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remains trapped among the exposed roots
and stems (Fig. 11-4).
• Chemical - adsorption, ion exchange, and
precipitation may aid in the removal and
storage of heavy metals from water traveling
through wetlands. Metal accumulations and
chemical processes of removal are depend-
ent on soil conditions including pH, cation
exchange capacity, redox potential and the
presence of organic matter (Laing et al. 2008).
agricultural runoff, urban sewage, storm-water
overl ows, and industrial efl uents remain a
threat to fresh- and salt-water quality the world
over (Fig. 11-5). Such nutrient overloads are
often responsible for hypoxic dead zones at the
mouths of major rivers, estuaries, and enclosed
bays, leading to the widespread disruption of
aquatic life. A recent study identii ed more than
200 such dead zones worldwide, with the largest
covering 120,000 km 2 in the Baltic Sea (Schrope
2006). These areas cause signii cant economic
disruptions to vital i sheries, as seen both in the
case of the blue crab industry in the Chesapeake
Bay of the eastern United States and shelli sh
beds and coastal i sheries at the mouth of the
Mississippi River in the Gulf of Mexico. While
wetlands alone cannot ameliorate this problem,
they provide an important service in improving
water quality through the processes outlined
above.
Point and non-point introductions of pollutants
including nutrients and heavy metals from
11.3.3 Water storage and diversion
Wetlands have for millennia served as reservoirs
of fresh surface and ground water to meet
human and ecosystem demands. The storage
capacities of wetlands depend on a host of
factors including their size and their watersheds,
which comprise the wetland basin and catch-
ment areas. Storage capacity is also inl uenced
by the height of the water table. Generally, the
Figure 11-4. Dense stand of Avicennia marina (gray
mangrove) along the west coast of India. Notice debris
and litter which gets entangled and stranded among
the branches and roots during low tide. Photo by
Firooza Pavri.
Figure 11-5. The highly polluted Mithi River estuary l owing through suburban Mumbai along coastal western
India at low tide with a stand of Avicennia marina (gray mangrove), a highly salt-tolerant species which also
withstands relatively high levels of pollution. Photo by Firooza Pavri.
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