Layering & Overturn

LAKES

Lakes vs. pondsIn lakes light doesnt reach the whole way to the bottomNo photosynthesis = no O2 producedMany organisms on or near the bottom require oxygen. How does it get there?OverturnOverturn is the process by which currents circulate the water in a lake or pond resulting in a movement of oxygen and nutrientsSpring OverturnMixing of water in the springtimeCaused by melting and the cold water sinking to the bottomThe water is then mixed easily by wind Temperature is the same throughoutSame temperature = same densitySame density allows for easier mixingBrings nutrients up from bottom to the topSpring Overturn

Spring Overturn

Summer StagnationIn summer the sun heats the top layer faster than the wind can circulate it.3 layers resultDifferent densities = 3 different levelsThese layers dont mix well

Summer Stagnation

Summer Stagnation

EpilimnionUpper layer warmerWater circulates freelyHypolimnionCool layerDoes not circulateNo oxygen from the topThermocline or metalimnionMiddle layerTransition from warm to coldFall OverturnEpilimnion (top layer) cools The fall winds cause another mixingThe entire lake becomes a uniform temperature againFall Overturn

Fall Overturn

Fall Overturn

Winter StagnationThe upper layer (epilimnion) cools further and sinksIncreased cooling = increased densityNow the thermocline is reversed. The water on the bottom is near 4C and the water at the top is 0C (cold to warm)Top layer freezesResult = 3 layersEpilimnion is iceHypolimnion is above or at 4C

Winter Stagnation

Winter Stagnation

ZONES OF A LAKE

Trophogenic ZoneUpper layer where most oxygen is producedOrganisms here produce food through photosynthesis

Tropholytic Zone

Lower part of the hypolimnion with high CO and a low OOrganisms feed on decomposing matter

Compensation depth

Zone between trophogenic and tropholytic zonesOxygen production and use are equal

Limnetic zoneSubzone of trophogenic zonePart of open water where light penetrates the waterPhytoplankton are the main producers

Littoral zone

Subzone of trophogenic zoneRegion where light reaches the bottom (near shore)

AGING OF A LAKE

EutrophicationThe process of agingThe increasing productivity of a lakeMeans adequate foodNutrient richAccumulations of nutrients from runoff, sewage wasteComparing new and old lakes

Eutrophic

Adequate food or nutrient richOxygen levels decrease greatly at the bottomEutrophic

OligotrophicLacking foodNutrient poorOxygen decreases only slightly with depthYoung biological lakeBedrock of limestone or dolomite (basic)The pH normally drops as a lake agesDecay releases acids into the water (carbonic acid)Carbonic acid forms when the carbon dioxide from respiration reacts with the water

Oligotrophic

MesotrophicMiddle stage of lake lifeMore acidicThe pH usually rises and becomes more basicYoung biological lakeMiddle aged biological lakeOld biological lakebasicmore acidicmore basicOligotrophicMesotrophic

EutrophicGEOPHYSICAL PROPERTIES OF LAKESTemperatureNature of the bottomColorTurbidityTransparencyTemperatureEach species of organism has its own optimum or preferred temperatureSome can stand a variation in temperatureBut if it goes too far in a range = Death or move elsewhereIncrease of 5 C can be lethalFish killsOxygen demand exceeds the available oxygenDeath occursTLM Tolerance Limit Median - based on time to get out of a bad situationHigh temperatures will increase the toxic effects of chemical pollutants in the water

Nature of the BottomDetritusSubmerged PlantsColorSuspended materialWater is green = phytoplanktonWater is yellow or brown = dead algae, runoffTurbiditySuspension of solidsPhytoplankton and zooplanktonDead wastes sewageInorganic clay and siltSecchi Disk

TransparencyHow well light passes through waterSuspended solids in waterSecchi DiskLow reading (1m) = high level suspended matterHigh reading (8m) = quite clearBased on compensation depth