Climate Change and the European Water Dimension - Agri ...

system, their physiology and internal structure have been modified to sustain an optimal
growth rate in a saline environment. No more than 60 species, organized in 12 genera,
are obligate halophytes, distributed worldwide except for Antarctica. Seagrasses occur
as large meadows concentrated within a narrow coastal band, limited off-shore by the
available sunlight at the bottom for photosynthesis, and on-shore by the species
tolerance to turbulence and wave action.
In addition to be highly productive (ca. 0.6 Gt C.yr -1 ; Duarte and Chiscano 1999),
seagrass meadows provide habitat for a wide variety of organisms, as well as nursery
ground for many fish species, and shrimps. An entire ecosystem is preserved within the
seagrass beds owing to the accumulation of detritus and the regeneration of nutrients at
the bottom layer by anaerobic bacteria in the sediment. Seagrass leaves are also an
important food source for several large-sized consumers such as sea turtles, birds and
dugongs. However, a large fraction of their photosynthetic production remains unused
and subject to low decomposition rate (Cebriàn and Duarte 1997). As a result, seagrass
beds represent efficient carbon storage, ca. 15% of the carbon storage in the ocean
(Duarte and Chiscano 1999). In addition, seagrass beds play a significant role in
stabilizing the bottom sediment, preventing thus from coastal erosion, and improve the
water quality by filtering and retaining suspended matter in the water column.
Climate change expressions such as level rise, increase in atmospheric CO2 and UV-B
radiations, global warming, intensification of storm events, all can affect seagrass
distribution, productivity, and community composition at various degree (Short and
Neckles 1999). Single effects can be beneficial (e.g. increase CO2 and inorganic nutrient
availability for photosynthesis) or detrimental (e.g. increasing turbidity and reduction of
light penetration) to the development of seagrass meadows. The net effect combining all
forcing factors are still difficult to assess, although a global decline of seagrass
populations is becoming more and more factual as a result of climate change (Short and
Neckles 1999) and direct human disturbance (Duarte 2002).
In early 30’s, 90% of the North Atlantic eelgrass population (Zostera marina) was
destroyed possibly by an epidemic infection due to some organisms. According to
Rasmussen (1977), however, increased temperature in the North Atlantic relative to
other ocean basins and the Mediterranean Sea is at the origin of the seagrass decline,
becoming then more sensitive to organisms attacks such as bacteria, fungus and other
species. Significant biomass recovery in affected areas (e.g. Danish Coastal waters)
took place 30 years after, in early 60’s, and is still experiencing a positive trend in spite of
a strong inter-annual variability (Frederiksen et al. 2004). On the contrary, eelgrass
biomass, in areas unaffected by the ‘wasting disease’, shows a negative long-term trend
over the entire time-series (starting in the mid-50’s; Frederiksen et al. 2004).
The physiological responses of seagrass species to different levels of physical gradients
are flexible, tolerating in most cases moderate changes over reasonable adaptative time
scales. For example, the single effect of an increase of a few degrees Celsius in water
temperature over the next century as a result of global warming would likely have little
effect on seagrass growth and distribution. On the other hand, global warming is
expected to intensify extreme weather events in different parts of the world, causing
dramatic consequences on seagrass population through storms, wave actions, resuspension
of sediment in the water column, as well as sudden pulses of freshwater
water runoff. After such events, the re-colonization can take several tens of years, or
even centuries, particularly with species having less successful sexual reproduction such
as Posidonia oceanica (Balestri and Cinelli 2003).
Changes in the amount and quality of light have also important implications for
seagrass habitats. Sea level rise and higher water column turbidity reduce the
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