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Abstract: The response of Posidonia oceanica (Linnaeus) Delile to the warm-water episode of summer 1999 was studied by means of the technique of lepidochronology. Study sites include three sites affected by the mass mortality event of benthic invertebrates and one not affected. The results showed a significant decline in some parameters (number of leaves and/or rhizome growth) for the three sites affected by the mass mortality event for the year following the warm-water episode (1999–2000). A similar decline was not observed for the unaffected site. The fact that high temperatures could have a negative impact on deep Posidonia oceanica near its cold limit of distribution is an unexpected result. To cite this article: N. Mayot et al., C. R. Biologies 328 (2005). [Copyright &y& Elsevier]

Abstract: A shallow Mediterranean brackish lagoon (Urbinu, Corsica), 700ha in surface area, characterized by low freshwater input and permanent communication with the open sea, and therefore by relatively stable salinity (usually 30–38), was subject in late 1993 to an exceptional rainfall event occurring on an average once every 50 years: 450mm in 48h (compared to the average annual precipitation of 650mm). The volume of freshwater that poured into the lagoon corresponds to 36% of its volume. As a result, salinity dramatically dropped while turbidity increased. The seagrass Cymodocea nodosa and other habitats were mapped before (1990) and after (1994, 1996, 1999) the rainfall event, and the sea urchin Paracentrotus lividus stock was estimated together with its population structure. In 1994, after the rainfall event, the surface area of seagrass meadows moderately declined, but it cannot be ruled out that this loss may be within their usual inter-annual fluctuations. The sea urchin stock dropped by 50% (6–3 million individuals). Low salinity, turbidity and siltation were probably the reasons for the changes in sea urchin population in addition to variability of dynamic population parameters (e.g. recruitment, mortality). The recovery of sea urchin stock was completed within a few years (six years or less). The high population dynamics and the high recruitment potential of sea urchins may act as a mechanism to maintain sea urchin populations in this highly variable habitat. These results reflect the resilience and high adjustment stability of the system. [Copyright &y& Elsevier]

Coastal marine vegetation has been recently highlighted for its highly efficient carbon storage capacity. Among the sixty-four species of seagrass, Posidonia oceanica , a Mediterranean endemic species, appears to be the most effective in carbon fixation and storage. Based on new data from the study of one of the largest P. oceanica meadows in the Mediterranean Sea (100 km of coastline, 20 425 ha), and a synthesis of available data from the whole of the Mediterranean basin, the aim of this work is to evaluate the amount of carbon fixed each year by P. oceanica and sequestered in the matte, in relation with the mitigation of the impact of climate change (carbon sink). The mean total carbon fixation (blades, sheaths and rhizomes) per year varies between 33.5 and 426.6 g C.m−2 and the mean carbon sequestration (long-term sink in the matte), corresponding to the sheath and rhizome tissues, varies between 7.7 and 84.4 g C.m−2, with a clear decreasing trend according to depth because of the meadow density decrease. The synthesis of a hundred measurements made throughout the Mediterranean Sea and at depths between 0.5 and 32.0 m provides a basis for estimating the average annual carbon fixation and sequestration rate throughout the Mediterranean basin. The fixation of the blades is estimated at 1 024 t C.ha−1.yr−1, that of the sheaths at 220 t C ha−1.yr−1 and that of the rhizomes at 58 t C ha−1.yr−1; i.e. a total fixation rate of 1 302 t C ha−1.yr−1 and sequestration rate (dead sheaths and rhizomes) of 278 t C ha−1.yr−1. This annual carbon fixation represents only 0.61% on average of CO 2 emissions/releases for all Mediterranean countries but in the large Mediterranean islands this fixation is on average 3.1% and can reach almost 14.4% for Corsica. Moreover, the major advantage of the P. oceanica meadow lies in its capacity to store carbon from annual carbon sequestration for centuries to millennia and can be compared to several terrestrial ecosystems considered to be efficient in carbon storage (peatlands). • First assessment of carbon fixation and sequestration by Mediterranean seagrass. • 5 681 206 tons of CO 2 emissions, are fixed annually by Posidonia oceanica. • 21% of this carbon is sequestered, annually, by the Posidonia oceanica soil (matte). • For Mediterranean islands, carbon fixation can reach up to 14.5% of carbon emissions. [ABSTRACT FROM AUTHOR]