BIOL4660_Lecture 3

VERTICAL GRADIENTS IN LAKES

  • Properties of Water and Lake Basins
    • Temperature, dissolved gases, dissolved solutes, light, heat

VERTICAL STRATIFICATION IN LAKES

  • Temperature-Density Relationship
    • Maximum water density occurs at 4 °C.
    • A small temperature change in warmer water leads to a larger density decrease compared to cooler water.

THERMAL GRADIENTS IN LAKES

  • Formation of a Thermal Gradient
    • Sun heats water, creating layers based on temperature:
    • Epilimnion: upper layer, warmer, less dense
    • Metalimnion: middle layer, temperature changes occur here
    • Hypolimnion: lower layer, cooler, more dense
    • Wind and convection cause mixing, especially in summer.
    • Stability of thermal stratification increases with larger temperature differences between layers, affecting mixing ability.

FACTORS INFLUENCING THERMAL STRATIFICATION AND MIXING

  • Key factors include:
    • Time of year
    • Lake depth
    • Topography
    • Solute concentration
    • Wind fetch (distance wind travels)

MIXING PATTERNS OF LAKES

  • Holomictic Lakes: Mix from surface to bottom at least once a year.
    • Types:
    • Monomictic: One mixing period yearly (cold or warm)
    • Dimictic: Two mixing periods yearly (temperate regions)
    • Polymictic: Shallow lakes mix continuously through ice-free periods.
    • Amictic: No mixing; perpetually frozen.
    • Meromictic: Deep layers do not mix due to strong density gradients created by solute concentration.

LAKE NYOS: A DANGEROUS MEROMICTIC LAKE

  • Located on the slope of a volcano, it holds a significant concentration of CO2 (over 1 liter per liter in hypolimnion) due to magma activity.
  • In 1986, a sudden CO2 release resulted in the death of 1,700 people and animals.

OXYGEN IN WATER: DIFFUSION EQUILIBRIUM

  • Henry’s Law:
    • Cs = KH imes P_t
    • Where:
    • $C_s$: gas dissolved (mmol/kg)
    • $K_H$: solubility coefficient
    • $P_t$: partial pressure of gas
    • Example:
    • Oxygen concentration at 20°C: 0.28 ext{ mmol/kg} = 9.03 ext{ mg/L}

OXYGEN GRADIENTS IN DIMICTIC LAKES

  • Profiles:
    • Positive Heterograde: O2 maximum at metalimnion due to photosynthetic activity.
    • Negative Heterograde: O2 minimum due to decomposition at the density gradient.
    • Orthograde: Oligotrophic lakes maintain high O2 in hypolimnion; lower O2 in warmer epilimnion.
    • Clinograde: Eutrophic lakes show reduced hypolimnetic O2 due to high decomposition rates.

BIOLOGICAL IMPACT OF ANOXIA

  • Winter Fish Kills: Occur in shallow lakes under ice cover due to oxygen depletion.
  • Process:
    1. Ice forms, preventing gas exchange with atmosphere.
    2. Vegetation decay consumes limited O2.
    3. Snow blocks sunlight, halting photosynthesis.
    4. Fish suffocate as O2 levels decline.

LIGHT GRADIENTS IN WATER

  • Light Attenuation Equation: Iz = I0 e^{-kz}
    • Where:
    • $I_z$: light intensity at depth $z$
    • $I_0$: surface light intensity
    • $k$: extinction coefficient for the water
  • As depth increases, clear lakes allow more light penetration, leading to higher temperatures in the epilimnion.

PHOTIC ZONE

  • Depth at 1% Light Penetration: Determines distribution of aquatic plants/algae.
  • Aphotic Zone: Below photic zone; sunlight is scarce.

COMPENSATION POINT OF LIGHT**

  • Depth where photosynthesis equals respiration, below which plant survival is compromised.

LAKE TYPES AND PHOTOSYNTHESIS**

  • Eutrophic Lakes: Most photosynthesis occurs in epilimnion.
  • Oligotrophic Lakes: Photosynthesis primarily below the thermocline, typically cooler.