1. CIV 913 Environmental Assessment and Sustainability
Eutrophication
Eutrophication of Freshwaters - Harper D
Freshwater Ecology - various
Limnology - various

2. Eutrophication Objectives Definition
Causes
Limnology and Lake Ecology
Effects
Control Strategy
Definition
The enrichment of waters by inorganic plant nutrients.

3. Eutrophication Cause - sources of Nitrogen and Phosphorous.
External
Municipal and Industrial wastewaters.
(Main source of Phosphorous)
Land run-off.
(Main source of nitrogen)
Atmospheric Deposition.
Internal
Nutrient regeneration from bottom sediments.
Groundwater seepage (sub-surface flow)
Historical incidence
recent (demographic growth - consumerism)

4. Eutrophication Limnology
Lake vs River
renewal time years vs days
Stratification in Lakes.
EPILIMNION
THERMOCLINE
HYPOLIMNION

5. Eutrophication Limnology
Stratification.
Formed by temperature gradient.
Most of the heat from light penetration is absorbed in top 1 or 2 metres.
Wind gives rise to mixing to form:
epilimnion at the top
hypolimnion at the bottom
a transitional zone, the metalimnion. in which a thermocline exists.
Temperature range may be:
20`C to 4`C in temperate lakes.
29`C to 25`C in tropical lakes (but can be equally stable stratification)

6. Eutrophication Limnology
Nutrients in lakes
Nitrogen
fixation, sediment denitrification
Internal Phosphorus Cycling
Forms of P
bound to Ferric hydroxides
bound to Calcite (CaCO3) or hydroxyappetite (Ca5OH(PO4)3
bound to clay
released by extreme pH, change in redox (anaerobic)

7. Eutrophication Limnology
Trophic classification of Lakes
Ultraoligotrophic
Oligotrophic
Mesotrophic
Eutrophic
Hypertrophic
Numerical Classification
Trophic State Index (TSI)
scale
by Secchi depth - 64m= 0; 32m= 10; 16m=20; etc
see OECD
categories

8. Eutrophication Limnology
OECD Trophic Categories
CATEGORY
Ultraoligotrophic
Oligotrophic
Mesotrophic
Eutrophic
Hypertrophic P 4
 10
100
Chl.
 1
 2.5
8 - 25
25
Max
Chl.
 2.5
 8
8 - 25
75
Secchi
(m)
12 6
6 - 3
 1.5
Secchi (min)
(m) 6 3
 0.7

9. Eutrophication Productivity
Rates of Primary Production in Lakes.
Oligotrophic Eutrophic
Natural Polluted
Mean rates in
growing season.
(mgC/m2/d)
Annual Rates
(mgC/m2/d)

10. Eutrophication Prediction of Water Treatment Plant Problems.
UK study in 1960s by Lund to predict effects:
Winter maximum PO4 > 5g/l
Winter maximum NO3 > 300 g/l
Produces Algae > 3000 cells/ml
Models in 1960’s by Vollenweider
where
TP is total phosphorus
L is surface loading of P
z is depth
p is flushing (renewal per year)
O is sedimentation rate coefficient of P

11. Eutrophication Predicting Permissible P Loading Using OECD Formulae.
Developed relationships between:
Chlorophyll A (annual mean and maximum),[Chl]
P inlet concentration [P]i and
hydraulic residence time Tw
[Chl]mean = [P]i / (1+(Tw)0.5) mg/m3

12. Eutrophication Effects. Freshwater. Fish diversity reduced.
Low/no DO in hypolimnion, hence reduced fauna and flora diversity.
Algal blooms and adverse aesthetics.
Algal blooms and water treatment difficulties.
affects drinking water quality and treatment costs.

13. Eutrophication Effects
Seawater.
Algal blooms.
Red tides (phaecocystis) and toxins affect coastal fisheries.
Corals.
Suffocated by algal sedimentation.
Macrophytes in shallow coastal waters.
Increased biomass (fish).

14. Eutrophication Adverse Effects of Algae in Water Treatment
physical blocking of filters
3000cells/ml detrimental
polysaccharides
chelate Fe and Al ions (enter treated water)
THM production
Taste and odour
toxins
animal infestation in distribution system
industrial
ion exchange poisoned
deposits block valves

15. Eutrophication Water Quality Objectives for Lakes.
Must take account of intended use.
Develop a nutrient load control strategy.
Using algal biomass as a trophic response indicator:
set target for mean algal biomass
set target for peak algal biomass
Determine phosphorous load to be removed.
Control point sources, then diffuse sources.

16. Eutrophication Typical Controls. Municipal sewage treatment.
chemical precipitation
biological removal
combinations.
Pre-reservoirs (>15 day HRT, aerobic)
Chemical precipitation in the lake.
High flow-through lake day HRT.
Hypolimnetic aeration.
Artificial water circulation.
Land use practices.
Removing polyphosphates from detergents
Flushing
Dredging
see UWWT Directive

17. Nutrient Removal - Standards -
UWWT Directive (1991):
Pop >10, N<15mg/l P<2mg/l
Pop >100,000 N<10mg/l P<1mg/l
or 80% removal of Total P
% removal of Total N
(The above applies to “sensitive waters”)