Environmental Engineering Reference
In-Depth Information
and the seasonal burning of vegetation to improve fodder quality is the
most common management practice. However, the rapidly growing coastal
population, the massive clearance of upland forests for agriculture, the
poorly controlled industrial effl uents, and the untreated sewage discharge
from coastal cities are major threatens to the coastal environment.
Concluding Remarks
Coastal wetlands have naturally evolved in response to global changes.
Sea level, the major environmental driver on coastal wetlands evolution,
has fl uctuated by an order of 100 meters over the past 18,000 years, and
salt marshes still persist. The examples shown in this chapter reveal the
complex nature of the responses of coastal wetlands to global changes,
as well as the need for long-term studies to separate the effects of short
term catastrophic events and widespread chronic human alterations from
''natural background change”.
Numerous studies show the resilience of coastal wetlands to natural
disturbances. However, changes in climate and sea level, coupled with
anthropogenic changes on sediment loads, species introduction, nutrient
enrichment, and other human alterations are likely to have a disproportionate
impact on mangroves and salt marshes. Projected rates of human-induced
change are unprecedented, and there is no outcome of global change
that we can reliably predict (Schneider 1993). We actually have a poor
understanding of species' tolerances and the relationships between species
diversity and ecosystem function (Ray et al. 1992). Ecosystem processes are
often characterized by complex nonlinear interactions involving numerous
biological, chemical, and physical components, and predicting the effects
of global change on complex ecosystem functions may be a real challenge
given the little knowledge that we have about thresholds of nonlinearity in
the responses of coastal wetlands to future stresses (Mintzer 1992).
Most coastal wetlands export reduced nitrogen compounds, organic
matter and other energy-rich substances to deeper waters supporting the
receiving ecosystems, as suggested in the “outwelling” hypothesis (Odum
1980). Many commercially important species use coastal wetlands as
nurseries and foraging areas during their early life stages (Robertson and
Duke 1987, Van der Welde et al. 1992). Shallow protected waters fringed
by wetlands are commonly rich in phytoplankton, which supports dense
populations of suspension feeders and other consumers. All these functions
make wetlands valuable environments for humans that harvest shellfi sh
and other highly appreciated stocks. As shown in previous section, salt
marsh plants and sediments retain contaminants, including heavy metals
and chlorinated hydrocarbons. As long as any pollutant from land is
buried in marsh or mangrove sediments, these wetlands are preventing
