Ch. 8 Water movements

Lake Mixing Classification

  • Dimictic Lakes: mix twice a year

    • spring and fall overturn

  • Holomictic Lakes: mix totally sometimes

  • Amitic Lakes: permanently sealed off by ice = no wind and no mixing

  • Cold Monomictic Lakes: circulate once per year; Tmax is less than 4oC

    • mix once a year when ice is gone

    • no stratification in spring or fall

  • Warm Monomictic Lakes: circulate once per year; Tmin is greater than 4oC

    • no ice cover

    • very stratified because of heat

    • when cooled the wind will mix

  • Oligomictic Lakes: rarely circulate; Tmin is very much greater than 4oC

    • under intense heat all year long

    • very rare that the lake will mix

  • Polymictic Lakes: many mixings per year

    • shallow lakes = wind can mix lake very easily many times

  • Meromictic Lakes: part of the water column almost ‘never’ mixes because they have a deep level of salt layer

Chemical Stratification

  • Dissolved chemical substances (e.g. salts) can greatly affect the density of lakes (much more than temperature)

    • doesnt take too much salt to change the density

  • Ex. floating in the Dead Sea because of the high salinity and the dense water

Salinity and Density

  • a salt concentration of 1g/L increases the density of water by about 0.0008

  • a change in temperature between 4oC and 5oC changes the density of water about 0.000008

  • So, there are about 2 orders of magnitude (100x) more change with the increase in salinity

  • Water of higher salinity is more dense = water will sink

Meromictic Lakes

  • Meromictic lakes contain a dense chemical gradient

    • Monimolimnion layer: lower, more dense, concentrated, deep portion of the lake that almost never mixes

      • will not get oxygen

      • major habitat of bacteria (NO higher eukaryotes because of no oxygen)

    • Mixolimnion layer: upper, less dense part of the lake that mixes like a regular dimictic lake

    • Chemocline layer: zone/barrier marking the chemical gradient between the mixolimnion and the monimolimnion

  • Lakes that have bigger depths are more susceptible to meromixis, whereas lakes with minimal depth are less susceptible

  • Lakes that have some sort of barrier around the lake (like trees or tall buildings) will be more susceptible to meromixis because the barrier does not let wind get through to mix the lake

Chemical Meromixis

  • For meromixis to occur, they need 3 sources of salts to set up the chemical gradient:

    • 1) Ectogenic meromixis: external source of dissolved material (salts)

      • e.g. salts coming from outside the lake

      • ex. marine incursions (lakes near the ocean) or human activities that add salt to lakes (salting icy roads)

    • 2) Crenogenic meromixis: chemical gradient from saline spring

      • groundwater can be salty because the longer the water satys in contact with the rocks at the bottom of groundwater, the more salt will be added to the lake

    • 3) Biogenic/Endogenic meromixis: formed by processes within the lake basin, from biological sources

      • e.g. when organisms die in the lake, they release the salt contents within their body when decomposed, adding more salt into the deep waters of the lake when they sink

Surface Water Movements

  • Surface Waves (progressive waves)

    • mainly affects near-shore environments

    • once the waves break at the bottom of the shore they will affect deeper waters

    • ex. wave erosion

  • Surface Currents

    • generated by external sources

    • ex. Coriolis effect - northern hemisphere waters shifts to the right

  • Langmuir Circulation & Streaks

    • when the wind blows it will mix as deep as the epilimnion layer goes, then streaks will be caused by convergence and collect algae and other particles

Internal Wave Movements

  • Water movements affecting the whole lake:

    • Surface seiche

      • wind is pushing water on one side of the basin (drying out one side of the land)

      • when wind stops it comes back and does it to the other side (goes back and forth)

    • Internal seiche

      • piles water on one side under water at the thermocline

    • Thermal bars

      • occurs during spring progression

    • Influence of rivers

    • Currents under ice

      • typically minor, but some currents may occur

      • slow currents occur die to release of heat from sediments (convective currents)

      • most of this heat was accumulated during the previous summer