Lakes Ecosystem

LAKES Ecosystem

Temperature-Density Relationship in Water • Density increases with decrease in temperature • Maximum density is approximately at 4oC • Below 4oC, water is less dense

Stratification of Lakes • Occurs when a warm surface layer of water develops over cooler, deeper water • A warming climate results in frequent and larger "dead zones"—areas of water depleted of oxygen and unable to support life

Stratification of Lakes • Persistent dead zones can result in – toxic algal blooms, – foul-smelling, – musty-tasting drinking water, – damage to fisheries, and – massive fish kills

Stratification of Lakes Lower oxygen and warmer temperatures promote – greater microbial decomposition and – subsequent release of nutrients and contaminants from bottom sediments • •

Phosphorus released is enhanced Mercury release and uptake by biota is likely to increase – exposing humans to higher mercury levels via fish consumption.

Seasonal Lake Stratification Spring • After the ice melts on a lake, temperature from the surface to the bottom is generally the same. • Wind allows circulation and mixing of the lake water. • Surface water can be pushed to the lake bottom and bottom water can rise to the surface • Large amounts of oxygen can reach the bottom of the lake.

Seasonal Lake Stratification Late Spring Air temperatures rise heats warms lake from surface down. Warm water is less dense and floats over the cold water. Epilimnion – layer of warm water Hypolimnion – cold layer below Thermocline or metalimnion – layer between epilimnion and hypolimnion – rapidly changes temperature with depth.

Seasonal Lake Stratification Summer Epilimnion reaches maximum depth. Warm water and abundant sunlight provide ideal environment for algae growth. Algal blooms tend to give the epilimnion a greenish hue Wind circulates the surface water Water is only mixed in the epilimnion. • Bottom is deprived of oxygen

Seasonal Lake Stratification Autumn • Epilimnion becomes shallow • Hypolimnion increases in thickness • Lake loses its stratification • Generally uniform temperatures

Seasonal Lake Stratification Surface water cools faster than water below • Cold, dense water sinks and further helps to mix the lake • Complete mixing occurs • Oxygen and nutrients are replenished throughout the lake. • This process is called autumn overturn.

Seasonal Lake Stratification Winter • Surface water is cooled below 4°C (0oC) • Water no longer sinks and solidifies. • Ice covers the surface and prevents wind from mixing the lake water • A layer of low density water colder than 4°C, but warmer than 0°C forms just under the ice. • Winter stagnation

Annual Cycle of Thermal Stratification

Temperature Profile

Density Stratification

Stratification in Lakes

Stratification in Lakes

Lake Zones

Typical Food Chain

Typical Food Chain in Lakes