Water as an Environment Properties of Water Vertical Stratification in Lakes Water Movements Part 1

1. Water as an EnvironmentProperties of WaterVertical Stratification in LakesWater MovementsPart 1

2. Water is unique among all liquids in terms of • Density • Melting & Boiling Points • Viscosity • Specific Heat • Surface Tension • Solvent Properties • Absorption of Radiation

3. Structure of Water • H2O is a polar molecule • Oxygen end is relatively negative • Hydrogen end is relatively positive • H atoms form hydrogen bonds with the O atoms of adjacent water molecules • In liquid water, H-bonds break and form constantly, lasting only 10-12 seconds • In liquid water, on average 3 of four possible H-bonds are formed U. Texas

4. In ice, 4 of 4 possible H-bonds are formed and molecules are locked in a hexagonal crystalline lattice • Polarity and H bonds result in most of the other unique properties of water Ice lattice Madsci network www.makingthemodernworld.org.uk/

5. Density • For most substances, density increases as the liquid phase cools to become a solid • In water, density decreases as liquid water freezes to become a solid

6. Density (cont’d) • With decreasing temperature , the density of water increases to a maximum (1 gm/cm3) at about 4 oC. • Below 4 oC, density decreases slowly, then rapidly, as ice forms. Significance? • Lakes freeze from the top down instead of from the bottom up. This preserves life. Note that above 4 oC, the Temp vs. density curve is not linear. At 30 oC, a 1 degree increase in temperature causes a change in density 7x greater than a 1 degree increase at 5 oC. (See Dodson, Fig. 2.5) This has significance for tropical lakes.

7. Factors that affect density • Temperature • dissolved salts • particulates • dissolved gases - increase (CO2) or decrease (CH4) depending on partial molar volume • pressure - water not very compressible, but P results in Tmax so that at depths of (~1000 m) the temperature of the density maximum is reduced to ~2.91C.

8. A note on Units • In the metric system, water is used to bring together units of weight, length, volume, and energy. • This is extremely convenient for aquatic ecologists!

9. = 1g 1cm 1cm 1cm 1cm 1cm 1cm = 1ml Also Units • 1 gram is defined as the mass of 1 cubic centimeter of pure water (at 4oC)

10. This makes for easy conversions • For example: • 1 liter of water weighs 1 kilogram • 1 cubic meter of water contains 1000 liters and weighs 1 tonne (metric ton) • 1 liter of water weighs 1 million milligrams, therefore the units “mg/L” and “parts per million” (ppm) are equivalent. Also, a calorie is the energy needed to raise the temperature of 1 ml of water by 1 oC (at 15 oC)

11. Thus, a Snickers bar (273,000 cal) contains enough energy to heat 273,000 ml of water from 15 oC to 16 oC =

12. Properties of water (cont’d) • High boiling point means that water can exist as liquid over a wide range of temperatures • High Specific Heat means that water bodies tend to maintain a more constant temperature than their surroundings – useful in agriculture.

13. Properties of water (cont’d) • High Surface Tension allows a community of organisms to live on (and just under) the water surface. R. Suter Water strider Mosquito larvae Chemistryland.com

14. High absorption of infrared radiation causes surface of water to heat faster than deeper layers. Causes thermal layering. www.lsbu.ac.uk/water/vibrat.html

15. Solvent Properties. Many inorganic molecules (salts) dissolve in water as well as polar organic compounds (amino acids, sugars, alcohols) Wiley.com

16. Vertical Stratification in Lakes

17. Temperature Cycles & Lake Stratification • Most lakes mix during some seasons and become stratified during other seasons. • These terms refer to the vertical circulation of water: Mixing = circulation, Stratification = lack of mixing (development of layers) • The mixing pattern has a large effect on lake chemistry and the biota • Lakes have traditionally been classified according to their annual mixing pattern or mixing regime (amictic, monomictic, dimictic, etc.)

18. Temperate zone Dimictic Lake Stratified Mixing Stratified Mixing

19. Thermal zones in a stratifed lake Metalimnion

20. Allens Lake, MISept 7, 2007 Epilimnion Metalimnion Hypolimnion

21. Seasonal Cycle in a Temperate Dimictic Lake • After ice melts in spring, the lake is cold and isothermal (same temperature from top to bottom) Note, Z = depth 0 Z Zmax 4 Temperature

22. Seasonal Cycle in a Temperate Dimictic Lake • As air temperature and solar radiation increase, there may be a period of isothermal warming, where warmer surface waters are mixed downward by wind and wave energy 0 Z Zmax 4 Temperature

23. Seasonal Cycle in a Temperate Dimictic Lake • Eventually, the heating of the surface water will outpace the capacity of wind and waves to mix the heat downward 0 Z Zmax 4 Temperature

24. Seasonal Cycle in a Temperate Dimictic Lake • The warm surface layer (epilimnion) floats on the colder, denser layer (hypolimnion) 0 Z Zmax 4 Temperature

25. Seasonal Cycle in a Temperate Dimictic Lake

26. Seasonal Cycle in a Temperate Dimictic Lake • Over the summer the epilimnion may continue to warm, but the hypolimnion temperature will change very little 0 Z Zmax 4 Temperature

27. Seasonal Cycle in a Temperate Dimictic Lake • Wind from strong storms can have an effect on the thermal profile, causing “storm thermoclines”. 0 Z Zmax 4 Temperature

28. Seasonal Cycle in a Temperate Dimictic Lake • In the fall, the epilimnion begins to cool, and the process goes in reverse. The thermocline will deepen. 0 Z Nutrients mixed upwards Zmax 4 Temperature

29. Seasonal Cycle in a Temperate Dimictic Lake • Fall Overturn followed by isothermal cooling 0 Z Zmax 4 Temperature

30. Seasonal Cycle in a Temperate Dimictic Lake • Inverse stratification and ice formation 0 Z Zmax 4 Temperature

31. Seasonal Cycle in a Monomictic Lake

32. Mixing Regimes • Dimictic: Mixes in spring and fall (stratified in summer and winter) • Monomictic: • Cold – high latitudes or elevation, Mixes all spring summer and fall. Stratified under winter ice. • Warm – never freeze in winter. Mixes all fall, winter, spring. Stratified in the summer. (Great Lakes as well) • Amictic: never mix. Antarctic lakes always ice covered and inversely stratified • Polymictic: Mix many times annually. Usually shallow lakes

33. Winter Conditions:Temperature at which lakes freeze depends on wind energy 4 Temperature 0 2 10 Mean Fetch (km) Fig 11.11 Kalff

34. Effective Fetch Prevailing winds N

35. Thermal Bars • In spring, near-shore areas of lakes heat faster than offshore areas. Also, inflowing water from tributaries is usually warmer than winter lake temperatures. • These two factors lead to Thermal Bars, which are usually spring features of large lakes.

36. Lake Ontario Thermal Bar http://www.on.ec.gc.ca/solec/nearshore-water/paper/images/fig3.gif

37. Lake Michigan Thermal Bar 1982 1994 Beletsky and Schwab, 1991

38. Southern Lake Michigan Thermal Bar and Sediment Plume • Thermal bars have a large effect on the ecology of L. Michigan • DOC and nutrient-rich tributary water held near shore and Bottom sediments resuspended by storms cause • Resuspension of contaminants (PCBs, PAHs) • Blocks light (reducing phytoplankton productions) • Increased bacteria growth • Increased zooplankton growth (feeding on bacteria) • Ecology for entire year depends on extent of spring storms and thermal bar

39. Stability of Stratification • Thermal Stability refers to the physical energy (wind mixing) required to completely destratify a lake. • Stability is related to the difference in density between the eplilimnion and hypolimnion. • Stability is also related to the depth of the thermocline 0 Ze Z Ze Zmax Temperature

40. Thermocline Depth • Thermocline Depth of lakes is largely influenced by regional wind strength. • Lakes located in windy regions (New Zealand, Scotland, Argentina, etc) will have a greater average depth of themocline than otherwise similar lakes in less windy regions. • Within a region (similar wind strength) thermocline depth is influenced by lake morphometry, fetch, and water transparency Log Thermocline depth (m) Secchi Depth (m)

41. Effects of Zebra Mussels on Thermocline depth • Small resevoir in Ohio was invaded by zebra mussels in 1993 • ZM are filter-feeders, consumed phytoplankton, reducing algal biomass by 1995

42. Effects of Zebra Mussels on Thermocline depth • Reduced algal biomass resulted in greater transparency (secchi depth)

43. Increased transparency permitted deeper penetration of light energy, resulting in a deeper thermocline • Zebra mussels as “ecosystem engineers” 2003 light 2005