ES 105 2025 Lecture material week 2

Properties of Water

Importance of Light in Water

  • Required for photosynthesis

  • Influences organism activity (serves as a sensory cue)

  • Heats aquatic systems, leading to stratification in lakes

Electromagnetic Spectrum

  • Photosynthetically Active Radiation (PAR)

Behaviors of Light in Water

Types of Light Interaction

  • Reflection: Small fraction of visible light that is reflected based on the sun's angle and surface waves.

  • Scattering: Some light is lost back to the atmosphere due to suspended particles.

  • Absorption: Transformation of light into heat, influenced by:

    • Water itself

    • Dissolved organic materials

    • Seston (suspended organic particles)

    • Sediment

    • Plankton (living organisms)

  • Transmission: Spread of light with distance

Fate of Light Striking Water

  • Photic Zone: Area with enough light for photosynthesis.

  • Aphotic Zone: Area with insufficient light for photosynthesis.

Light Penetration in Lakes

Light Attenuation

  • Describes how light is absorbed with depth.

  • Example light attenuation coefficients:

    • Oligotrophic Lake: L. Tahoe = 0.002 m-1 (99% reaches 5m)

    • Mesotrophic Lake: Lawrence L. = 0.39 m-1 (14% reaches 5m)

    • Eutrophic Lake: Wintergreen L. = 1.00 m-1 (0.7% reaches 5m)

  • Chlorophyll a: Absorbs blue and red light.

Heat Dynamics in Lakes

  • Light absorption leads to conversion to heat.

  • High specific heat of water allows for dissipation of absorbed light as heat, primarily warming lakes.

  • Water serves as a more important heat source than air or stream inputs.

Limnology

Definition of Limnology

  • The study of structural and functional interrelationships of organisms in inland waters and how they are affected by dynamic physical, chemical, and biotic environments. (Wetzel, 2001)

Types of Inland Waters

  • Includes lakes, streams, rivers, wetlands, marshes, and groundwater.

Watershed

Definition of a Watershed

  • An area of land where all water that falls in it drains to a common outlet (USGS).

Stream and River Dynamics

Importance of River Flow

  • Streams transport materials; estimated at 15-20 billion tons of materials carried to oceans annually.

Stream Order

  • Developed by Strahler (1940s) to assess position along water flow continuum.

  • Begins at first order; subsequent orders are determined by merging of streams.

Thermal Stratification in Lakes

How Lakes Form

  • Lakes are non-flowing bodies of water collected in depressions in the landscape.

  • Types of lake formation:

    • Tectonic: Formed by Earth's crust movements (e.g., Lake Baikal).

    • Glacial: Formed by glacial processes (e.g., kettle lakes).

Stratum Layers

  • Lakes exhibit stratification due to differing density, affecting mixing:

    • Epilimnion: Warmer surface layer.

    • Thermocline: Rapid temperature change layer.

    • Hypolimnion: Colder, deeper layer.

Seasonal Changes in Stratification

  • Fall Turnover: Cooler temperatures lead to sinking of denser surface waters, mixing the lake.

  • Spring Turnover: Ice melting allows surface warming, mixing water layers swiftly.

Summary of Key Concepts

  • Factors affecting stratification and mixing in lakes:

    • Time of year, lake depth, lake shape, wind fetch, topography, inflow/outflow dynamics, solutes, and seiche.

  • Stratification impacts oxygen levels, nutrient availability, and organismal activity.