BL1006 Lecture 9: Standing Waters

BL1006 Lecture 9: Standing Waters

Types of Lakes

  • Tectonic Lakes: Formed by geological action.
    • Rift Valley Lakes: Examples include:
    • East African Rift Valley lakes
    • Lake Baikal
    • Loch Ness
    • Volcanic Lakes: Formed by volcanic activity, e.g., Lake Taupo in New Zealand.
    • Tectonically Uplifted Lakes: Includes isolated lakes from ancient seas such as Lake Aral and the Caspian Sea.

Origin of Lakes

  • Lakes can form through various geological processes:
    • Rivers create lakes wherever water flows downhill over land.
    • Lakes can originate via tectonic movements, volcanic activity, glacial action, and other geological events.

Specific Lakes

Lake Tanganyika

  • A major example of an East African Rift Valley lake.
  • Bathymetry:
    • Depth: Ranges from 0 m to a maximum of -1500 m.
    • Context provided by the African Plate boundaries.

Lake Baikal

  • Characteristics:
    • World's deepest lake with a maximum depth of 1,642 ext{ m}.
    • Largest freshwater lake by volume: contains 23,615 ext{ km}^3 of water, holding approximately 23 ext{%} of the world's fresh surface water.
    • Oldest lake, estimated to be between 25 to 30 million years old.
  • Endemic species:
    • Baikal Seal (specific to Lake Baikal).
    • Two species of grayling: Thymallus baikalensis and T. brevipinnis.
    • Baikal Oilfish, Comephorus baicalensis.

Loch Ness

  • A rift valley lake located within the Great Glen Fault.
  • Popularly known for Nessie, the Loch Ness monster legend. An individual reportedly spent 32 years living in a van at Loch Ness to search for this creature.

Lake Taupo

  • Formed from a volcanic eruption approximately 2000 years ago.
  • Considered one of the largest volcanic eruption sites in the last 5000 years.
  • Eruption comparisons:
    • Produced 60-100 ext{ cubic kilometres} of volcanic material.
    • Eruptions compared with St. Helens (1980), Pinatubo (1991), and Krakatau (1883).

Tectonically Uplifted Lakes Related to Shrinking Seas

  • Includes:
    • Black Sea
    • Caspian Sea (currently shrinking rapidly)
    • Aral Sea (now almost completely dried up).

Formation of Lakes by Glacial Action

  • Ice Scour Lakes: Formed by glaciers scraping bedrock, leading to steep-sided lakes.
  • Glacial Moraine Lakes: Formed in depressions caused by glacial drift, often including kettle holes.
  • Kettle Holes: Created when ice chunks from retreating glaciers are buried in till deposits. As they melt, steep-sided water bodies, or kettle lakes, form.

Lakes Formed Through Various Processes

  • Solution Lakes: Emerge from water dissolving soluble rocks.
  • Rivers: Lakes created by erosion and sediment deposition within river floodplains.
  • Shoreline Lakes: Created by marine actions blocking river delta sediment flows.
  • Beaver and Human Activity: Reservoirs can also lead to lake formations.

Example Lakes

  • Loughareema, Co. Antrim: A 'vanishing lake' formed by rainwater dissolving surrounding limestone.
  • Lower Yangtze, China: Lakes originating from river activities.
  • Beaver Dams and Reservoirs: Created through human and animal activity.

Lake Organisms and Communities

Lentic Organism Classification

  • Based on size:
    • Net Plankton: >200 ext{ μm}
    • Microplankton: 20-200 ext{ μm}
    • Nanoplankton: 2-20 ext{ μm}
    • Picoplankton: 0.2-2 ext{ μm}
    • Femtoplankton: 0.02-0.2 ext{ μm}
  • Based on their location in the lake:
    • Littoral Zone: Near the shore.
    • Limnetic Zone: Open water.
    • Benthic Zone: Bottom of the lake (depth varies).
  • Trophic Levels:
    • Heterotrophic Decomposers: Microbes breaking down material.
    • Primary Producers: Plants contributing to primary productivity.
    • Primary Consumers: Small animals feeding directly on producers.
    • Secondary Consumers: Larger animals feeding on primary consumers.

Lake Zonation

  • Euphotic Zone: Depth at which ext{1 ext{%}} of light is available for photosynthesis (extends to the nutrient-rich top layers).
  • Aphotic (or Profundal) Zone: Depth beyond which light is insufficient for photosynthesis (less than 1 ext{%}).
  • Littoral Zone: Where the euphotic zone reaches the lake bed.
  • Limnetic Zone: Below the euphotic depth to the lake bottom.
  • Benthic Zone: The lake bed area at any depth.

Nutrient Cycling in Lakes

  • Key nutrients are Phosphorus (P) and Nitrogen (N).
    • Limiting Nutrients: Set productivity limits throughout the year (eutrophic, mesotrophic, oligotrophic classification).
  • Characteristics of eutrophic lakes: Often have abundant nutrients, leading to high primary productivity and dense phytoplankton, creating a green water appearance.

Seasonal Productivity Patterns

  • Spring: Increased productivity due to high light and temperature, combined with abundant nutrients.
  • Summer: Decline in productivity due to nutrient limitation in surface layers during stratification.
  • Autumn: Nutrient-rich water rises to the surface due to wind mixing, leading to an autumn peak in productivity.
  • Winter: Low light and temperature result in reduced productivity, termed the winter minimum.

Factors Governing Lake Ecosystems

Light

  • Critical for plant growth, limits depth of plant and algal growth due to effective absorption by water.
  • Secchi Depth: Measure of light penetration in water, assessed with a Secchi disc.

Heat

  • Thermal Stratification: Temperature layering affects aquatic ecosystems.
    • Surface layers warm above denser cold water.
    • Warmer layers insulate cooler depths, affecting nutrient cycling and organism alignment in water layers.
  • Temperature's effect on water density varies, leading to important stratification phenomena.

Nutrients

  • Primary productivity is influenced by available nutrients. Nutrient depletion occurs through biological activities and sediment accumulations, leading to stratified nutrient cycles throughout seasons.

Wind

  • Influences lake structure and productivity, as well as sediment distribution along shorelines. Wind speed and direction inform littoral zone characteristics and water mixing.

Conclusion

Understanding the myriad factors including geological formation, light availability, temperature gradients, nutrient dynamics, and wind effects are essential for comprehending lake ecosystems and their productivity throughout seasons.