Density Stratification of Lakes • Water masses will layer in order of increasing density with depth. • Any denser water at the surface will sink to a depth of equal density or to the lake bottom. • Sinking denser water (convection currents) and wind waves create turbulent mixing of water masses. • Establishment and sustainability of distinct layers requires low turbulence: • - Calm, low wind, conditions • - Stable daily temperature regime I Causes & Profiles Seasonal Circulation Patterns Lake Classification
The Thermocline as a Barrier! Large amounts of energy are needed to “breakdown” a well established thermocline. It is a barrier to eddy transport of solutes (like nutrients) from the hypolimnion and slows the sinking of particles from the epilimnion.
Pressure Effect on Density: • Greater pressure decreases the temperature of maximum density. • Both water depth and altitude (relative to sea-level) will change pressure. • The bottom of a deep freshwater lake may be colder than 3.98ºC (max. density at 1 atm). Changes by about 0.1ºC per 100 m. • A high altitude lake may have slightly warmer bottom temperature than an identical lake experiencing identical climate at sea level. • Similar influences of temperature and pressure on biological processes will happen at different depths for identical lakes at different altitudes (all other factors being constant).
Salinity Effect on Density: • Solutes also decrease the temperature of maximum density and decrease the freezing point. In fact, density will continue to increase in salt water up to the colder freezing point (e.g. -1.9ºC in ocean seawater). • Some lakes and all estuaries experience changes in both temperature and salinity; which layers will be at which depths depends on the combined effects on density. • Caballing is an unexpected consequence when water masses of equal density (yet due to different properties) mix together. The new water mass sinks!
Inverse Thermal Stratification due to Salinity (TDS) Effects Pure H20 Pond The TDS profile would show an increase with depth. This is a desert pond, as water evaporates a the denser saltier brine sinks to the bottom, taking heat with it.
Suspended Sediment Effects: • Water laden with suspended sediments is denser than clearer water with otherwise similar temperature, salinity, and pressure. • E.g. (right) Rio Negro flowing in over the sediment laden Amazon River. • E.g. (below) Lake with turbid stream flowing into it. FLOW Relative Values → T z↓ SS
Seasonal Circulation Patterns: E.g. Mountain Lake, VA, a temperate (30º- 60º N or S) dimictic lake • Stable statification versus “turnover” are the extremes of lake water circulation. • Plot of isotherms, lines of constant temperature at 1ºC increments. • Note summer thermal stratification and winter inverse stratification. • Note two turnover events (spring and fall); hence, dimictic.
Lake Classification by Circulation • Amixis refers to a lake that does not ever turnover; called an amictic lake. • Holomixis refers to lake turnover. The frequency of mixis over annual to decadal time scales is used in classifying lakes. • Dimictic lakes follow the pattern described for temperate lakes; spring and fall turnovers. • Oligomictic lakes turnover every few years. • Monomictic lakes turnover once: (careful here!) • Cold monomixis: colder lakes that inverse-stratify only under winter ice cover and mix the remainder of the year. • Warm monomixis: warmer lakes that mix during winter (never freeze over) and stratify in summer. • Polymictic lakes turnover frequently throughout the year.
Meromictic Lakes Temperature (ºC) • Meromictic lakes experience incomplete mixing at the surface layer; the bottom layer is high in solute concentration & stable. • Strata from surface to bottom: • Mixolimnion, • Chemocline, • Monimolimnion • Classified by source/cause of meromixis: • Biogenic (accumulation of calcium bicarbonate; hardness) • Ectogenic (inputs of fresher or saltier) • Crenogenic (subsurface saline inputs) Z (m) Conductivity is the inverse of current resistance; more ions (salts) then greater values.