In August of 2014, residents of several Ohio cities that use Lake Erie as a drinking water source were warned not to drink tap water because of the presence of toxins released by blue-green algae (cyanobacteria). Which of the following is the most likely cause of recent blooms of blue-green algae in Lake Erie? Increased recreational use of Lake Erie Increased recreational use of Lake Erie A Cooler average summer temperatures Cooler average summer temperatures B Phosphorus contained in agricultural runoff Phosphorus contained in agricultural runoff C Carbon in wastewater from fracking operations Carbon in wastewater from fracking operations D Chlorine from sewage treatment plants along Lake Erie’s shoreline

Phosphorus contained in agricultural runoff

Explanation:

The term eutrophication refers to the enrichment of surface water with nutrients. This process is related to human pollution activities.

Eutrophication is the biological process that generates in water by an oversupply of nutrients or organic matter. This matter, mainly compounded with nitrogen or phosphorous, favors the phytoplankton multiplication (microscopic plants growing on the water surface) and other aquatic plants. This overgrowth leads to an enhance in primary productivity. Nitrogenated compounds might come from human and animal wastes, while phosphorated compounds might come from fertilizers and other chemicals used in agriculture, that penetrates the soil and reaches water bodies.  

Little by little begin sedimentation is caused by the death of some vegetable organisms that sink in the bottom. These sediments rich in organic matter suffer decomposition in the presence of oxygen.

When oxygen disappears, anaerobic bacteria act and produce fermentation.

On the surface, plants and phytoplankton keep growing, creating a mantle that impedes the light reach deeper areas. The phytoplankton overgrowth on the water surface and the fermentation processes in the bottom originate toxic compounds that damage the animals. Animals die, creating more sediments in the bottom where there are anaerobic conditions.  

Decomposition and fermentation processes increase, producing a bad smell.

The result of eutrophication is the stratification of the water mass:

• First superficial layer, the overgrowth of blue-green algae, produces toxins and interrupts the pass of light to deeper areas. The surface then becomes warmer.

• The second layer might have oxygen available for the animals.

• In the third layer, there is no oxygen, so life is impossible for animals or plants.

• In the fourth layer, there are animals and plant remains in the process of sedimentation.

• At the bottom, there are organic matter and anaerobic bacteria in charge of decomposition and fermentation, through which they produce toxic gases and smell.