Water, Light, and Lakes Joe Conroy 31 March 2004
Goals • To explore the properties of water germane to Limnology • To appreciate the importance of lake water in the global water economy • To understand the role of light in providing energy to aquatic ecosystems Overreaching Goal: Explore Ecological Consequences of Topics
Goals • To explore the properties of water germane to Limnology • To appreciate the importance of lake water in the global water economy • To understand the role of light in providing energy to aquatic ecosystems Overreaching Goal: Explore Ecological Consequences of Topics What does this mean for. . .?
Properties of Water • “fulcrum of biochemical metabolism” • Chemistry and Physics = Boundary Conditions • Hydrogen Bonding • Ionic and Covalent character • What properties are important? • Specific Heat • Density • Viscosity • Surface Tension Photo Credit: M. Chaplin For more info and good pics: http://www.lsbu.ac.uk/water/
Specific Heat • “Heatability” or thermal capacity • Extremely high for water • Due to H-bonding • 1.0cal g-1°C-1 for liquid water, 0.5 for ice @ 0°C • Heat of Fusion = 79.72cal g-1 • Heat of Evaporation = 540cal g-1 Einput melts ice -or- E loss forms ice
Specific Heat Ecological Consequences? • Stability of aquatic environments • Temperature fluctuations occur seasonally, not daily
Density • Molecular properties w/ interesting results • Vibrational energy • Bond length • Maximum Density @ 4°C • Ice has open spaces = less dense • Warmer water has greater vibrations = less dense • Density difference per degree lowering • Changes at different temperatures
Density Figure 2-3, p. 12
Density Figure 2-3, p. 12
Density Ecological Consequences? • Work is needed to mix liquids w/ different ρ’s When is less work required to mix water? Winter? Summer? • How does density increase?
Viscosity • Resistance to flow (think honey) • w/ temperature (flows more easily) • Implications for passive organisms (plankton) • Determined by interactions w/ density, exposure area, speed Stokes’ Law
Viscosity Ecological Consequences? • Sinking rates of aquatic organisms • Succession of organisms?
Surface Tension • At air-water interface, inward bonding causes water to adhere to itself = Tension
Surface Tension Table 2-2, p. 13
Surface Tension Ecological Consequences? • Habitat for Neuston at air-water interface
Properties of Water - Bibliography • Wetzel – Chapter 2, pp. 8-14 • Class Notes – pp. 16, 20
Sources of inflow Precipitation Surface water from watershed Groundwater seepage Spring Sources of outflow Drainage Seepage Evaporation Evapotranspiration Water Economy LAKE = INFLOW – OUTFLOW Closed Lake: no outlet -versus- Open Lake: outlet or seepage
Water Economy Watershed = drainage basin or catchment area Lake Erie & Lake St. Clair Watersheds Photo Credit: Dr. R.J. Schaetzl, MSU Geology 333
Where is all the Water? Figure 4-2, p. 45
Water Economy Ecological Consequences? • Lake/River water = 0.017% of ocean water • Lake/River water = 0.9% of ice But. . . extremely important for aquatic organisms and humans!
Aside: Storage vs. Turnover • Important concepts to distinguish • Components that store more turnover slowly • Ocean (water) • Fish (biomass) • Components that store less turnover rapidly • Atmosphere (water) • Plankton (biomass)
Water Economy - Bibliography • Wetzel – Chapter 4, pp. 43-48
Light in Lakes Light = Solar Radiation = Energy (Heat) • Lake optical properties affect light penetration • Properties of Energy • Can do work • Neither created or destroyed • Able to be transformed from one form to another • Radiant Energy Potential Energy through biochemical pathways of organisms
Electromagnetic Spectrum • Sun’s radiation is of various wavelengths Figure 5-1, p. 50
Energy and Wavelength • Where: h = Plank’s constant (6.63x10-34J s) c = speed of light (3x108 m s-1) λ = wavelength of light ν = frequency of light • wavelength frequency E • wavelength frequency E
Net Radiation • Not all of the sun’s radiation hits lake surfaces • 17% absorbed by the atmosphere • Ozone, water vapor, CO2, O2 • Light impinging on lakes • Depends on latitude and season • 5-10% reflected back from the lake surface • 5% scattered once it penetrates a lake • Refraction creates a longer light path length in lakes • Equation on Wetzel, p. 56
Light Attenuation and Absorption • Attenuation = decrease of radiant energy with depth by both scattering and absorption • Absorption = decrease of light energy with depth due to transformation to heat • Important in lakes b/c light at depth determines E input
Measuring Light in Lakes • Percent Transmission • Compares light at depth to light at surface • %Transmission = 100 * ((I0 – Iz)/I0) where: I0 = surface light Iz = light at depth z • Intensity of Radiation at depth • Exponential decrease in light intensity Iz = I0*e-ηz where: η = extinction coefficient z = depth
Extinction Coefficient • Measures how quickly light is attenuated • Composite of factors: ηt = ηw + ηp + ηc where: ηt = total extinction coefficient ηw = water extinction coefficient ηp = particulate extinction coefficient ηc = colored compounds extinction coeff.
Absorption and Extinction Coefficients Table 5-2, p. 57
Absorption and Extinction Coefficients Figure 5-10, p. 60
Special Light Measures • Photosynthetically Active Radiation (PAR) • Portion of the EM spectrum available for Ps • Approximately 400-700nm • Measured using a PAR meter • Compensation Depth • Water depth at which photosynthesis and respiration are just balanced • Approximately 1% Surface Light • Measured using a PAR meter or Secchi Disk
Light Penetration, Secchi Depth, and Productivity Secchi Disk
Light Penetration, Secchi Depth, and Productivity Figure 5-11, p. 61
Light Penetration, Secchi Depth, and Productivity Figure 5-17, p. 66
Light and Lakes Ecological Consequences? • Light provides most energy to lakes • Lake position (globally), watershed, and geology impact biota
Light and Lakes - Bibliography • Wetzel – Chapter 5, pp. 49-69 • Class Notes – pp. 16, 20-23
QUESTIONS?
