1. Factors Determining the Distribution of Freshwater Biota: 2
ES3053 Freshwater Hydrobiology
Factors Determining the Distribution of
Freshwater Biota: 2
Professor Nick Gray
Trinity Centre for the Environment
Trinity College Dublin
© Tigroney Press

2. Dissolved Oxygen and Temperature
Water at normal temperature holds very little oxygen compared to the atmosphere.
Gas molecules diffuse from an area of high concentration to an area of low concentration.
In the same way oxygen diffuses across the air-water interface into water where it becomes dissolved.

3. The solubility of oxygen depends on:
At the same time oxygen is diffusing in the opposite direction.
When the volume of oxygen diffusing in either direction is equal the water is classed as being in equilibrium and so saturated with oxygen.
The solubility of oxygen depends on:
Temperature
Pressure
Concentration of dissolved minerals

4. Freshwaters at one atmosphere pressure at 20oC contains 9
Freshwaters at one atmosphere pressure at 20oC contains 9.08g of O2 per m-3.
As the temperature increases so the saturation concentration decreases
Temperature Saturation concentration
4oC O2 mg l-1
14oC O2 mg l-1
24oC O2 mg l-1

5. Variation in oxygen saturation concentration with temperature in freshwater at one atmosphere pressure

7. Increasing concentrations of dissolved minerals also lowers saturation concentration.
So seawater always has a lower saturation concentration of dissolved oxygen compared to freshwaters at any given temperature.
For example:
At 15oC Freshwater 10 mg l-1
Marine mg l-1

8. Rate of diffusion is directly proportional to the oxygen deficit
How quickly is oxygen replaced in water?
Depends on oxygen concentration in the water in relation to the saturation concentration (i.e. the oxygen deficit)
Rate of diffusion is directly proportional to the oxygen deficit
So the larger the deficit the faster the rate of oxygen transfer.
When water is saturated with oxygen and is equilibrated then it is 100% saturated regardless of temperature.
So in monitoring percentage saturation is used.

9. No Mixing: Oxygen gradient set up which slows transfer rate.
Turbulence: Breaks up gradient, maintains maximum oxygen deficit, rapid transfer rate
Overall amount transferred: Depends on surface area to volume ratio. So a shallow wide river will reoxygenate faster than a deep narrow river with the same deficit.

10. Supersaturation?
Oxygen concentration can become supersaturated up to 200% under conditions of:
Agitation: After waterfalls, weirs. Permanent area of supersaturation but quickly returns to 100% downstream.
Eutrophication: Bright sunlight causes oxygen to be released due to photosynthesis during the day. Leads to diurnal variation in oxygen concentration.
Animals, plants and micro-organisms are all using oxygen and so contributing to the oxygen deficit.

11. Difference in oxygen concentration (y-axis) at different rates of reaeration

14. Solids in Water
Type and concentration of suspended solids controls transparency and turbidity in water which affects insolation and overall productivity.
Suspended solids comprised of silt, clay and fine inorganic/organic particles, organic matter, plankton and micro-organisms
Particles range 10 nm to 0.1 mm diameter
Measured as fraction of suspended solids retained on a filter paper of pore size 0.45 ųm.

15. Solids in Water
Particles < 1 ųm do not settle unaided – colloidal fraction.
Turbidity caused by scattering and absorption of incident light by particles. Measured by nephelometry (light scattering) - NTU are nephelometric turbidity units.
On storage flocculation occurs, settlement of particles is enhanced, precipitation if pH shifts. So turbidity is measured in the field.
Transparency measured using a Secchi disc.

16. Method: http://water.epa.gov/type/rsl/monitoring/vms58.cfm

17. Erosion, transportation and deposition of particulate matter is a function of:
Shear stress
Turbulence
Particle size
Particle density
At high flows smallest particles suspended at surface larger particles near bottom of water column: clay-silt-sand

20. Sediment in lakes arise from a number of different sources:
Riverine inputs
Shoreline erosion
Lake bed erosion
Airborne inputs
Autochthonous material

21. Ions and cations are primarily from weathered rocks and soils
Dissolved minerals
Ions and cations are primarily from weathered rocks and soils
Atmospheric deposition and activities of man also important (e.g. agriculture, drainage, urban runoff, waste disposal etc.)
These dissolved ions determine the chemical properties of water such as conductivity, acidity, alkalinity and hardness. Also buffering of water.
In turn these affect the physical properties such as colour, taste and odour, as well as the ability to sustain aquatic life.

22. Conductivity Measure of water to conduct electric current
Linked to concentration of mineral salts in solution
Measured using electrode in ųScm-1
TDS (mgl-1) = conductance x factor
Factor is between (increasing with sulphate). Once established for particular river very stable. Salinity (ppm) = ųScm-1 x 1.56
Natural freshwaters ųScm-1 . Excess values indicate pollution or saline intrusion.

23. pH
Negative of the logarithm to the base 10 of the hydrogen ion concentration.
pH5 is 1000 times more acidic than pH8
In unpolluted water pH is primarily controlled by the balance between CO2, CO3 and HCO3 as well as any natural organic acids present

24. Acidity-Alkalinity
Measures the acid neutralizing capacity of waters. If no buffering capacity available then related to pH. But normal waters contain range of acids and bases, so need to measure ANC.
Acidity controlled by strong mineral acids, weak acids (carbonic, humic, fulvic), hydrolyzing salts of metals (e.g. Fe and Al).
Measured by titration with strong base up to pH 4 (free acidity) and pH 8.3 (total acidity).
Alkalinity controlled by sum of titratable bases (e.g. carbonate, bicarbonate, hydoxides).
Determined by titration to lower pH to 8.3 (free alkalinity) and pH 4.0 (total alkalinity).

25. Acid Neutralizing Capacity (ANC)
ANC =  base cations   strong acid ions (eq l-1)
ANC = [Ca2+ + Mg2+ + Na+ + K+]  [SO42- + NO3- + Cl-]
Lake Acidification Classification
ANC (eq l-1) Sensitivity
0 Acidified
<1 – 40 Very sensitive
>40 – 200 Sensitive
>200 Insensitive

26. Buffering of surface waters
Carbon dioxide is highly soluble and is closely linked to the chemical processes that determine acidity and alkalinity. It dissolves in water to form carbonic acid.
CO2 + H2O H2CO3
Carbonic acid (pH 5.6) dissociates readily to produce free hydrogen ions and bicarbonate ions
H2CO H+ + HCO3-
Acidity of water is determined by the activity of the hydrogen ions.

27. Bicarbonate ions further dissociate:
HCO H+ + CO32-
These ions can further react with water to produce hydroxide ions.
HCO3- + H2O H2CO3 + OH-
CO32- + H2O HCO32- + OH-
In clean water pH is controlled by the balance between CO2, HCO3- and CO32- as well as any organic acids present.

29. Hardness Varies from place to place reflecting geology
Surface waters generally softer than ground waters
Expressed in mg CaCO3 l-1
Caused by divalent metal cations only

30. Principal cations causing hardness
Ca HCO3
Mg SO4
Sr Cl
Fe NO3
Mn SiO3
Total hardness taken to be Ca + Mg
Usually minor component and ignored

31. Degree of Hardness

33. Effects of Hardness Profound effect on fauna present
Osmotic regulatory problems in very soft water making fauna more susceptible to contaminants
Some species are more common in, or restricted to, hard waters (e.g. molluscs, flatworms, crustaceans, caddis and leeches)
Many insects are indifferent to hardness thus found in greatest abundance in soft water
Public heath concerns over increased cardiovascular disease in soft water

34. Hardness calculated from ionic analysis
Hardness = ion (mg l-1) x Equiv. weight of CaCO3/Equiv. weight of ion
If water sample contains 18 mg Mg l-1; 75 mg Ca l-1
Hardness = [18 x 50/12.16] + [75 x 50/20.14] =
= 260 mg CaCO3 L-1

35. Significant physical, chemical and biological differences between soft and hard water rivers.
Hard waters
Higher Ca and pH
Rise as springs
Springs in bed
Constant clarity, discharge rate , and temperature throughout year
Soft waters
Lower Ca and pH
Surface runoff from mountains
Usually flashy with sudden droughts and floods
Different fauna and flora due to physico-chemical nature
Impoverished fauna with low productivity
High concentration of humic material – colour
Usually turbid, high solids during high flows

37. References: Further information in Chapter 2 of the course text.
Monitoring
Water quality data
Further information in Chapter 2 of the course text.