Causes of Eutrophication
Eutrophication in the Sea of Azov. Source: SeaWiFS Project,
NASA/Goddard Space Flight Center and ORBIMAG
Developed by Richard Sandford with
contributions from Martin Bloxham and Paul Worsfold,
Eutrophication – Causes
2.1 Enhanced nutrient inputs
Eutrophication is caused by enhanced
nutrient inputs (N and P) as a result of:
Inorganic fertilisers and animal wastes in crop
Combustion of fossil fuel
production
Wastes from animal production
Combustion of fossil fuel
Forestry practices
Agricultural practices
Phosphate detergents
Sewage discharges
Aquaculture
Draining of wetlands
Sewage discharges
Soil and sediment erosion, remobilisation,
leaching (from agriculture, mining, construction)
Example: Effects of Agricutlure
Wastes from animal production
Soil erosion
on lakes
Eutrophication – Causes
2.2 Increased recycling/mobilisation of nutrients
Eutrophication is also caused by Increased
recycling/mobilisation of nutrients as a result
of:
Forestry practices
Water impoundment
Forestry practices
Draining of wetlands
Water system impoundment (e.g. reservoirs)
Soil and sediment erosion, remobilisation,
leaching (e.g. As a result of agriculture, mining,
construction)
Question: What is a Causal
Chain?
Soil erosion
Draining of wetlands
Eutrophication – Causes
2.3 Trapping of nutrients (e.g. in river impoundments)
Eutrophication is also caused by Trapping of
nutrients (e.g. in river impoundments)
as a result of:
Water system impoundment
Soil and sediment erosion, remobilization,
leaching (e.g. As a result of agriculture, mining,
construction)
Soil erosion
Water impoundment
Water impoundment
Question: What uncertainties
are there when determining
causes?
Eutrophication – Causes
There are over 600 small lakes in Northern
Ireland. Research on the sediments in six of
these has reconstructed a remarkable record
of changes in total phosphorus concentration
over the past 150 years that appears to be
typical of lakes in most developed countries.
The data shown above is for three lakes that
have no point sources (e.g. sewage) draining
into them, only the inputs from surrounding
agriculture. Each of the lakes shows an
increase in phosphorus, initially as a result of
land clearances (ploughing releases
phosphorus) and then a more pronounced
increase since the 1950s. This recent large
change is due to land drainage, fertiliser use
and the indirect impact of rural sanitation.
Impacts of these activities are accelerating the
demise of the lakes. Each of the lakes is
affected in a slightly different manner
according to the characteristics of the
particular drainage basin. Recent decreases
may reflect more prudent use of fertilisers.
source: redrawn from Anderson, J.
Freshwater. Biol. 38, 427-440, Fig. 9
Eutrophication – Causes
Causal chains
Immediate
causes
a. Enhanced
Nutrient
Inputs
b. Increased
recycling/
Mobilization
c. Trapping of
nutr ients (e.g.
River
impoundments)
Secondary causes
Tertiary causes
1. Use of fertilizers
in crop production
(a)
2. Use of animal
wastes in
agriculture (a)
3. Wastes from
animal production
(a)
4. Combustion of
fossil fuel (a)
5. Forestry/
Agricultural
practices
(a,b)
6. Phosphate
detergents (a)
7. Sewage
discharges (a)
8. Aquaculture (a)
9. Draining of
wetlands (a,b)
1. Enhanced food
production via use
of fertilizers (a)
2. Intensification
of animal
production (a)
3. Enhanced
energy demand
(a,c)
4. Urbanization (a)
5. Enhanced
aquacultural
production (a)
6. Land-use
changes (a,b)
7. Enhancement
of navigation,
dredging of
waterways and
harbours (b)
8. Enhancement
of water supplies
(b,c)
9. Hydrolelectric
power
development (b,c)
Policy
management
failur es
1. Lack of
internalization of
costs of
environmental
degradation
2. Inadequate
development
and/or
enforcement of
regulations
Enhanced nutrient input is the
Immediate Cause of eutrophication.
The Secondary Cause could be the use
of fertilisers in crop production. The
Tertiary Cause may be the
intensification of animal production. This
could be as a result of Policy
Management Failures. This stepwise
process is known as a Causal Chain.
A Causal Chain is a series of
statements that demonstrate and
summarise, in a stepwise manner, the
linkages between problems and their
underlying or 'root' causes.
Eutrophication – Causes
Uncertainties in determining causes
1. Relative importance of given sources in a given situation (temporal and spatial
variance)
2. Level/input rates of nutrients that give rise to exceptional algal blooms
3. Limitations of information on the incidence and bio-availability of forms of
nutrients (N & P)
4. Uncertainties in the precision of comparisons among options
5. Uncertainty in the prediction of the outcome of management interventions