Climate change and Mediterranean seagrass meadows: a synopsis for environmental managers

International audienceThis synopsis focuses on the effects of climate change on Mediterranean seagrasses, and associated communities, and on the contribution of the main species, Posidonia oceanica, to the mitigation of climate change effects through sequestering carbon dioxide. Whilst the regression of seagrass meadows is well documented, generally linked to anthropogenic pressures, global warming could be a cause of new significant regression, notably linked to the introduction of exotic species, the rise of Sea-Surface Temperature (SST), and relative sea level. Seagrass communities could also be affected by climate change through the replacement of high structural complexity seagrass species by species of lower complexity and even by opportunistic introduced species. Although it is currently very difficult to predict the consequences of these alterations and their cascade effects, two main potential conflicting trends in the functioning of seagrass ecosystems are acceleration of the herbivore pathway or the detritivore pathway. The mean net primary production of the dominant species, Posidonia oceanica, is relatively high and can be estimated to range between 92.5 to 144.7 g C m-2 a-1. Around 27% of the total carbon fixed by this species enters the sedimentary pathway leading to formation, over millennia, of highly organic deposits, rich in refractory carbon. At the Mediterranean scale, the sequestration rate might reach 1.09 Tg C a-1. The amount of this stored carbon is estimated to range from 71 to 273 kg C m-2 , which when considered at the Mediterranean scale would represent 11 to 42% of the CO 2 emissions produced by Mediterranean countries since the beginning of the Industrial Revolution. The greatest value of the P. oceanica ecosystem, in the context of mitigation of global climate change, is linked to this vast long-term carbon stock accumulated over millennia, and therefore, efforts should be focused on preserving the meadows to keep this reservoir intact. Introduction Seagrass meadows constitute a major component of coastal marine ecosystems, which provide goods and services to the growing human populations in coastal areas. However, seagrass meadows are presently experiencing a decline globally (Orth et al., 2006); a loss rate of 7% a-1 since 1990 has been computed, which places seagrass meadows among the most threatened ecosystems on earth (Waycott et al., 2009). Worldwide, seagrasses are subjected to all five of the most serious threats to marine biodiversity (Norse, 1993): overexploitation, physical modification, nutrient and sediment pollution, introduction of non-native species, and climate change. Threats to seagrass meadows from climate change include increases in sea surface temperature, sea level, and frequency and intensity of storms (Short & Neckles, 1999), which are predicted to have deleterious effects on seagrasses and represent a pressing challenge for coastal management (Brouns, 1994)