Knowledge and implications of global change in the oceans for biology, ecology, and ecosystem services - Ocean Sustainability in the 21st Century - page 138

4.3.2 Seagrass ecosystems
Seagrasses cover about 0.1-0.2% of the global ocean, and develop highly productive eco-
systems which fulfil a key role in coastal ecosystems. A conservative estimate of the total
area covered by seagrass is 177,000 km 2 (Spalding et al., 2003 ), and the economic value
of all ecosystem services provided by seagrass approaches US$3.8 trillion per year globally
(Costanza et al., 1997 ). Seagrass ecosystems provide key ecological services, including OC
production and export, nutrient cycling, sediment stabilization, enhanced biodiversity, and
trophic transfers to adjacent habitats in tropical and temperate regions (Orth et al., 2006 ) .
Changes in sea level, salinity, temperature, atmospheric CO 2 , and UV radiation can alter
seagrass distribution, productivity, and community composition. In turn, potential changes
in distribution and structure of seagrass communities may have profound implications for
local and regional biota, near-shore geomorphology, and biogeochemical cycles (Short and
Neckles, 1999 ) . Widespread seagrass loss results from direct human impacts, including
mechanicaldamage(bydredging,fishing,andanchoring),eutrophication,aquaculture,silta-
tion, effects of coastal constructions, and food web alterations; and indirect human impacts,
including negative effects of climate change (erosion by rising sea level, increased storms,
increased ultraviolet irradiance), as well as from natural causes, such as cyclones and floods
(Duarte, 2002 ). Present losses are expected to accelerate, particularly in south-east Asia and
the Caribbean, as human pressure on the coastal zone grows. Positive human effects include
increased legislation to protect seagrass, increased protection of coastal ecosystems, and en-
hanced efforts to monitor and restore the marine ecosystem. However, these positive effects
are unlikely to balance the negative impacts, which are expected to be particularly promin-
ent in developing tropical regions, where the capacity to implement conservation policies is
limited.