Readings: Snucins & Gunn 2000

1. Lec 2: Light and Heat I. Light and Transparency II. Stratification:Vertical Temp. Gradients III. Circulation Readings: Snucins & Gunn 2000 1

2. All-important influence on in-lake conditions Solar Spectrum: Differing wavelengths and intensities Solar Radiation • Variation in the solar spectrum • PAR: PhotosyntheticallyActive Radiation • Infrared: Main heat source PAR 2

3. 50% of remaining light is absorbed for each additional meter, yet: 30% blue light remains after 70m 6% yellow light remains after 70% 0% orange light remains after 17 m 0% of red light remains after 4 m Selective Absorption of the Solar Spectrum by 1 meter of Pure Water UV V B G Y O R IR 100 50 % Absorbed 0 300 400 500 600 700 3 Wave Length, nanometers

4. Transparency of water depends on: Wave length (water is differential in its absorption of certain wave lengths) Suspended materials Dissolved materials Different lakes tend to have different light absorption characteristics Percent Incident Light 0.1 0.5 1.0 5 10 50 100 Selective Light Transparency in Different Lakes Little Triste, AZ 1 Saguaro, AZ Itasca, MN 2 Montezuma Well, AZ Depth (m) 3 Seneca, NY 4 Crystal, WI 5 Long, MN Tahoe, CA-NV Secchi Disk 4

5. Transparency Secchi Disk (20cm diameter) - measures depth of 95% light absorption – range <5 cm - >40 m (Crater Lake, Oregon, has the greatest transparency of any North American Lake) Light meter typically measures in photons or calories (lakes have light profiles just as they have oxygen & temperature profiles) Determination of Transparency 5

6. Based on Bouguer’s (a.k.a. Lambert’s) & Beer’s laws, where:Io = intensity of entering light Id = intensity of light at depth Z e = base of natural logarithms (approx. 2.7) k = extinction coefficient The “Extinction Coefficient” (k) is the proportion of the original light absorbed at a depth The proportion of light transmitted through a depth is called the “Transmission Coefficient” (k is more commonly used) 6 Quantification of Transparency Extinction Coefficient S “Secchi Depth” k = 1.7 / Zsd

7. Turbidity is a measure of water’s cloudiness Caused by suspended materials in water Often varies seasonally, affected by: Water movements Stream discharge Plankton populations Settling time for suspended materials vary: sand: 10 cm/second (still water) colloids: <0.5 cm/year (still water) Turbidity 7

8. Light is Attenuated More Rapidly in Eutrophic Lakes 8

9. Photic Zone • Affected by water clarity • Important for system metabolism • Important habitat determinant -Heat -Dissolved oxygen (DO) Compensation Depth Possible to calculate from Secchi Depth? 9 9

10. Light and heat represent a continuum with wave lengths e.g. >700 nm (infrared) = heat Water selectively absorbs in the infrared at 820 nm 91% absorbed within the 1st meter 99% absorbed within the 2nd meter Based on the absorption of light, you would expect the following temperature profile of a body of water at uniform temperature exposed to the sun: Thermal Characteristics of Lakes Temperature Depth 10 10

11. Lakes generally do not show heat distributions that directly reflect the relative absorption of light with depth Many lakes (esp. deep)stratify during part of the year Thermal Characteristics of Lakes • This results in a characteristic • thermal profile: • Epilimnion (upper water) • Metalimnion (middle water) • Hypolimnion (lower water) 11

12. Layering based on differences in density (temperature or salinity) Stratification alters biogeochemistry and ecology Lake with all same temperature called isothermal Thermal stratification into three layers Stratification 12

13. A Thermally Stratified Lake 0 Temperature ( C) 0 2 4 6 8 10 12 14 16 18 0 Epilimnion 5 Defined by at least 1OC / m Metalimnion (thermocline) 10 Depth (m) 15 Hypolimnion 20 25 13

14. Thermal Characteristics of Lakes • Principles governing thermal stratification1. Heat enters and leaves the lake (mostly) from the surface2. Temperature affects water density3. Warmer water has a much greater difference in density per degree change than cold water 14

15. Amictic - no mixing. Applies only to lakes that permanently are ice covered. Arctic climates only Cold Monomictic - Temperature never exceeds the temperature of maximum density (4OC). Ice covered from late fall through late spring, mixes all summer. In very cold climates. Dimictic - Spring and Fall mixing periods. Lake surface freezes in winter, lake is thermally stratified in summer Warm Monomictic - Lake never freezes. Mixes over winter. Stratified from early spring through late fall Oligomictic - Circulates irregularly. Mostly in the tropics Polymictic - Continually circulates at low temperatures Lakes at high elevations near the equator Classification of Lake Mixing Regimes Function of latitude, elevation, morphometry 15

16. Lake Thermal Profile - Time and Depth 16

17. 17 Diagrammatic Representation of Dimictic Mixing Regime

18. Wind Epilimnion Metalimnion Hypolimnion Circulation Patterns in a Stratified Lake 18 -Wind mixes surface heat down -Density differences cause resistance to vertical mixing -The work need to mix depends on the different desities of the strata -However, much more work is needed to mix 25o to 15o vs. 15o to 5o Why? When is mixing most likely to occur? Effect of wind fetch?

19. Annual Temperature Cycle of a Dimictic Lake Represented as Temperature-Depth Profiles Temperature 0O 4O 0O 4O 0O 4O 0O 4O Ice Depth Summer Stratification Fall Overturn Winter Stratification Spring Overturn 19

20. Annual Temperature Cycle of a Warm Monomictic Lake Temperature 0O 4O 20O 0O 4O 20O 0O 4O 20O 0O 4O 20O Depth Summer Stratification Fall Winter Mixing Spring 20

21. High heat retention – due to specific heat of water Most biological processes have Q10 values of 2-3 Influence on DO concentrations (Important habitat variable) Determines who, when, & where re: community composition and ecosystem processes Importance of Heat and it’s Distribution 21

22. Horizontal Lake Zones and Biota psammon macrophytes benthos Shallow & deep water emergents Floating plants Submerged plants Sublittoral zone Profundal zone 22