Environmental Engineering Reference
In-Depth Information
of metabolic toxins (Mendelssohn and Morris 1999), is heavily altered by
diking. While drained marsh soils become acidifi ed and organic matter
is more rapidly oxidized (Portnoy 1999), toxic sulfi des accumulate in
undrained areas behind dikes. Lack of sediment inputs and increased
decomposition also causes subsidence of the marsh surface (Portnoy and
Giblin 1997). Many areas that were originally reclaimed for agriculture were
transformed into urban, residential, and industrial land. Large coastal cities
such as Boston, San Francisco, Amsterdam, Rotterdam, Venice, and Tokyo
expanded on former coastal wetlands (Pinder and Witherick 1990).
Land reclamation is the most striking transformation of wetland
areas, but there are other human disruptions of natural hydrology that
signifi cantly change the structure and functions of wetland communities.
Ditching is a common practice used by farmers to accelerate tidal fl ushing
and to increase salt marsh plants production (Sebold 1998). The invention of
ditch-digging machines accelerated ditch construction, and many marshes
were extensively ditched throughout the world with different purposes
(Bourn and Cottam 1950, Resh 2001). The immediate effect of ditches is
to ameliorate waterlogging stress in the high marsh, and to drain organic
soils up to 5 m away from ditches. By increasing tidal fl ushing frequency,
ditches ameliorate anoxic stress and increase plant productivity near ditch
banks (Valiela 2006). Ditches have caused shifts in salt marsh vegetation
that favor high marsh species, which are better below-ground competitors
when anoxic and saline stresses are reduced (Bertness and Ellison 1987). In
response to intensive ditching, it is expected that low marsh grasses, well
adapted to anoxia, give place to species often found on well-drained soils
(Bourn and Cottam 1950).
Although the landscape level effects of ditches have been well described,
ecosystem level impacts of ditching on biogeochemical cycles and marsh
accretion processes have not been thoroughly explored. Tidal restrictions are
another common hydrologic alteration, occurring as a consequence of roads
and railway crossings that limit tidal infl uence on upriver wetland areas
and restrict sheet fl ow across marshes (Buchsbaum 2001). Tidal restrictions
limit marine infl uence upriver, and impounded marshes become fresher
and receive less input of inorganic sediments. Complete restriction converts
impounded salt marshes to freshwater wetlands (Roman et al. 1984). Tidal
restrictions have negative impacts on biogeochemical cycling, leading to
whole marsh subsidence in similar ways to dikes, endangering animal
wetland species (Soukup and Portnoy 1986) and ecosystem functioning
(Portnoy 1999).
The combination of land claim and hydrologic alterations has had
an important impact on the long-term persistence of coastal wetlands.
The fact that many former and extensive perimarine wetlands are largely
absent from European coastal lowlands is primarily due to land claim (e.g.,
Search WWH ::




Custom Search