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

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.

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

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 13.1 O2 mg l-1 14oC 10.3 O2 mg l-1 24oC 8.4 O2 mg l-1

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

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 8 mg l-1

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.

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.

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.

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

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.

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.

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

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

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

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.

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 0.55-0.75 (increasing with sulphate). Once established for particular river very stable. Salinity (ppm) = ųScm-1 x 1.56 Natural freshwaters 10-1000 ųScm-1 . Excess values indicate pollution or saline intrusion.

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

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).

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

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 H2CO3 H+ + HCO3- Acidity of water is determined by the activity of the hydrogen ions.

Bicarbonate ions further dissociate: HCO3- 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.

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

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

Degree of Hardness

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

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] = 74 + 186 = 260 mg CaCO3 L-1

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

References: Further information in Chapter 2 of the course text. Monitoring http://water.epa.gov/type/rsl/monitoring/ Water quality data http://www.epa.ie/water/wm/rivers/results/#.VLPCmdKsXfA http://water.epa.gov/type/rsl/monitoring/riverssurvey/ Further information in Chapter 2 of the course text.