Eutrophication and algal boom
Young
lakes (and man made reservoirs) usually have low levels of nutrients and
correspondingly low levels of biological activity. Such lakes are referred to a
being oligotropic from the Greek
work oligos meaning little or few. Literally oligotrophic means little-nourished.
Old
lakes usually have high levels of nutrients and correspondingly high levels of
biological activity. Such lakes are referred to as being eutrophic from the Greek word eu meaning well. Literally eutrophic means well-nourished.
The
natural time scale for the aging of a lake from being oligotrophic to eutrophic
is of the order of thousands of years. However, a high rate of input of
nutrients (from human activities) can increase the rate of aging significantly
resulting in eutrophic conditions developing after only a few decades. This artificial eutrophication has already
happened in many parts of the world including the Norfolk Broads and parts of
To
renew all the water in a lake may take up to a hundred years compared to a few
days for the renewal of the water in a river. Consequently, lakes are
particularly susceptible to pollution such as artificial eutrophication.
The causes of eutrophication
The
main causes of eutrophication are
In
most freshwater lakes the limiting nutrient is phosphorus, so an input of
phosphorus in the form of phosphate ions (PO43-) results
in an increase in biological activity.
The effects of eutrophication
The
main effects of eutrophication are an
Why Should We
Be Concerned?
Although eutrophication is a natural process in the aging of lakes and some
estuaries, human activities can greatly accelerate eutrophication by increasing
the rate at which nutrients and organic substances enter aquatic ecosystems
from their surrounding watersheds. Agricultural runoff, urban runoff, leaking septic
systems, sewage discharges, eroded streambanks, and similar sources can
increase the flow of nutrients and organic substances into aquatic systems.
These substances can overstimulate the growth of algae, creating conditions
that interfere with the recreational use of lakes and estuaries, and the health
and diversity of indigenous fish, plant, and animal populations.
Algal blooms hurt the system in two ways.
First, they cloud the water and block sunlight, causing underwater grasses to
die. Because these grasses provide food and shelter for aquatic creatures (such
as the blue crab and summer flounder), spawning and nursery habitat is
destroyed and waterfowl have less to eat when grasses die off. Second,
when the algae die and decompose, oxygen is used up. Dissolved oxygen in the
water is essential to most organisms living in the water, such as fish and
crabs. Increased eutrophication from nutrient enrichment due to human
activities is one of the leading problems facing some estuaries in the
mid-Atlantic.
Algal blooms
Algal blooms are one of the more insidious
consequences of eutrophication. In addition to being unsightly and smelly,
masses of blue-green algae can literally choke the life out of a lake or pond
by depriving it of much needed oxygen.
At
first glance this may seem like something of a paradox - since blue-green algae
undergoes photosynthesis, it should produce more oxygen than it consumes.
However, after large concentrations of algae have built up, aerobic processes
such as respiration and the decomposition of dead algal cells becomes
increasingly significant. Under extreme conditions a eutrophic lake or pond may
be left entirely devoid of fish.
The cause of algal blooms
Algal bloom: the rapid excessive growth of algae,
generally caused by high nutrient levels and favourable conditions. Blooms can
result in deoxygenation of the water mass when large masses of algae die and
decompose, leading to the death of aquatic plants and animals.
Algal
blooms are not a new phenomenon - Captain Cook recorded an algal bloom during
his voyage in 1770!
Algae
are a natural component of aquatic environments, and even when they are
abundant, it is not necessarily a problem. Often a proliferation of microscopic
algae can have beneficial effects on fisheries and aquaculture industries such
as oyster or mussel farms by increasing the amount of food available.
Macroalgae provide sheltered habitat for juvenile fish. In fact, the local
fishery production in the Peel-Harvey estuary almost doubled in the 1970s when
weed (macroalgae) growth in the estuary was at its peak, without a similar
increase in fishing effort.
The effect of algal blooms
However,
when algal blooms increase in intensity and frequency, the results can cause
community concern, health problems, and in some cases can be catastrophic to
the environment. The impacts are ecological, social and economic.
Algal
blooms upset the delicate natural balance of plant and animal ecosystems in a
waterway or wetland. They can degrade recreation, conservation and scenic
values, and interfere with economic uses such as fisheries and tourism. Weed
that washes ashore and forms rotting piles on beaches can cause offensive
smells and become a health problem for nearby residents as well as a nuisance
to beach users and fishers. An over-abundance of algae can choke a body of
water such as a river, clog irrigation pipes, and block out the light other
plants, such as seagrasses, need to produce food. Excessive weed growth can
eventually kill seagrass beds. When an algal bloom dies the process of decay
can use up all the available oxygen in the water, effectively suffocating other
aquatic life. This can kill fish, crabs and other animals, especially those
that are attached or sedentary (do not move around). Some species of algae produce
toxins.
Furthermore,
algae are a natural and critical part of our
Algae
may become harmful if they occur in an unnaturally high abundance or if they
produce a toxin. A high abundance of algae can block sunlight to underwater bay grasses,
consume oxygen in the water leading to fish kills, produce surface scum and
odors, and interfere with the feeding of shellfish and other organisms that
filter water to obtain their food. Some algal species can also produce
chemicals that are toxic to humans and aquatic life. Fortunately, of the
more than 700 species of algae in
The
deterioration of water quality and microbial hazards
There are several substances because of unsuitable treatment that cause the deterioration of water. They are as fallows:
Most sewage contain organic materials and
waste which are oxygen demanding. The natural system can undergo
self-purification processes in which the organic materials and waste are broken
down by bacterial and other biological activities naturally. During the
process, oxygen is consumed. Later, oxygen can be 'refilled' gradually through
a number of re-oxygenation processes.
However, natural systems have a limited
capability to accommodate self-purification. If the oxygen required for
biodegradation was greater than the oxygen present in water, oxygen depletion
will occur. The activities of organisms will slow down or stop. Such situation
usually happens due to high anthropogenic input of organic materials (like
domestic and industrial sewage discharge) into water bodies.
There are a lot of disease-causing
micro-organisms entering the aquatic system by means of contamination from
human and animal waste. Human can be affected through different water contact
activities, like swimming, water drinking, etc. Diseases like cholera and
typhoid may spread.
These disease-causing micro-organisms may
spread to water only sporadically. And once there, they do not survive long.
Thus, it is difficult to test their existence in water. Usually, we use fecal
coliform as indicator species, such as Eschericia coli, to identify the
problem of water contamination by disease-causing micro-organisms.
Thermal pollution
It would be cost-effective for industrial
and commercial sectors to use water as agent for cooling. However, the
discharge of such cooling water may artificially heat up the aquatic
environment. Many aquatic organisms can only live within a narrow range of
temperature. Different species live in different temperature ranges.
Apart from killing them, artificial heating
of water can increase the physiological stresses to some species and interfere
the natural life processes such as growth rates, respiration, reproduction, and
distribution. Some species can survive in the new environment will become
dominant in the ecosystem while previous dominant species become less dominant
or even excluded. Heating up water can also reduce the dissolved oxygen which
may directly kill aquatic species through asphyxiation.
In water, toxic chemicals can become
attached, or absorbed, to sediment particles and then transported to and
deposited in other areas. These pollutants can affect the organisms which live
there and can build up toxin in organisms that feed on them, and so be passed
along the food chain, causing problems all along the way. Sediments can also
reduce the sunlight penetrating into water and thus the affecting
photosynthesis for aquatic plants.
Plant nutrients, like nitrate and phosphate
fertilizers are important to plant growth. However, too much fertilization
encourages the overabundance of plant life. Eutrophication is the process by
which the nutrient content in water is increased suddenly. It is a natural
phenomenon until human accelerates it.
The prime contaminants are nitrates and phosphates,
they overstimulate the growth of algae, causing unsightly scum and unpleasant
odors, and depleting the DO of water that is vital to other aquatic life.
Moreover, the algae cloud the water and block sunlight, causing underwater
plants to die. Because these plants provide food and shelter for aquatic
creatures (such as the blue crab and summer flounder), spawning and nursery
habitat is destroyed and waterfowl have less to eat when plants die off.
In urban area, plant nutrients (nitrates
and phosphates) enter the natural environment through anthropogenic sources,
like sewage treatment effluents. In rural area, they enter the environment
through applying fertilizer.
Petroleum hydrocarbons in water can cause
smothering in water, and also result in adverse health impacts on living
organisms, loss of habitat, etc. Human activities introduce a great amount of
petroleum hydrocarbons into water, including daily sewage, leaching, oil
spills, underground tank leaks, etc.
Inorganic chemicals and minerals
Inorganic chemicals and minerals
pollutants, such as acids and heavy metals, are directly or indirectly
discharged from industrial and mining processes into sediments and water
bodies. Acid rain and runoff not only threaten aquatic life directly through
the effects of lowering pH, but also indirectly leaching, which may increase
the heavy metal burden on receiving water bodies.
There are huge amount of synthetic organic
compounds worldwide, including plastics, solvents, detergents, pesticides, PCBs
(Polychlorinated Biphenyls), pharmaceuticals, paints, etc. They are
accidentally or intentionally discharged into the aquatic system. Some of the
pure products, by-products, and waste products have a powerful and acute effect
(quick impacts) or a long-term, chronic effect on the aquatic ecosystems. Since
they are synthetic, these compounds may be resistant to biological breakdown
and they may persist and accumulate in the environment or organisms.
Causes of Water Pollution in
There are three basic causes of water pollution in Hong Kong, including:
There are many sources of water pollution, including anthropogenic
(household, commercial, industrial, farming, etc.) and natural sources.
Moreover, these sources are close to one another and in huge amount. Dredging
and sea dumping activity can also lead to water pollution.
Sewage from
In city, there are some illegal connections of waste water pipes to storm-water
pipes. In this way, the waste water will be discharged directly into sea
without treatment.