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Introduction
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Fig. 1. Conceptual diagram showing comparison of healthy ecosystem with
eutrophic ecosystem. In a healthy ecosystem, nutrient inputs (e.g.
nitrogen and phosphorus) takes a key role of maintaining the balance
between the growth of macroalgae (red algae, brown algae, and/or green
algae) and phytoplankton and their predator species as well as high
water clarity. Appropriate density of phytoplankton and algae species
contributes to the optimum DO level for fish species, and consequently,
humans can take benefits from the marine environment.
On the other hand, in a eutrophic ecosystem, large amount of emissions
and nutrient inputs from farms, urban areas, factories, and sewage
treatment plants are into the marine environment triggers rapid growth
of phytoplankton, exceeding the consumption capacity of zooplankton and
fish. This growth causes lower water clarity and decreased light
penetration, lower DO, and decreased hydrophytes. Consequently, harmful
algae blooms abnormally occur as well as water contamination and fish kill.
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Eutrophication is a phenomenon in which conditions of certain water
bodies such as seas, lakes and rivers change from being oligotrophic to
eutrophic. Originally, this term explains a natural process occurring in
ponds and lakes by non-periodic changes of biotic communities under
certain environmental conditions; oligotrophic water with low nutrient
concentration and low production activities by few plankton and fish
changes to being eutrophic with high nutrient concentration and high
production activities. However, recently this term has been used more
often to indicate increase of nutrient inputs to water bodies by
anthropogenic activities. Sewage, runoffs from agriculture and livestock
industries, and industrial waste water are considered as causes of this
type of eutrophication.
Man-induced eutrophication changes the structure of marine communities
and decreases the biological diversity. In an extreme case, it may lead
to occurrence of red tides or fish kills. Therefore, eutrophication has
been widely recognized as one type of pollutions or environmental
problems. The water with progressing eutrophication has abundant
nutrients, so primary production near the surface is increased by
photosynthesis, and specific phytoplankton tends to grow rapidly. In
addition, when abnormally-grown plankton communities are dead, they
deposit at the sea bottom. Oxidative degradation of organic matters
leads to drastic decrease of dissolved oxygen (DO) and hypoxic water
masses are developed. Moreover, this negative process leads to stagnant
of organic degradation, sediments of sludge then, anaerobic bacteria,
which grows without oxygen, become dominant with odor. When these
hypoxia or anoxia water masses are mixed with surrounding waters, it may
lead to fish kills. Thus, in the eutrophic environment, primary
production by photosynthesis increases; however, it cannot always lead
to increase of fish catches. Damages from eutrophication became more
when red tides occur over fish cages of mariculture field.
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Ministry of the Environment , Northwest Pacific Region Environmental Cooperation Center
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