Estuary Processes and Managment - River Dynamics and Integrated River Management - page 502

8.4.4 Eutrophication Control and Management
Control of eutrophication is mainly affected by manipulating the nutrient input. So far many suggestions
have been proposed to mitigate the effects of eutrophication, ranging from developing monitoring programs
that provide early detection of algal blooms, to introducing potential pathogens to cure blooms. Because
any action taken to control or cure blooms may have enormous impact on the environment, it is generally
more cost-effective to take early preventive measures to control eutrophication than to develop curative
strategies after the water quality has already deteriorated.
Common restoration methods mainly include diversion of nutrient input to another water body where
the impact is less, and reduction of nitrogen or phosphorus by chemical and biological means (e.g., water
treatment). It is usually sufficient to reduce only the most important limiting nutrient (Schindler and Fee,
1974). Control of point sources of pollution from municipalities and industries is usually given priority
as it is the most cost-effective measure. In many cases, however, effective prevention of eutrophication,
or restoration of eutrophic waters cannot be achieved without controlling non-point source pollution.
Considering the difficulty for controlling non-point sources, the improvement in all aspects should be
encouraged: ķ control of agricultural practices contributing nutrients to water bodies, with special
reference to waste from animal husbandry, irrigation, fertilizer application, and fish culture; ĸ integrated
approaches to control urban runoff and storm overflows, which normally by-pass conventional treatment
plants; Ĺ preserving natural buffer strips or constructing artificial wetlands to reduce the amounts of
nutrients reaching water bodies from runoff, since they are based on the capacity of self purification of
nature and are usually much cheaper to maintain and operate.
Considerable effort has been directed towards the development of tools for a better understanding of
the causes and consequences of massive algal blooms. An interdisciplinary study is usually required for
assessing the risks and impacts of eutrophication. In general, three approaches have been adopted: setting
of thresholds based on available data and expert judgment, establishment of empirical cause-effect
relationships, and development of numerical modeling (Nixon et al., 1996; OECD, 1982; Painting et al.,
2007). An assessment standard may consider many environmental indicators of eutrophication, including
primary production, nutrient, DO, turbidity, submerged vegetation, and macroalgae. Extensive research on
empirical relationships has greatly improved our understanding of the causes and effects of eutrophication.
Mathematical models that incorporate all the major interactions (growth kinetics, hydraulic transport, and
environmental parameters) of the ecosystem have also proved to be useful in predicting trends of
eutrophication and evaluating the effectiveness of alternative methods of control (e.g., waste reduction,
flow augmentation).
Case study— Hong Kong is situated at the mouth of the Pearl River Estuary (Fig. 8.41) in southern
China. The high incidence of red tides and algal blooms is a unique feature in the sub-tropical coastal
waters around Hong Kong, which is believed to be related to the eutrophication caused by excessive and
concentrated organic discharges. The reported number of red tides reached a peak of 88 outbreaks in
1988, but decreased to current frequencies of around 20 per year after the introduction of the Tolo
Nutrient Export Scheme in the mid-1990s (Fig. 8.42). Hong Kong's population and organic sewage loads
are centered around Victoria Harbor; before the Harbor Area Treatment Scheme came into operation in
2001, some 1.5 million m 3 of untreated sewage and industrial wastewater were discharged daily into
Victoria Harbor. The annual non-point source pollution load into Hong Kong's coastal waters is estimated
to be about 8000 tons of total nitrogen (TN) and 1500 tons of TP (Li et al., 2003). Besides, the background
pollution is contributed by the organic load carried by the Pearl River estuary.
Fish kills are often associated with the observed red tides. The 1998 red tide (due to the dinoflagellate
Karenia digitatum ) was the most serious red tide in Hong Kong's history. It was recorded that 30 red tide