💧Limnology Unit 2 – Physical Properties of Freshwater Systems

Key Concepts and Definitions

• Limnology studies the biological, chemical, and physical properties of inland waters (lakes, rivers, wetlands, and groundwater)
• Freshwater bodies contain water with low concentrations of dissolved salts and other total dissolved solids
  • Typically have a salinity less than 1 ppt (parts per thousand)
• Lentic ecosystems refer to standing waters such as lakes, ponds, and wetlands
• Lotic ecosystems encompass flowing waters like rivers, streams, and springs
• Trophic state describes the biological productivity of a water body
  • Oligotrophic: Low nutrient levels and low productivity
  • Mesotrophic: Moderate nutrient levels and productivity
  • Eutrophic: High nutrient levels and high productivity
• Stratification is the formation of distinct layers in a water body due to differences in water density
• Thermocline (metalimnion) is a layer of rapid temperature change separating the epilimnion and hypolimnion

Physical Characteristics of Freshwater Bodies

• Morphometry describes the physical dimensions and shape of a water body
  • Includes surface area, volume, maximum depth, mean depth, and shoreline length
• Bathymetry is the measurement of water depth and the mapping of underwater topography
• Catchment area (watershed) is the land area that drains water into a freshwater body
  • Influences water quantity, quality, and nutrient inputs
• Residence time is the average time water spends in a lake or reservoir before being replaced
  • Calculated as the volume of the water body divided by the inflow or outflow rate
• Thermal stratification occurs when a water body forms distinct layers with different temperatures
  • Epilimnion: Upper, warmer layer
  • Hypolimnion: Lower, colder layer
• Mixing regimes determine the frequency and extent of water column mixing
  • Dimictic: Mixes twice a year (spring and fall)
  • Monomictic: Mixes once a year
  • Polymictic: Mixes multiple times a year

Water Chemistry and Composition

• pH measures the acidity or alkalinity of water on a scale from 0 to 14
  • Neutral pH is 7, values below 7 are acidic, and values above 7 are alkaline
• Dissolved oxygen (DO) is essential for aquatic life and influences chemical processes
  • Oxygen solubility decreases with increasing temperature and salinity
• Nutrients such as nitrogen and phosphorus are crucial for aquatic plant and algal growth
  • Excess nutrients can lead to eutrophication and harmful algal blooms
• Dissolved organic matter (DOM) consists of organic compounds from decomposing plant and animal material
  • Affects light attenuation, nutrient cycling, and microbial processes
• Alkalinity is a measure of the water's capacity to neutralize acids and buffer pH changes
  • Determined by the presence of carbonates, bicarbonates, and hydroxides
• Major ions in freshwater include calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and sulfate (SO₄²⁻)
  • Concentrations vary depending on the geology and human influences in the catchment area

Temperature Dynamics and Stratification

• Water temperature influences physical, chemical, and biological processes in freshwater ecosystems
• Thermal stratification occurs when a water body forms distinct layers with different temperatures
  • Density differences between warm and cold water create a stable layering
• Epilimnion is the upper, warmer, and well-mixed layer
  • Receives the most sunlight and has the highest productivity
• Metalimnion (thermocline) is a layer of rapid temperature change separating the epilimnion and hypolimnion
  • Acts as a barrier to mixing between the upper and lower layers
• Hypolimnion is the lower, colder, and often darker layer
  • Receives less sunlight and has lower dissolved oxygen levels
• Seasonal mixing events (turnover) occur when the thermal stratification breaks down
  • Allows for the redistribution of nutrients, oxygen, and heat throughout the water column
• Climate change can alter thermal regimes and stratification patterns
  • Warmer temperatures can lead to earlier stratification, longer stratified periods, and reduced mixing

Light Penetration and Optical Properties

• Light availability determines the depth of the photic zone, where photosynthesis can occur
  • Photic zone extends from the surface to the depth where light intensity is 1% of the surface value
• Secchi depth is a measure of water transparency using a Secchi disk
  • Indicates the depth at which the disk is no longer visible from the surface
• Light attenuation (extinction) is the decrease in light intensity with depth due to absorption and scattering
  • Influenced by water molecules, dissolved substances, and suspended particles
• Photosynthetically active radiation (PAR) is the portion of the light spectrum (400-700 nm) used by plants and algae for photosynthesis
• Dissolved organic matter (DOM) and suspended sediments can strongly absorb and scatter light
  • Leads to reduced water clarity and a shallower photic zone
• Spectral composition of underwater light changes with depth
  • Blue and green wavelengths penetrate deeper than red and infrared wavelengths
• Euphotic depth is the depth at which light intensity is sufficient for net primary production
  • Typically corresponds to the depth where PAR is 1% of the surface value

Water Movement and Circulation Patterns

• Wind-driven circulation creates surface currents and mixing in the epilimnion
  • Influenced by wind speed, direction, and fetch (distance over which the wind blows)
• Langmuir circulation consists of parallel, counter-rotating vortices aligned with the wind direction
  • Enhances vertical mixing and redistribution of nutrients and plankton
• Internal waves (seiches) are standing waves that oscillate within a stratified water body
  • Caused by wind stress, changes in atmospheric pressure, or seismic activity
• Inflows and outflows affect water residence time and circulation patterns
  • Rivers, streams, and groundwater contribute water and nutrients to the lake
  • Outlets remove water and influence the flushing rate of the lake
• Coriolis effect influences large-scale circulation patterns in large lakes
  • Causes deflection of currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
• Upwelling brings deep, nutrient-rich waters to the surface
  • Occurs when surface waters are displaced by wind or diverging currents
• Horizontal and vertical mixing processes distribute heat, nutrients, and dissolved gases throughout the water column

Sediment Processes and Lake Morphology

• Sedimentation is the deposition of particles on the lake bottom
  • Influenced by particle size, water velocity, and flocculation (aggregation of particles)
• Resuspension is the reintroduction of sediment particles into the water column
  • Caused by wind-induced turbulence, bioturbation (disturbance by organisms), or human activities
• Sediment focusing is the preferential deposition of fine sediments in deeper parts of the lake
  • Results from the lateral transport of particles by currents and gravity
• Sediment accumulation rates vary spatially and temporally
  • Depend on factors such as catchment erosion, primary productivity, and human land use
• Sediment cores provide a historical record of lake conditions and catchment processes
  • Used for paleoenvironmental reconstructions and dating of sediment layers
• Benthic boundary layer is a thin layer of water directly above the sediment
  • Characterized by reduced flow velocities and increased solute gradients
• Sediment-water interface is a site of active biogeochemical processes
  • Nutrient cycling, oxygen consumption, and microbial activity occur at this interface

Measurement Techniques and Tools

• Temperature and depth profiles are measured using a CTD (Conductivity, Temperature, Depth) probe
  • Provides high-resolution data on thermal stratification and water column structure
• Dissolved oxygen is measured using an oxygen sensor or the Winkler titration method
  • Helps assess the oxygenation status and potential for anoxia in the water column
• Secchi disk is used to measure water transparency and estimate the depth of the photic zone
  • Lowered into the water until it is no longer visible, and the depth is recorded
• Water samples are collected using a Van Dorn or Niskin bottle
  • Allows for the collection of water from specific depths for chemical and biological analyses
• Acoustic Doppler Current Profiler (ADCP) measures water velocity and direction
  • Uses sound waves to determine the speed and direction of water movement at different depths
• Sediment cores are obtained using a gravity corer or a piston corer
  • Provides a vertical profile of sediment layers for analysis of physical, chemical, and biological properties
• Remote sensing techniques (satellite imagery, aerial photography) provide large-scale data on water quality and catchment characteristics
  • Used for monitoring algal blooms, sediment plumes, and land use changes in the catchment area