Aquatic Ecology Freshwater - Part 2

1. Aquatic Ecology Freshwater - Part 2 Prof. Dr. N. De Pauw AECO Laboratory of Environmental Toxicology and Aquatic Ecology Aquatic Ecology

2. Course Contents • Place of limnology in natural sciences • Historical development of limnology • The water cycle, distribution, age and genesis of inland waters • Structure and physical properties of water • Physical relationships in natural water bodies • Communities of living organisms in natural waters • Materials budget in natural waters I • (= gasses, solid and dissolved substances, importance of sediments) • 8. Materials budget in natural waters II • (= production, consumption, decomposition)

3. 4. Structure and physical properties of water Contents 4.1. Properties of water 4.2. Water molecules and aggregate formation 4.3. Density en density anomaly of water 4.4. Adhesion and cohesion 4.5. Surface tension 4.6. Viscosity and kinematic viscosity 4.7. Thermal properties of water

4. 4.1. Properties of water • Water exhibits some unique properties which derive from : • the structure of the water molecule • the tendency to form aggregates • The specific properties affect the existence of single organisms and the biocoenoses. • The life cycle of organisms in natural waters is influenced by: • the density • the density anomaly • the thermal properties of water

6. 4.2. Water molecules and aggregate formation Water molecules are strong dipoles owing to spatial arrangements of atoms. Without pronounced dipole character water would not be liquid! Pronounced dipole moment of molecule (= charge x distance)

7. Water moleculesexperience strong attraction to each other and aggregate to form : hyper-molecular linear, areal and spherical clusters Elektrostatic attraction of 2 molecules leads to hydrogen bridge formation Bonding energy of hydrogen bridges is smaller than energy needed for the formation of covalent bonds WATER = ACTIVE CHEMICAL COMPOUND Water remains liquid at normal temperatures despite its low molecular weight. In normal ice a rigid crystalline structure is formed.

11. 4.3. Density of water • Dependent on: • DISSOLVED SOLIDS : increase dissolved solids results in increase density • Continental waters : dissolved solids < 1 g/L • Exceptions : mineral waters, salt lakes, deep water of lakes • Chemical density differences in lakes result in stable stratification • PRESSURE : influence small • Only effect in deep lakes • TEMPERATURE DIFFERENCES : important • At 1 atm. greatest density of water at 4 °C • Colder and warmer water are lighter: have a distinct bouyancy relative • to water at 4°C. • Density differences increase with increasing temperature = important • for stability of thermally stratified water bodies

17. 4.3. Density anomaly of water • Density of water is maximal at temperatures higher than freezing point =consequence of aggregate formation of hydrogen bridging of water molecules • Importance of the density anomaly: • Bottom water of deep lakes cannot be colder than water at its • density maximum, which is about 4°C • Layer of ice protects the deeper water from freezing • = protection of organisms • Maximum density also dependent on : • Salt content of water • Seawater has a maximum density at -3.5 °C, and freezes at -1.91 °C • Pressure • Increase in pressure (e.g. a depth of 100 m) : temperature of maximum • density decreases with ca. 0.1 °C  temperature bottom water of deep • lake << 4 °C

18. 4.4. Adhesion and Cohesion • Behaviour of water molecules relative to solid surface : has far reaching biological consequences • ifcohesion < adhesion  boundery surface becomeshydrophylic= wetted • ifcohesion > adhesion  boundary surface becomeshydrophobic= water repellent

19. 4.4. Adhesion and Cohesion Hydrophobic properties of body surface • Essential for water-based animals taking up atmospheric oxygen at the surface : need dry connection (e.g. many insects, water spider) • Reduced water uptake in body of many water-living organisms • Less colonisation by surface attached organisms • Osmoregulatory function(insects) • Hydrophilic properties of body surface • For animals breathing under water: gills or entire body surface • hydrophilic • Reduced friction when moving

20. 4.5. Surface tension • Surface tension of water varies with: • Temperature • Dissolved solids content • Temperature dependence biologically unimportant • Presence of tensio-active substances (e.g. detergents)  pronounced decrease of surface tension • Reduction in surface tension ‘oil patches’ • = areas of smooth water which form at the surface of a water mass • Surface tension has to be allowed for by all those organisms which • inhabit the boundery zone between water and atmosphere : • NEUSTON = Micro-organisms • + • PLEUSTON = Larger organisms

21. Surface tension : molecules in the surface layer only interact with molecules below and besides them  molecules in the liquid phase however interact in all directions, the energy is thus more dispersed.

22. 4.6. Viscosity and kinematic viscosity • Viscosity= resistance of the water during free flow or other changes in physical form • = Force needed to move a mass of 1 kg over 1m in 1 sec. in the medium (Pa s of N/m2,s) • Dependent on :salt content  negligible • temperature important • Temperature influences swimming and floating • Water of 25 °C has a viscosity only half of that of water at 0 °C • Kinematic viscosity = • Frictional forces + pondering effects dependent on : • Velocity of motion • Shape of body • Viscosity of water • Example : at 25 °C organisms sink 2 x faster then at 0 °C Viscosity density Biological meaning not fully understood

24. 4.7. Thermal properties of water • Specific heat capacity exceptionally high :4.18 kJ kg-1 °C-1 • Specific heat capacity of ice : 2.04 kJ kg-1 °C-1 • Specific heat capacity of air : 1.00 kJ kg-1 °C-1 • Means that a heated water body stores a large quantity of heat ! • Thermal capacity: • = Ratio of supplied heat and the resulting temperature increase • Quantity of heat needed to evaporate 1 kg of water is exceptionally high ! • water has a high thermal buffer capacity • water has a low thermal conductivity = amount of heat passing in 1 second between opposite faces of a cube of 1 cm edge when a temperature difference of 1°C is maintained between 2 faces Heat transport in water bodies takes place as a result of the movement of the water itself.

25. 4.8. Dielectric constant of water • The dielectric constant of a substance indicates how much electrostatic energy can be stocked per unit volume of a substance when a tension of 1 volt is applied • Water has a very high dielectric constant • (=80,08 at 20°C) • = expressionof dissociating action of water on • heteropolar bonds in an electrolyte • (two bodies with opposite loadings attract each other • with a smaller force then in a vacuum. Upon this • principle the possibility is based that in water anions • and kations can freely exist side by side) • The dielectric constant decreases when the temperature • increases

26. Course Contents • Place of limnology in natural sciences • Historical development of limnology • The water cycle, distribution, age and genesis of inland waters • Structure and physical properties of water • Physical relationships in natural water bodies • Communities of living organisms in natural waters • Materials budget in natural waters I • (= gasses, solid and dissolved substances, importance of sediments) • 8. Materials budget in natural waters II • (= production, consumption, decomposition)

27. 5. Physical relationships in natural water bodies • Thephysical properties of water manifest themselves in ways typical of either stagnant or running water • Water bodies of some depth are characterized by • vertical gradientsof : • temperature • pressure • light • chemical substances • Physical factors of basic importance are : • Radiationintensity • Heat balance • Motion and exchange processes dictate biological structure affect materials balance

28. 5. Physical relationships in natural water bodies 5.1. Radiation climate in a water body 5.2. Heat budget of water bodies 5.3. Water movement and water exchange in natural waters

29. 5.1. Radiation climate in a water body • Global radiation received on surface of a water body = short wave radiation: 300–3000 nm • Radiation consists of : • ultra-violet radiation : 300 – 380 nm • visible light : 380 – 750 nm • infra-red radiation : 750 – 3000 nm • Global radiation made up by : • direct sunlight • diffuse radiation • Light impinging on the water surface subjected to 3 processes : • 1. reflection • 2. scattering within water mass • 3. absorption within water mass

33. Reflection of light • Reflection of light at the water surface(Fresnel’s formula) : • Amountof reflected light depends on position of the sun • and varies according to time of the day and season I = angle of incidence r = angle of refraction

34. Light penetrating the water • Selective scattering • Absorption • Extinction = amount of radiation retained • Transmission = amount of radiation which emerges during passage through water Iz = radiation intensity at depth Z Io = incident radiation intensity  = extinction coefficient

39. Expressing radiation intensity • Einsteins (1 E = 6.02 x 1023 fotonen) • J / cm² • W / m² • Amount of scattered energy in water depends on number of suspended • particles in the water • Example : in lake Lutz 1 % • in lake Leopoldstein 9 % • Uncoloured clean water appears blue in thick layers because short- • wave radiation most intenselyscattered • Scattered radiation biologically important: use of visible light by • photo-autotrophic organisms • Extinction and transmission dependent on wave-length of incident • radiation • Optical properties of water affected by: • dissolved inorganic and organic substances and particles of all kinds • Absorption increases with content of brownish-yellow humic substances

42. Radiation climate in running waters • Same factors of importance as in lakes • Differences : • reflection at surface is higher • in a clear mountain stream light penetrates as far as • the streambed • in turbid rivers light can be completely absorbed • organic effluents reduce light transmission

43. Colour of water • Dependent on optical characteristics which are influenced by: • Selective transmission of light • Content of suspended matter and • dissolved substances • Colour of surroundings - reflection • Blue is the most strongly transmitted colour • Phytoplanktoncan impart a green, brown, • green-blue and even red colour • Blue = colour of ‘desert’ water • Humic acidsgive a yellow-brown colour • Wastewater can can lead to a grey or black • colour • Mud or siltcan colour the water yellow, brown • or red

44. The clear, unproductive waters of a high-altitude lake in the Sierra Nevada, California

45. The Rio Negro flows into the main branch of the Amazone which is heavily loaded with silt. The water of the tributary is darkly coloured by humic substances present in the soil of its forrested catchment area.

46. ‘Red tide’

47. Snow and ice • Ice has the same optical properties as • distilled water • Layer of clear ice : no effect on input of • radiation energy • Snow : prevents passage of radiation in • water (e.g. 20 cm snow = 99 % reduction)

49. Ice coveredwith snow hardly transmits any light

50. 5. Physical relationships in natural water bodies 5.1. Radiation climate in a water body 5.2. Heat budget of water bodies 5.3. Water movement and water exchange in natural waters