2012年12月19日星期三

Eutrophication

 

 

Introduction

Eutrophication is the process of excessive nutrient enrichment of waters that
typically results in problems associated with macrophyte, algal or cyanobacterial
growth.


While an enormous amount of literature has been published on this topic, a detailed
treatment is outside the scope of this report. A recent local review [Walmsley, 2000]
provides important perspectives on eutrophication of surface waters with particular
emphasis on policy and research needs in South Africa. It contains a very useful list of
references which can be consulted for more detailed information.

The causes and effects of eutrophication are complex. This chapter only summarises
briefly the current state of knowledge. Internationally, much research work is in
progress that aims at furthering our knowledge of the intricate interrelationships involved
in eutrophication of water resources. A paper by Rast and Thornton (1996) can be
consulted for more information on research trends.




Causes





 


In natural lakes a distinction is sometimes made between ‘natural’ and ‘cultural’
(anthropogenic) eutrophication processes (e.g. Rast and Thornton (1996)). Natural
eutrophication depends only on the local geology and natural features of the catchment.

Cultural eutrophication is associated with human activities which accelerate the
eutrophication process beyond the rate associated with the natural process (e.g. by
increasing nutrient loads into aquatic ecosystems). In South Africa where impoundments
are man-made, the conceptual difference between ‘natural’ and ‘cultural’ seems less
appropriate.

Increased nutrient enrichment can arise from both point and non-point sources external
to the impoundment as well as internal sources like the impoundment’s own sediments
(that can release phosphate).

The adjacent figure illustrates some of the factors that drive the eutrophication process
in an impoundment.





Trophic status


Eutrophication is a process and it is useful to be able to characterise the stage at which
this process is at any given time in a particular water body. The ‘trophic status’ of the
water body is used as a description of the water body for this purpose. The following
terms are used.




Oligotrophic - low in nutrients and not productive in terms of aquatic animal and plant
life.

Mesotrophic - intermediate levels of nutrients, fairly productive in terms of aquatic
animal and plant life and showing emerging signs of water quality problems.

Eutrophic - rich in nutrients, very productive in terms of aquatic animal and plant life
and showing increasing signs of water quality problems.

Hypertrophic - very high nutrient concentrations where plant growth is determined by
physical factors. Water quality problems are serious and almost continuous.
It is convenient to associate the trophic status in impoundments with total phosphorus
and chlorophyll a measurements. The following relationships between trophic status
and these variables are used. These are essentially those of Van Ginkel et al. (2000),
which were based on the work of Walmsley and Butty (1980) and Walmsley (1984).
These have been shown to be applicable to South African impoundments.



Impacts


Eutrophication is a concern because it has numerous negative impacts. The higher the
nutrient loading in an ecosystem the greater the potential ecological impacts. Increased
productivity in an aquatic system can sometimes be beneficial. Fish and other desirable
species may grow faster, providing a potential food source for humans and other
animals (though this is not a common situation in South Africa). However, detrimental
ecological impacts can in turn have other adverse impacts which vary from aesthetic
and recreational to human health and economic impacts. This is summarised in the
following figure.



  • Ecological impacts

Macrophyte invasions and algal and cyanobacterial (blue-green) blooms are themselves
direct impacts on an ecosystem. However, their presence causes a number of other
ecological impacts.

Of critical concern is the impact of eutrophication on biodiversity. Macrophyte invasions
impede or prevent the growth of other aquatic plants. Similarly, algal and cyanobacterial
blooms consist of species that have out-competed other species for the available
nutrients and light.

Their impact on animal biodiversity is also of concern. By generally lowering the
ecological integrity of an ecosystem, only the more tolerant animal species can survive.


  • Aesthetic impacts

Algal and cyanobacterial blooms, and particularly surface scums that might form, are
unsightly and can have unpleasant odours. This is often a problem in urban
impoundments where people live close to the affected water body.

If the water is being used for water treatment purposes, various taste and odour
problems can occur. These lower the perceived quality of the treated water, although
do not cause human health problems.



  • Human health impacts


An infestation of water hyacinth (Eichhornia crassipes) can be a health hazard. It can
provide an ideal breeding habitat for mosquito larvae and it can protect the snail vector
of bilharzia.

Of all the cyanotoxins currently known, the cyclic peptides represent the greatest
concern to human health, although this may be because so little is known about the
other cyanotoxins [Chorus and Bartram, 1999]. The concern exists primarily because
of the potential risk of long term exposure to comparatively low concentrations of the
toxins in drinking water supplies. Acute exposure to high doses may cause death from
liver haemorrhage or liver failure. Other short term effects on humans include
gastrointestinal and hepatic illnesses. A number of adverse consequences have been
documented for swimmers exposed to cyanobacterial blooms. Chronic exposure to low
doses may promote the growth of liver and other tumours. Nevertheless, many
cyanobacterial blooms are apparently not hazardous to animals.

It is also possible that people exposed to odours from waterways contaminated with
decaying algae of cyanobacteria may suffer chronic ill-health effects.


Economic impacts

Nearly all of the above mentioned impacts have direct or indirect economic impacts.
Algal or cyanobacterial scums increase the costs of water treatment in order to avoid
taste, odour and cyanotoxin problems in the treated water. Excessive blooms can clog
filters and increase maintenance costs.

Human and domestic and wild animal health impacts due to cyanotoxins in water have
obvious direct economic impacts.

Once significant eutrophication has occurred, the costs of corrective action can be
enormous. Macrophytes may need to be sprayed or brought under control by biological
or other costly treatment processes.




 

MANAGEMENT


The basis of eutrophication management is often the ‘limiting nutrient concept’
[Walmsley, 2000]. The rate and extent of aquatic plant growth is dependent on the
concentration and ratios of nutrients present in the water. Plant growth is generally
limited by the concentration of that nutrient that is present in the least quantity relative
to the growth needs of the plant. Minimisation of eutrophication-related impacts
therefore tends to be focussed on efforts to reduce nutrient (particularly phosphorus)
inputs. This approach therefore deals directly with the primary cause of eutrophication
(namely, nutrient enrichment).
Typically, limiting nutrients entering an impoundment exhibiting a high degree of
eutrophication will first focus on point sources. These are easier to quantify, simpler to
manage and often contribute the highest nutrient load. Following this, non-point sources
are managed and then internal (“in-lake”) management options can be implemented.


没有评论:

发表评论

发表评论