Non-contaminated lakes are “aerobic” (oxygenated) and maintain “oligotrophic” (very low levels of nutrients) condition. Limited number algae grow in such lakes due to limited amount of nutrient available to them. Such lakes are also suitable for the fish population and support aerobic microbes. Facultative anaerobic organism cannot survive in such environment.
A facultative anaerobic organism is an organism, usually a bacterium that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation. In contrast, obligate anaerobes die in the presence of oxygen.
Respitation: Aerobic CH2O + O2 = CO2+ H2O
Phosphate is commonly used as a fertilizer. Phosphate may be added to the lake through sewage, fertilizer and other sources. The excess nutrients stimulate the algal growth in the lake. The algae grow and die that becomes the food source for the aerobes. In such condition, the aerobe population in the lake multiplies exponentially consuming oxygen at a faster rate than it could diffuse through water. This leads to anaerobic condition in the lake. In such situation, facultative anaerobes thrive killing entire fish population of the lake.
Respiration: Anaerobic 2CH2O + SO4-2 = 2HCO3 + H2S
Eutrophication can be human-caused or natural. Untreated sewage effluent and agricultural run-off carrying fertilizers are examples of human-caused eutrophication. However, it also occurs naturally in situations where nutrients accumulate (e.g. depositional environments), or where they flow into systems on an ephemeral basis.