Caulerpa taxifolia: Invasive algae in Posidonia oceanica seagrass in the mediterranean

     Facultad de Ciencias Del Mar, 2º de Grado, Botánica Marina                  16 de Noviembre 2011     

INTRODUCTION:

Nowadays, the Mediterranean Sea suffers a terrible problematic with the algae called Caulerpa taxifolia (Vahl), a green algae from the Clorophyta division, with blades ranging 2-15cm. C. taxifolia has a tropical origen, with presence in warm seas. It reaches the Mediterranean Sea, most likely due to the water draining from the Oceanographic institute of Monaco, although it was also discussed that the invassive algae was carried on the hull of a boat. The first record of C. taxifolia was near Monaco, in 1984. Right now, it occupies about 15 000 000 square meters of seabed and it represents a threat for the ecosystem since one of its methabolic prodcuts, the caulerpine, is toxic for the mediterranean organisms, cause for wich is called killer algae or green plague. One of the worst consequences of the existence of C. taxifolia in the mediterran is the tendence to occupy the space of Posidonia oceanica (Linnaeus), a flowering underwater plant endemic to the Mediterranean, with elongated and plaane leaves up to 1 m long. That plant is forced to move, causing the same to the fauna hosted by P. oceanica acoge, because the caulerpine avoids the depredation of C. taxifolia (except for the fish Sarpa salpa). The ecological consequences can be terrible, and that's why they are doing awareness campaigns for the elimination and non-spreading of the C. taxifolia.

The success of Caulerpa taxifolia expanding resides in its capacity of colonization the sandy bottoms, the quickness which it reaches them and its high regeneration power; it can regenerate an entire algae from a piece, being that an asexual reproduction method.



Meanwhile, Posidonia oceanica can live in soft and rocky bottoms, but in constrat of C. taxifolia, is not able to live in polluted water, so it is perjudicied by the agricultural and urban residues, as well as it is also depleted by the bottom trawls and the ship anchors. The P. oceanica seagrass are a haven for many marine organisms, as moluscs, sea urchins, polichaeta and other algae; the ecological importance of this flowering plant is really significative and it is very important to preserve it. Unfortunately, it is in a deep recession. The main problem is that Posidonia needs more time to colonizy than Caulerpa and while C. taxifolia can live where P. oceanica does, the plant cannot, possibly because of the caulerpine.

There have been done campaigns to exterminate Caulerpa taxifolia (both entire colonies and isolated foots) manually, using aspirating pumps, hot water, electrolysis with copper or the use of chlorine. The biological control is still in an experimental fase, it could be done using moluscs (nudibranchs), but the introduction of non-autochthonous is a delicate issue and needs very detailed studies. (http://www.grid.unep.ch/product/publication/download/ew_caulerpa.en.pdf). It has also been conducted awarness campaigns, emphasizing the importance of not tearing to avoid more propagation. As well as leting the people know who to inform in case of finding any colony of the green killer algae.

DISCUSSION:

There have been several studies about the effect of Caulerpa taxifolia on Posidonia oceanica seagrass, and one of the most significant is that conducted by the coastal marine environment laboratory from Nice-Sophia university, in Antipolis. They studied over 10 years (1995-2005) the effect of Caulerpa taxifolia in the shallow P. oceanica seagrass. They bassed the ressearch in mesures of density of C. taxifolia from 2 invaded zones and the number of holdfasts, the number of ortotrop (perpendicular to the ground) and plagiotrop (prostrated) holdfasts and the branching ratio of P. oceanica.

Decline of density in  P. oceanica populations:

During the 10 years of the Nice-Sophia University study,  P. oceanica didn't disappear in the researched zones, but it was observed a decrease in density in the seagrasses invaded by C. taxofolia. During the year 2000, the decrease of P. oceanica was substancial, induced probably by a drastic increase of the temperatures of the summer before (C. taxifolia develops better with high temperatures). In this period, C. taxifolia increased substancially it's size in the invaded seagrasses. During the next 5 years, C. taxifolia reduced its size considerably, even so, P. oceanica didn't recovered completely.

Ortotrop rhizomes are converted to plagiotrop rhizomes:

It has been observed that in most of the seagrass invaded by C. taxifolia, the number of rhizomes plagiotrops is higher than ortotrops. The thing is that ortotrop rhizomes are converted to plagiotrop, something that also happens when P. oceanica is damaged by anchors. That behaviour is the way of P. oceanica to recolonize the lost territory.

Rhizomes:

The rhizomes, up to 1 cm of width, can grow both horizontally (plagiotrop rhizomes) and vertically (ortotrop rhizomes). The plagiotrop ones, thanks to the presence of lignified roots up to 15cm long, stick the plant to the substrate. The ortotrop rhizomes are responsible of the increase of heigh and their function is combating the burial caused by the sedimentation. Both kinds of growht result in the "matte", a terrace formation consisting in a network of rhizoma layers, roots and trapped sediments. Thus, the posidoneas colonize an environment that algae could hardly occupy due to their lack of roots.

Annoxic soil:

P. oceanica was not replaced by C. taxifolia during the 10 years of study. It is thought that the dense P. oceanica seagrass act as barriers for C. taxofolia, it is not normal that C. taxifolia infect dense seagrass. However, when it occurs, the C. taxifolia reaches its maximum expansion, seeding in the seagrass, causing anoxic soil. This process drives to a decrease in the seagrass density, that induces a decrease in the barrier, and then a progress of C. taxifolia. Even so, it has been seen that the progress stops because the plagiotrop rhizomes, that replaces the ortotrops, have negative efects in C. taxifolia development and growth, the movement of this rhizomes removes C. taxifolia.

It has been observed in some invaded seagrass the complete desapearance of  C. taxifolia. This can be associated to cold periods (not endured). Furthermore, posidoniaq can get back to tho space thanks to the plagiotrop rhizomes, but it takes much time.

P. oceanica seagrass have a huge ecological importance for the Mediterranean Sea, being C. taxifolia one of their threats. According to the information found, it seems not to be clear how far C. taxifolia harms P. oceanica. Nevertheless, it is a fact that its invasion weakens P. oceanica population, although it seems not to be completely replaced.

Bibliography and interesting related literature:

·         Anderson LWJ (2002) Caulerpa taxifolia in the United States: rapid response and eradication program. In Williams E, Grosholz E (eds), International Caulerpa taxifolia conference proceedings. California Sea Grant College, University of California – San Diego, La Jolla, CA, 169-189. (Consultado 15 de Octubre 2011).

·         P. Castro-Díez¹, F. Valladares², A. Alonso³, La creciente amenaza de las invasiones biológicas, Ecosistemas Revista Científica y Técnica de Ecología y Medio Ambiente,  Vol.  13, págs. 61-68. Septiembre del 2004. Disponible en:  http://www.revistaecosistemas.net/articulo.asp?Id=32 .

·         Juan Carlos Calvín, Comunidades de fondos arenosos. Apartado: Comunidad de pradera de Posidonia oceanica. Región de Murcia Digital. Disponible en: http://www.regmurcia.com/servlet/s.Sl?sit=c,365,m,2624&r=ReP-16216-DETALLE_REPORTAJESABUELO (Consultado el  11 de Noviembre 2011).

·         DELGADO O., RODRIGUEZ-PRIETO C., GACIA E., BALLESTEROS E., 1996. Lack ofsevere nutrient limitation in Caulerpa taxifolia (Vahl) C. Agardh, an introduced seaweed spreading over the oligotrophic northwestern Mediterranean. Botanica marina, 39 : 61-67.

·         Enrique Ballesteros, Caulerpa taxifolia [En Línea].Boletín informativo del Centro de Estudios Avanzados de Blanes, 16 de Enero del 2011. Disponible en: http://marenostrum.org/vidamarina/algalia/verdes/caulerpa/ (Consultado  28 de Octubre del 2011).

·         GRAVEZ V., BOUDOURESQUE C.F., MEINESZ A., VERLAQUE M., COTTALORDA J.M., GUITTON L., BERNARD G., ESCOFFIER B., BONHOMME P. Caulerpa taxifolia : l'expansion d'une algue tropicale en Méditerranée. Conséquences pour l'environnement et les activités humaines. Conseil régional Provence-Alpes-Côte d'Azur et GIS Posidonie publ. (ISBN 2-905540-26-5): 1-25.

·         Nuria Marbá Bordalba, Expansión de Caulerpa prolifera, C. taxifolia y C. racemosa en el Mediterráneo: Dinámica cloral, producción y destino de la producción. [En Línea]. Instituto Mediterráneo de Estudios Avanzados 2005. Disponibilidad en: http://www.cti.uib.es/digitalAssetn/128/128306_caulerpacart.pdf (Consultado 28 de Octubre del 2011).

·         Heike Molenaar, Alexandre Meinesz, Thierry Thibaut, Alterations of the structure of Posidonia  oceanica beds due to the introduced alga Caulerpa taxifolia. Scientia Marina, Vol. 73, Nº 2 (2009) [En Línea]. Consejo Superior de Investigaciones Científicas. Disponible:  http://scientiamarina.revistas.csic.es/index.php/scientiamarina/article/view/1065 (Consultado 29 de Octubre del 2011).

·         S. Kluser, G. Giuliani , A. De Bono, P. Peduzzi. Caulerpa taxifolia, a growing menace for the temperate marine environment. Environment  Alert  Bulletin. Enero del 2004. Disponible en:

http://www.grid.unep.ch/product/publication/download/ew_caulerpa.en.pdf . (Consultado  13 de Noviembre  del 2011).