Dynamique et èvolution de l'herbier à Posidonia oceanica en Méditerranée

Abstract

The natural growth of a Posidonia oceanica meadow is the result of a long process of development (series of events), but the dynamics are often difficult to establish because of the lack of a zero state and of sufficiently long chronological series. In addition, while it is sometimes possible to observe major regressions, the phenomena of recolonisation and of small-scale fluctuations are rarely detected. Nevertheless, with the emergence of new investigatory techniques (lepidochronology, microcartography, monitoring networks, accurate dating techniques, etc.,) considerable progress has been made in this field. The development in space and time of the seagrass beds is dependant on hydrodynamic action and sedimentary deposits. This means that in sheltered bays Posidonia oceanica may form original structures such as barrier reefs or micro-atolls, while in turbulent waters one observes the formation of spectacular structures resulting from erosion (intermatte, “rivière de retour”). A disequilibrium in sedimentation may result in dramatic regression in some seagrass beds. The multiplicity of prevailing environmental conditions results in the occurrence of a range of meadow types: “herbiers de plaines”, “herbiers de collines”, “herbiers tigrés”, “herbiers en escalier”. The impact of human pressure over the past few decades has not been insignificant. Coastal development constructions that have altered the direction of currents and the sedimentation, sewage outfalls that have resulted in an increase in the turbidity of the water and the mechanical action of anchors or trawl nets which have damaged certain areas of seagrass meadow have caused extensive regression. Conversely, the rehabilitation of previously degraded areas has made it possible to monitor the process of recolonisation.

Key Words:

• Dynamic
• Mediterranean
• Posidonia oceanica
• Regression
• Seagrass bed
• Structure