Environmental Engineering Reference
In-Depth Information
cycling of nitrogen and phosphorus through land clearing, applications of
fertilizer, discharge of human waste, animal production, and combustion
of fossil fuels (e.g., Nixon 1995). At some locations, waters reaching the
coastal zone have dramatically increased its contents of N and P compared
to concentrations in the middle of the 20th century (Cloern 2001). This
human driven fertilization of coastal wetlands has serious environmental
consequences, because it stimulates plant growth and disrupts the balance
between the production and metabolism of organic matter in the coastal
zone.
Nutrients reaching the coastal zone are captured and transformed by
sediments and wetland plants, which function as long-term nutrient sinks
(Valiela and Teal 1979). Most salt marsh plants are nitrogen limited under
natural conditions, and relief of nitrogen limitation results in an increase
in aboveground plant height and biomass (Kiehl et al. 1997, Valiela et al.
1976). However, the limiting role of nitrogen is not always the case for every
primary producer, in every coastal wetland, and nutrients retained in the
produced biomass will eventually be released, at a longer time scale.
Coastal wetlands can be generally considered as sinks for the excess
of nutrients, but eutrophication has the potential to alter performance and
competitive ability of wetland plant species, changing patterns of plant
zonation (Levine et al. 1998). In New England salt marshes, the effects of
nutrient-rich runoff from human activities has been pointed as a major
cause in the observed landward expansion of S . alternifl ora and the seaward
invasion of P . australis (Silliman and Bertness 2004). In the Wadden Sea,
Elymus athericus , a typical high marsh species, expands over marshes
formerly dominated by Festuca rubra, and this long-term vegetation change
has also been attributed to eutrophication (Rozema et al. 2000).
In addition to nutrients, rivers reaching the coastal zone may carry
many other contaminants, including metals, petroleum hydrocarbons,
and synthetic organochlorines. Salt marshes and mangroves are
depositional environments for suspended particulate matter and associated
contaminants. In anoxic soils, metal ions precipitate as sulfi des of low
solubility, making deeper sediments stable storage for pollutants in the
absence of bioturbation or other sources of oxidation (Weis and Weis 2004).
Heavy metals have rarely been found to have negative effects on plants
(Giblin et al. 1980, Suntornvongsagul et al. 2007). The adaptations to salt
stress, such as cellular compartmentalization of solutes and excretion
through salt glands, may allow for higher tolerance of metals (Windham et
al. 2001). There are experimental results to support the idea that sediments
and vegetation can absorb and transform a large portion of deposited metals,
working as sinks for metal contamination (Leendertse et al. 1996). Though
salt marsh plants have proved resistant to metal pollution, there is concern
that resuspension during erosion events pollute marine systems and that
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