Question

Lake ecosystems: In the water column of a lake are different zones with respect to oxygen and light.

A. Describe the different zones and conditions briefly (the epilimnion, thermocline, and the hypolimnion)

B. Which of the following processes would you expect in which zone? (Note: some of the processes could take place in all zones, others might not happen at all.)

-Nitrification

-Anammox

-Denitrification

-Purple-sulfur bacteria

-Algae

-Nitrogen fixation

-Sulfate reduction

Answer 1

A. The epilimnion is the top-most layer in a thermally stratified lake. It is warmer and has a higher pH and dissolved oxygen concentration than the hypolimnion. It is also free to exchange dissolved gases such as oxygen and carbon dioxide with the atmosphere. Because this layer receives the most sunlight it contains the most phytoplankton.

Thermocline is a thin but distinct layer in large bodies of fluids. This layer is also known as Metalimnion. In this layer temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline divides the upper mixed layer from the calm deep water below.Thermoclines can also be observed in lakes. In cold climates, this leads to a phenomenon called stratification. During the summer, warm water will sit on top of colder, denser, deeper water with a thermocline separating them. Because the warm water is exposed to the sun during the day, a stable system exists and very little mixing of warm water and cold water occurs, particularly in calm weather.

Hypolimnion is the dense, bottom layer of lakes. This is the coldest layer in summers and warmest layer in winters. The bottom-most waters of the hypolimnion are typically close to 4 °C throughout the year.

B. Nitrification occurs in the euphotic or epilimnion zone of a lake.

Anammox in lake ecosystems plays a considerable role in N loss, based on statistical analysis of anammox rates in lake riparian zones and open waters.

Denitrification removes N from the water ecosystem by converting nitrate into gaseous nitrogen and it mostly occurs in the anoxic zones of lakes.

Purple-sulfur bacteria are generally found in illuminated anoxic zones of lakes where hydrogen sulfide accumulates. They are also found in “sulfur springs” where geochemically or biologically produced hydrogen sulfide can trigger the formation of blooms.

Algae is mainly found in the euphotic zone and other aquatic plants can also thrive. This is also the typical area of dense fish populations because oxygen levels are typically higher due to contact with the air.

Nitrogen fixation by planktonic organisms is often important in the eutrophic lakes but generally not important in the nitrogen economies of mesotrophic and oligotrophic lakes. Thus, we can say that nitrogen fixation is not an important aspect in every lake ecosystem.

Sulfate Reduction  is usually not limited by the sulphate in surface layers, as the concentration exceeds the Km but limitation may occur deeper in the sediment. The zone with the most active sulphate reduction is found in the top 10 cm of sediment. The importance of sulphate reduction in the decomposition of organic matter in lakes depends on the amount of organic matter present and the availability of sulphate.