💧Limnology Unit 1 – Lake and pond origins and morphology

Key Concepts and Definitions

• Limnology studies the biological, chemical, and physical features of inland waters (lakes, ponds, rivers, streams, wetlands, and groundwater)
• Lakes are large bodies of water surrounded by land formed through various geological processes
  • Typically have a closed basin with no direct connection to the ocean
  • Contain standing or slow-moving water
• Ponds are smaller bodies of standing water usually formed by natural processes or human activities (dam construction, excavation)
• Morphology refers to the physical structure and shape of a lake or pond including depth, surface area, shoreline development, and basin shape
• Trophic state classifies lakes based on biological productivity influenced by nutrient levels (oligotrophic, mesotrophic, eutrophic)
• Stratification is the formation of distinct layers (epilimnion, metalimnion, hypolimnion) in a lake due to temperature and density differences
• Turnover is the mixing of lake layers caused by seasonal changes in temperature and wind action
• Residence time is the average time water remains in a lake before being replaced by inflows or outflows

Formation Processes

• Tectonic lakes form due to the movement and interaction of Earth's crustal plates (East African Rift Valley lakes)
  • Faulting, folding, and volcanic activity can create depressions that fill with water
• Glacial lakes are created by the erosive action of glaciers during ice ages (Great Lakes, Finger Lakes)
  • Includes cirque lakes, kettle lakes, and moraine-dammed lakes
• Fluvial lakes result from the action of rivers and streams (oxbow lakes, floodplain lakes)
  • Meandering rivers can create cut-off loops that form oxbow lakes
  • Floodplains can have depressions that fill with water during high-flow events
• Shoreline lakes develop along coastlines due to changes in sea level or coastal processes (lagoons, barrier island lakes)
• Volcanic lakes form in volcanic craters or calderas after eruptions (Crater Lake, Oregon)
• Anthropogenic lakes and ponds are created by human activities such as dam construction, mining, or excavation for recreation or water storage
• Solution lakes form in areas with soluble bedrock (limestone, gypsum) where groundwater dissolves the rock creating depressions (karst lakes)
• Meteorite impact craters can fill with water to create lakes (Lonar Lake, India)

Types and Classifications

• Tectonic lakes (rift lakes, fault-block lakes)
• Glacial lakes (cirque lakes, fjord lakes, kettle lakes, moraine-dammed lakes)
• Fluvial lakes (oxbow lakes, floodplain lakes, levee lakes)
  • Oxbow lakes form when a meander loop is cut off from the main river channel
  • Floodplain lakes develop in depressions on the floodplain that fill during high-water events
• Coastal lakes (lagoons, barrier island lakes)
• Volcanic lakes (crater lakes, caldera lakes)
• Artificial lakes (reservoirs, impoundments, quarry lakes, pit lakes)
  • Reservoirs are created by damming a river or stream for water storage, flood control, or hydropower
  • Pit lakes form in abandoned mines or quarries that fill with groundwater or surface runoff
• Karst lakes (solution lakes, sinkhole lakes)
• Meteorite impact lakes
• Ephemeral lakes are temporary bodies of water that form in depressions or low-lying areas during wet periods and dry up during droughts (playa lakes)

Physical Characteristics

• Surface area is the total area of the lake's surface, influencing factors such as wind mixing, light penetration, and habitat availability
• Volume is the total amount of water contained in the lake, affecting thermal stratification, water residence time, and dilution of nutrients and pollutants
• Maximum depth is the deepest point of the lake, influencing stratification, light penetration, and habitat zonation
  • Mean depth is the average depth calculated by dividing the lake volume by its surface area
• Shoreline development is the ratio of the shoreline length to the circumference of a circle with the same area as the lake
  • Higher shoreline development indicates a more complex and convoluted shoreline
• Basin shape (profile and contours) affects mixing, stratification, and sediment distribution
  • Convex profiles have shallow margins and a deep central area, while concave profiles have a shallow central area and steep margins
• Water clarity is influenced by suspended particles, dissolved organic matter, and phytoplankton abundance
  • Measured using a Secchi disk to determine the depth of light penetration
• Thermal stratification occurs when lakes develop distinct layers with different temperatures and densities (epilimnion, metalimnion, hypolimnion)
  • Thermocline is the transition layer between the warmer epilimnion and colder hypolimnion

Geological Influences

• Bedrock geology determines the basin shape, depth, and potential for groundwater interactions
  • Resistant bedrock (granite) creates steep-sided, deep basins, while softer bedrock (shale) forms shallow, gently sloping basins
• Catchment area geology influences the chemical composition of inflows and the erosion potential of the surrounding landscape
  • Carbonate-rich bedrock (limestone) contributes to alkaline, well-buffered lake water
  • Silicate-rich bedrock (granite) results in acidic, poorly-buffered lake water
• Tectonic activity can alter lake basins through faulting, uplifting, or subsidence
  • Earthquakes can trigger subaqueous landslides or seiches (standing waves)
• Volcanic activity can create new lakes, alter existing ones, or introduce geothermal inputs
  • Volcanic ash and lava flows can change the basin shape or water chemistry
• Glacial history shapes lake basins through erosion and deposition
  • Glacial scouring creates deep, steep-sided basins (fjords, cirques)
  • Glacial deposits (moraines, eskers) can dam rivers or create kettle lakes
• Climatic factors (precipitation, evaporation, temperature) influence lake water balance and level fluctuations
  • Closed basins in arid regions are more sensitive to climatic changes
• Weathering and erosion of the catchment area contribute sediments and nutrients to lakes
  • High erosion rates can lead to rapid sedimentation and infilling of lake basins

Ecological Significance

• Lakes and ponds support diverse aquatic ecosystems with a wide range of habitats and species
  • Littoral zone is the shallow, near-shore area with rooted aquatic plants
  • Pelagic zone is the open water area dominated by plankton and fish
  • Benthic zone is the bottom sediment layer inhabited by invertebrates and microbes
• Primary productivity in lakes is driven by phytoplankton, algae, and aquatic plants
  • Nutrient availability (phosphorus, nitrogen) and light penetration control primary production
• Food webs in lakes include primary producers, primary consumers (zooplankton, herbivorous fish), secondary consumers (carnivorous fish, invertebrates), and decomposers (bacteria, fungi)
  • Trophic cascades can occur when changes in top predator populations affect lower trophic levels
• Nutrient cycling in lakes involves the exchange of nutrients between water, sediments, and organisms
  • Internal loading from sediments can delay lake recovery after external nutrient inputs are reduced
• Lakes act as carbon sinks, storing organic carbon in sediments and contributing to global carbon cycling
  • Methane production in anoxic sediments can be a significant greenhouse gas source
• Habitat connectivity between lakes, rivers, and wetlands is essential for the dispersal and migration of aquatic species
  • Fish use tributary streams for spawning and nursery areas
• Invasive species can disrupt lake ecosystems by competing with native species, altering food webs, or modifying habitats
  • Zebra mussels (Dreissena polymorpha) have caused significant changes in North American lakes

Human Impact and Management

• Eutrophication is the excessive enrichment of lakes with nutrients (phosphorus, nitrogen) from human activities
  • Agricultural runoff, sewage discharge, and urban development are major sources of nutrients
  • Symptoms include algal blooms, reduced water clarity, oxygen depletion, and fish kills
• Acidification occurs when lakes receive inputs of acidic substances from atmospheric deposition or acid mine drainage
  • Lowered pH can harm aquatic organisms and mobilize toxic metals from sediments
• Water level regulation for hydropower, irrigation, or flood control can alter lake ecosystems
  • Fluctuating water levels affect littoral habitats, fish spawning, and nutrient cycling
• Shoreline development (residential, recreational) can degrade water quality and habitat
  • Hardening of shorelines with seawalls or rip-rap reduces natural vegetation and erosion control
• Invasive species introductions through ballast water, aquarium releases, or bait bucket transfers can have cascading effects on lake food webs
  • Prevention through education, regulations, and early detection is crucial
• Fisheries management involves balancing recreational and commercial fishing with the conservation of native fish populations
  • Stocking, catch limits, and habitat restoration are common management tools
• Watershed management addresses land use practices and pollution sources in the catchment area to protect lake water quality
  • Best management practices (BMPs) for agriculture, forestry, and urban development can reduce nutrient and sediment loads
• Lake restoration techniques aim to improve water quality and ecosystem health
  • Nutrient inactivation (alum treatment), biomanipulation (fish removal), and aeration are examples of in-lake treatments
  • Watershed-based approaches are essential for long-term success

Case Studies and Examples

• Lake Baikal (Russia) is the world's oldest, deepest, and most voluminous freshwater lake
  • Contains unique endemic species adapted to its deep, oxygenated waters
  • Threatened by pollution, overfishing, and climate change
• Lake Victoria (East Africa) is the world's second-largest freshwater lake by surface area
  • Supports a highly productive fishery crucial for regional food security
  • Faces challenges from eutrophication, overfishing, and invasive species (Nile perch)
• Lake Tahoe (California/Nevada, USA) is known for its exceptional water clarity and alpine setting
  • Strict land use regulations and restoration efforts aim to protect its clarity from urban development and climate change impacts
• Aral Sea (Central Asia) has dramatically shrunk due to water diversions for irrigation
  • Ecosystem collapse, dust storms, and economic hardship for local communities
• Laguna Lake (Philippines) is a large, shallow lake vital for fisheries and agriculture
  • Rapid urbanization, industrialization, and aquaculture have led to water quality deterioration and conflicts among resource users
• Peyto Lake (Alberta, Canada) is a glacial-fed lake in Banff National Park
  • Its vibrant turquoise color is due to suspended glacial rock flour
  • Serves as an indicator of glacial retreat and climate change impacts on mountain lakes
• Lake Vostok (Antarctica) is the largest of over 400 subglacial lakes beneath the Antarctic ice sheet
  • Its unique environment and potential for harboring ancient microbial life have attracted scientific interest
  • Drilling efforts must balance research value with the risk of contaminating the pristine ecosystem