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

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

Selective Absorption of the Solar Spectrum by 1 meter of Pure Water UVIRVBGYOR % Absorbed Wave Length, nanometers 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 3

Selective Light Transparency in Different Lakes Tahoe, CA-NV Depth (m) Percent Incident Light 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 Long, MN Crystal, WI Montezuma Well, AZ Itasca, MN Little Triste, AZ Secchi Disk Saguaro, AZ Seneca, NY 4

Determination of Transparency Transparency –Secchi Disk (20cm diameter) - measures depth of 95% light absorption – range 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) 5

S Quantification of Transparency Extinction Coefficient Based on Bouguer’s (a.k.a. Lambert’s) & Beer’s laws, where: I o = intensity of entering light I d = 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) “Secchi Depth” k = 1.7 / Z sd 6

Turbidity 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) 7

Light is Attenuated More Rapidly in Eutrophic Lakes 8

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

Thermal Characteristics of Lakes 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: Temperature Depth 10

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

Stratification 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 12

A Thermally Stratified Lake Temperature ( 0 C) Depth (m) Epilimnion Metalimnion (thermocline) Hypolimnion Defined by at least  1 O C / m 13

Principles governing thermal stratification 1. Heat enters and leaves the lake (mostly) from the surface 2. Temperature affects water density 3. Warmer water has a much greater difference in density per degree change than cold water Thermal Characteristics of Lakes 14

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 (4 O C). 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

Lake Thermal Profile - Time and Depth 16

Diagrammatic Representation of Dimictic Mixing Regime 17

Wind Hypolimnion Metalimnion Epilimnion -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 25 o to 15 o vs. 15 o to 5 o Why? When is mixing most likely to occur? Effect of wind fetch? Circulation Patterns in a Stratified Lake 18

Annual Temperature Cycle of a Dimictic Lake Represented as Temperature-Depth Profiles 0O0O 4O4O 0O0O 4O4O 0O0O 4O4O 0O0O 4O4O Depth Temperature Summer Stratification Fall Overturn Winter Stratification Spring Overturn Ice 19

0O0O 4O4O 0O0O 4O4O 0O0O 4O4O 0O0O 4O4O Depth Temperature Summer Stratification FallWinter Mixing Spring 20 O Annual Temperature Cycle of a Warm Monomictic Lake 20

Importance of Heat and it’s Distribution High heat retention – due to specific heat of water Most biological processes have Q 10 values of 2-3 Influence on DO concentrations (Important habitat variable) Determines who, when, & where re: community composition and ecosystem processes 21

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