Environmental Engineering Reference
In-Depth Information
increased nutritional defi ciencies, among others (Harding and Addison
1986, Peakall 1986).
With the global reductions in use of DDT and PCBs during recent decades,
residues in sediments and concentrations in animals have diminished in
many coastal areas. However, a low but long-term concentration of the
degradation products persists in organisms in most places in the world.
Besides, there are of long term reservoirs in soils and aquatic sediments
that will slowly release DDT and PCB residues to the environment, and it
is not clear yet whether these low-concentration residues will have long-
term biological consequences.
Direct Human Transformations
There always has been an intense relationship between humans and
wetlands. Human impacts
on wetlands
date back to the end of the last
glacial age, when the combination of changing climates and expanding
and migrating human populations extinguished a considerable number of
wetland species (Martin and Wright 1967). In a study of 12 of the world's
largest estuaries, Lotze et al. (2006) found a 67% loss of coastal wetlands
during human history. Historical records describe people who depended
almost entirely on wetlands, like Fen Slodgers in the English Fenlands
(Wheeler 1896), or more recently, the Marsh-Arabs (Madan) of Southern
Iraq (Thesinger 1964). The use of salt marshes for fi shing and livestock
grazing date to the Neolithic in the North and Wadden Seas (Knottnerus
2005, Meier 2004), and the Mesopotamian tidal marshes are thought to be
birthplace of agriculture (Sanlaville 2002).
Grazing lands for cattle, sheep, goats, and horses has been probably the
most common use of salt marshes around the world (Knottnerus 2005), but
large scale wetland modifi cation and the conversion of wetland to upland
was historically undertaken for agricultural purposes (Glover and Higham
1996). Humans have directly converted wetlands into drylands to win areas
for intensive agriculture and forestry (Appleton et al. 1995). Moreover,
wetland destruction may result also from changing land use outside the
wetland boundaries. In the Netherlands, upland deforestation started
about 3,000 years ago, affecting river discharges of the Rivers Rijn, Maas
and Schelde. The consequent widening of the estuaries gave opportunity
for saline intrusion and led to massive erosion of the perimarine peatlands
(Pons 1992).
Salt marsh reclamation for agriculture began in the Netherlands and
France by the eleventh century, and probably earlier in China (Yoshinobu
1998), through diking for fl ood protection (Knottnerus 2005, Reise 2005).
Diking results in the conversion of wetland to upland. The biogeochemistry
of salt marsh soils, characterized by slow decomposition and tidal fl ushing
