Mixing & Waves in Aquatic Ecosystems
BIO321
Fall 2000

Reading: Chapter 5 in Horne & Goldman

I. Review - What are the major lake mixing types that we have talked about so far this semester?

amictic, dimictic, cold monomictic, warm monomictic, polymictic, oligomictic, meriomictic

II. Circulation within a lake system (but water is also always in motion - How?)

1. Wind is the major force of circulation and can be quite important

• distribution of heat
• distribution of dissolved substances
• distribution of organisms?

2. How does flow occur in a liquid medium like water?

• Laminar Flow occurs at very low velocity, is smooth, unidirectional, and results in little mixing
• Turbulent Flow is more typical and begins at low velocity and is enhanced by boundries
  • epi-metalimnion
  • air-water interface
  • density layers
  • transition to from laminar to turbulent flow occurs at low velocity
• Eddy formation occurs as a result of turbulent flow, resulting in some vertical mixing in eddy cells
  • eddy viscosity - determined by water molecules and state and is the transfer of motion between water molecules
  • eddy diffusion - distribution of solutes
  • eddy conductivity - distribution of heat

3. Wind caused waves = periodic current systems

if wavelength is greater than 2pi = 6.28cm they are referred to as gravity waves, while if < they are ripples

 

• Travelling surface waves are a result of friction between water and air currents
  • maximum wave height (h) =0 .105sqrt(Fetch) cm and is independent of depth Z
  • eddy cells result in little horizontal mixing
  • eddy cells result in some, but also not much vertical mixing
  • In shallow water, long waves occur (when wavelength³20xDepth) as on beaches and this is when mixing can be substantial

unimodal seiche	bimodal seiche	two layer seiche

• Standing waves (seiche) = dry shore = soupbowl waves

  • wind causes water to pile up on the leeward side of the lake, then the wind stops or declines and the lake "rocks" until dampened by friction with the lake basin
  • Usually these wavers are of very small amplitude (<1mm-30 cm), although in 1841 Lake Geneva had a seiche with an amplitude of 1.87 meters and a period of 73 minutes and the wave rocked for 7 days and 17 minutes. The period of a Lake closer to home is Lake Erie with 790 minutes.
  • Main causes are wind, earthquakes, and rapid drops in barometric pressure
  • The wave depends largely on the morphometry (basin shape and harmonics)
    • Wavelength(l) = 2L makes a unimodal seiche
    • Wavelength(l) = L creates a bimodal seiche with correspondingly increased mixing
    • Wavelength(l) = 2/n *L creates an n-modal seiche
  • Surface seiches are the most common and are usually small (1 - 2 mm)
  • Internal seiches may occur in a stratified lake where in reality there is an extra density interface between the layers, resulting in oscillation of the metalimnion.
    • Often these waves can be of greater amplitude and period
    • The two layers rock separately and often out of phase and so mixing can be increased, by the back and forth currents in opposing directions
    • Vertical and horizontal transport of het, dissolved substances, and sediment can have an impact on productivity

• Coriolis effect
  • In the N. hemisphere waves are pulled to the right - twisted waves to increase mixing

   

4. Non-periodic systems = currents

• wind streaks occur on lakes with very long stretches as Languir currents (special eddy cells) line up to leave particles in "windrows" leading to Langmuir circulation
• Density currents are caused by springs, streams, or rivers entering and/or passing through lakes.
  
• Mixing under ice
  • Sediment heating up leads to convection eddy cells mixing from bottom to top (Why?
  • Heat through clear ice = What would happen?
  • Ice freezing leads to density gradients as solutes precipitate out.
  • Oxygen use by sediments (why would the sediments use oxygen) can lead to "gas" diffusion currents
  • Horizontal mixing of up to 15-20 m per day may occur
• Vertical mixing is usually much less (about 1/3 as much)

Robert Kistler
Bethel College

version 2000