Chapter8_KEY

Types of Lakes

  • Ice block lakes: Formed by buried ice blocks melting after glacial retreat, creating kettle lakes.

  • Oxbow lakes: Created through differential erosion and sedimentation that leads to meandering rivers forming U-shaped lakes cut off from the river during flooding.

  • Dams of glacial lakes: Occur when natural drainage ways are blocked, forming a lake.

  • Lakes at confluence of two rivers: Formed by blockages in channels resulting in water buildup (e.g., Lake Pepin).

Lake Distribution in Minnesota

  • Very few lakes: Southern region (prairie).

  • Few lakes: Driftless region (southeast MN) not covered by recent glaciers.

  • Most lakes: Northern region.

  • Very shallow lakes: Prairie region.

  • Narrow lakes with steep rocky shorelines: Northeastern and BWCAW region.

Physical and Chemical Characteristics

  • Water chemistry influenced by:

    • Drainage basin (parent rock).

    • Runoff and groundwater.

  • Chemistry modified by:

    • Depth and size of the lake.

    • Soil, land use, pollution.

Nutrient Influence on Algae Growth

  • Phosphorus and nitrogen function as limiting resources for algae and aquatic plant growth.

  • Increased phosphorus leads to greater productivity in lakes.

Factors Affecting Light and Transparency

  • Reduced light in winter due to:

    • Snow cover.

    • Ice.

  • Reduced transparency caused by:

    • Water color (e.g., tea-like coloration from leaves).

    • Suspended material (sediment and algae).

  • Measurement of water clarity: Secchi disk method.

Oxygen Dynamics in Lakes

  • Oxygen sources/removers:

    • Summer: Photosynthetic organisms add oxygen; animals and heterotrophs remove it.

    • Winter: No addition; consumption by organisms occurs at all levels.

Temperature Profiles in Lakes

  • Summer Stratification:

    • Epilimnion: Warmed by the sun, mixed by wind.

    • Thermocline: Sharp temperature decline.

    • Hypolimnion: Cold, stable layer below.

  • Fall and Spring Turnover Events: Mixes temperature layers, especially as temperatures reach 39°F, where water is densest.

  • Winter behavior: Ice seals lakes, keeping sunlight and wind out.

Lake Zones and Ecosystem Roles

  • Labeling Zones:

    • Benthic, profundal, pelagic, and littoral zones.

  • Littoral Zone Benefits:

    • Provides habitat and infuses water with oxygen.

  • Consumers: Invertebrates (caddisfly larvae, scuds), small fish, snails, muskrats.

Ecological Role of Zooplankton

  • Example: Water fleas (Cladocera) feed on phytoplankton, increasing water clarity when abundant.

  • Filter feeders: Organisms collecting food while allowing water to pass through.

  • Examples: Copepods and rotifers.

Hypolimnion Environment

  • Conditions: Cold, dark (aphotic), lacking photosynthetic organisms.

  • Organisms: Fish preying on invertebrates, detritivores (e.g., protozoa, bacteria).

Lake Productivity and Aging

  • Types of lakes:

    • Oligotrophic: Deep, cold, high DO, low nutrients, high clarity, low productivity.

    • Eutrophic: Shallow, high nutrient levels, high productivity, lower clarity.

  • Aging process: Succession from oligotrophic to eutrophic, with sedimentation leading to increased nutrient levels.

Cultural Eutrophication and Restoration

  • Reducing eutrophication: Slowing nutrient input, restoring banks and wetlands.

Case Study: Lake Superior

  • Characteristics:

    • 1,333 feet deep; 21,280 sq. miles; largest freshwater lake in N. America.

    • Cold, well oxygenated, oligotrophic, low productivity.

Lake Contamination

  • Sources of contamination: Sediment, surface runoff, sewage, industrial wastes, atmospheric deposition, nitrogen, phosphorus from farmlands, invasive species.

  • Progress in pollutant reduction: Notable progress for point sources; nonpoint pollutants remain challenging.

Contaminants of Emerging Concern

  • Examples: Pharmaceuticals, personal care products (e.g., DEET, cotinine).

Lake Restoration Success - Chain of Lakes, Minneapolis

  • Restoration efforts: Clean Water Partnership focused on reducing sediment and nutrient pollution and increasing public awareness, leading to improved water quality in three of four lakes.