Duarte, C.M.

Coastal ecosystems and the services they provide are adversely affected by a wide variety of human activities. In particular, seagrass meadows are negatively affected by impacts accruing from the billion or more people who live within 50 km of them. Seagrass meadows provide important ecosystem services, includ- ing an estimated $1.9 trillion per year in the form of nutrient cycling; an order of magnitude enhancement of coral reef fish productivity; a habitat for thousands of fish, bird, and invertebrate species; and a major food source for endangered dugong, mana- tee, and green turtle. Although individual impacts from coastal development, degraded water quality, and climate change have been documented, there has been no quantitative global assess- ment of seagrass loss until now. Our comprehensive global assess- ment of 215 studies found that seagrasses have been disappearing at a rate of 110 km2 yr 1 since 1980 and that 29% of the known areal extent has disappeared since seagrass areas were initially recorded in 1879. Furthermore, rates of decline have accelerated from a median of 0.9% yr 1 before 1940 to 7% yr 1 since 1990. Seagrass loss rates are comparable to those reported for man- groves, coral reefs, and tropical rainforests and place seagrass meadows among the most threatened ecosystems on earth. 

Recent research has highlighted the valuable role that coastal and marine ecosystems play in sequestering car- bon dioxide (CO2). The carbon (C) sequestered in vegetated coastal ecosystems, specifically mangrove forests, seagrass beds, and salt marshes, has been termed “blue carbon”. Although their global area is one to two orders of magnitude smaller than that of terrestrial forests, the contribution of vegetated coastal habitats per unit area to long-term C sequestration is much greater, in part because of their efficiency in trapping suspended matter and associated organic C during tidal inundation. Despite the value of mangrove forests, seagrass beds, and salt marshes in sequestering C, and the other goods and services they provide, these systems are being lost at critical rates and action is urgently needed to prevent further degradation and loss. Recognition of their C sequestration value provides a strong argument for their protection and restoration; however, it is necessary to improve scientific understanding of the underlying mechanisms that control C sequestration in these ecosys- tems. Here, we identify key areas of uncertainty and specific actions needed to address them. 

The shoot demography and rhizome growth of Syringodium filiforme Kutz. and Halodule wrightii Aschers. were studied, based on plant dating techniques, to account for their role as pioneer in the succession sequence of Canbbean seagrasses. Results demonstrated that these spe- cies are able to develop dense meadows, supporting bio- masses in excess of 500 g DW m" They produced more than 2000 g DW m-2yr-' due to their high leaf (5.0to 8 5 yr.') and rhizome (20 to 3.3 yr-') turnover. Rhizome growth and branch~ngrates were very high, allowing these seagrasses to rapidly occupy the space they colonise. The rapid rhizome turnover involved, however, a high shoot mortality rate and low 11fe expectancy (average shoot life expectancy 100 to 180d).This implies that, while these pioneer species are able to rapidly occupy the space they colonise, their established shoots cannot occupy that space for as long as the more long- lived species Thalassia testudinum. We suggest, therefore. that the role of seagrass species as pioneer or climax species is independent of their capacity to support dense, productive populations, and is closely related to shoot longevity and rhi- zome turnover.