Estuary Processes and Managment - River Dynamics and Integrated River Management - page 530

vegetation (e.g., removal of nitrogen and phosphorus) and removal of heavy metals by adsorption/precipitation
reactions (e.g., aluminum, iron and calcium) in the soil. In view of the low cost and acceptable treatment
efficiency, application of untreated or treated wastewater to wetlands is regarded as a simple, efficient
and economical method for removal of wastewater pollutants. In addition, artificial wetlands—i.e. engineered
wetlands that reproduce some of the natural functions of wetlands—are attracting considerable interest in
river restoration and wetland compensation projects.
Mangrove wetlands are uniquely intertidal forested ecosystems, often found along the sediment-rich
sheltered estuarine shores of tropical and subtropical regions (largely restricted to latitudes between 30°N
and 30°S) that share some common characteristics: high temperature, fluctuating salinity, alternating
aerobic and anaerobic conditions, periodically wet and dry, unstable and shifting sub-stratum (Tam,
2006). Mangrove communities are made up of diverse groups of plants and animals, but in general some
mangrove associated plants (e.g., Kandelia obovata ) dominate mangrove communities owing to their
highly morphological and physiological adaptations to extreme conditions (Fig. 8.75). To survive in
transition zones between terrestrial and marine environments, mangrove plants evolved two unique
characteristics: ķ they are physiologically tolerant of high salt levels through a combination of salt
accumulation, excretion and exclusion—for instance, their root cell membranes can prevent the majority
of salt from entering the plants, their leaves can store salt at medium level, and their glands can secrete
excess salt; ĸ if the oxygen concentrations in soil become very low, mangrove plants can send forth
pneumatophores—a kind of aerial root growing upwards from the root above the mud—to absorb enough
oxygen for photosynthesis and metabolism.
Pollutants (e.g., nutrients and heavy metals) are apt to accumulate in mangrove ecosystems, because of
their absorptive capacity or rapid microbial activities. Therefore, mangrove wetlands are often used as
buffer zones to purify aquaculture wastewater prior to its entering the sea. For example, exchange of
coastal waters with shrimp ponds in brackish estuarine waters is important to assure high yields; on the other
hand, this will inevitably have a negative impact on the water quality of the coastal waters. Therefore, the
environmental-friendly "integrated pond-mangrove farming system" is often used to strip nutrients from
pond effluent. Ranges of “mangrove to pond area ratio” vary from 2 to 22 depending on the capacity for
nitrogen removal by mangrove system (Robertson and Phillips, 1995), but could be reduced to a range of
0.04-0.12 if de-nitrification occurs in the mangrove system (Rivera-Monroy et al., 1999).
In addition, the feasibility of using mangroves to remove pollutants from municipal and livestock
wastewater has also been studied. A 3-year field study in the Futian mangroves—a 370 hectare natural
mangrove intertidal wetland in Shenzhen (Fig. 8.75)—showed that primary-treated domestic sewage could
be purified if intermittently discharged to the landward region of the mangrove wetland during the low
tide period; the tidal water was not contaminated and negative impacts on plant growth were not detected
(Wong et al., 1997). Nitrification and de-nitrification processes are the most important pathways to remove
nitrogen because mangroves are periodically flooded by tidal water with alternating aerobic and anaerobic
conditions. Generally, the nutrient removal by wetland soils is most efficient at low nutrient loading rates,
and removal efficiency falls off rapidly as loadings are increased. The percentage of nitrogen lost from the
mangrove ecosystem is around 40% and the plant uptake varies from 12% to 68%, depending on the plant
species and salinity, and the organic loading. In contrast, phosphorus and heavy metals in wastewater are
mainly immobilized in sediments with little loss. At the end of the 3-year field study in Futian mangroves,
the amounts of nitrogen retained in mangrove sediments were much less than that of phosphorus, despite
the fact that more nitrogen was present in wastewater (Tam and Wong, 1995; Tam, 2006).
Mangrove plants are capable of transferring oxygen from the atmosphere to the roots and creating an
oxygenated zone for nitrification around the roots while the surrounding sediments are reduced thus
favor de-nitrification. The population sizes of various bacterial groups, in particular those related to the