Cystoseira forests


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Cystoseira species belong to the genus Fucaceae within the realm of chloroalveolata. They are widespread in the Mediterranean Sea and are very good bioindicators because of their sensitivity to the composition of the water. Here is on the left a picture of Cystoseira fibrosa and a photograph of Cystoseira amentacea on the right.

  

Geographic repartition

About 30 species of Cystoseira live in the Mediterranean Sea of which 20 are endemic. Cystoseira sp. inhabits the infralittoral level corresponding to a permanently immersed level (the top part can be emerged at low tide) down to the deepest limit where photophile algea can live. One forest of Cystoseira can be composed of one or several species mixed. These are ecosystems engineer.

Life history traits

The vegetative system has a seasonal cycle :
> Winter : The plant presents stems and bulbs containing reserves in the form of mannitol.
> Spring : Edification of photosynthetic branches from bulb’s reserves. These are withered.
> Summer : Photosynthetic products are exported from stems to new bulbs growing at a higher level.
> Autumn : Branches and old bulbs are deciduous.

The biocycle of Cystoseira is monogenetic meaning that it contains only one generation, and diplontic meaning that the diploid stage is multicellular and haploid gametes are formed, meiosis is « gametic ».
Cystoseira sp. are K strategists : they are long-lived (several dozen years),  zygotes are heavy and disseminated to some meters, the re-settlement  is very  slow, seedlings germination is photophile and some of them have defenses in the form of phoroglucinol.

Participating organisms and succession

Organisms that inhabit Cystoseira forests are distributed between different levels characterizing the ecosystem. Here is an exemple of organisms inhabiting a forest of Cystoseira funkii.
The tree layer is composed of Cystoseira funkii and Halopteris incurvus (rhodophyta). Padina pavonica and Halopteris scoparia (both stramenopiles) are part of the shrub layer. Sciaphilous and turf level contains Ceramium ciliatum, Peyssonnelia squamaria (both rhodophyta). Encrusting Lithophyllum (rhodophyta corallinaceae) constitute the encrusting level.  The bivalve mollusc Lithophaga lithophaga is endolithic level. Finally, Jania rubens and Falkenbergia rufolanosa (rhodophyta) form the epibiont level.
Here are images of (from right to left) Padina pavonica, Peyssonnelia squamaria, Lithophaga lithophaga and Jania rubens.

   

The Leaf Area Index (LAI) of Cystoseira is high : 20 m²/m², consequently these forests are an important support for epibionts as MPOs (multicellular photosynthetic organisms), cnidarians, bryozoa, ect.

The succession is the same as that of Posidonia oceanica (described in this article : Posidonia oceanica ), and it can continue until the P. oceanica pasture is a cutting settles. This is a case of nested climax. There is the succession :
After a few hours a bacterial film is deposited on the substrate. A few days/weeks after a turf of chromobiontes takes place. A few months/years after, chromobiontes Padina pavonica and Halopteris scoparia and the rhodobionte Jania rubens start to grow on the substrate. A few years/decades after rhodobionte Litophyllum and chromobionte Cystoseira grow on the substrate.

Bioindication and threats

Cystoseira depend on good water quality, and can be used for bioindication. These stands are very sensitive to pollution and the gradual substitution thereof by more resistant species stands with a broader ecological distribution is observed: Padina pavonica, Stypocaulon scoparium, etc.. Overgrazing by sea urchins can have very negative effects on the Cystoseira stands up to make them disappear completely if the population density is too high. The regeneration of these is slow, and several years may elapse before the Cystoseiretum reappears.

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