Describe stratification of environmental factors in lakes, and how this changes with eutrophication. Mention how eutrophication changes the location and abundance of different types of microorganisms living in lake systems.

Question

What will be an ideal response?

Answer 1

The depth profile of an oligotrophic lake includes the oxygenated zone (epilimnion) reaches to the thermocline, below which only anaerobes grow. Large amounts of nutrients can be deposited from runoff from agricultural fertilizer or septic systems
These nutrients include phosphates, nitrates, and organic pollutants. Because phosphorus is commonly a limiting nutrient (nutrient in shortest supply) for algae, addition of phosphates from detergents and fertilizers can lead to an algal bloom. Nitrogen from sewage effluents and agricultural fertilizer runoff can lead to algal blooms as well, by relieving nitrogen limitation of growth (sometimes, algae and cyanobacteria are limited by the availability of nitrogen). Organic pollutants from sewage effluents overfeed heterotrophic bacteria, depleting the epilimnion of oxygen.

In a eutrophic lake, the oxygenated epilimnion is much shallower, and microbial concentrations are tenfold higher than in an oligotrophic lake. This oxygen loss causes the anoxic hypolimnion to reach nearly up to the surface of the lake.

In general throughout lakes, as depth increases, minerals become increasingly reduced. Anaerobic forms of metabolism predominate, with microbes using various alternative electron acceptors.