stripes over time. Internally the leaves are decomposed enzymatically and destroyed. The destruction of eelgrass beds has a huge ecological impact. The primary production is reduced and habitat as well as food resources are destroyed. In the 30s of last century a dramatic decline of brant geese could be observed following the distruction of eelgrass beds.