Final answer:
Excess levels of nitrogen and phosphorus in lakes can cause algal blooms, reducing light penetration and depleting oxygen when the algae decay, leading to the death of aquatic life and creation of dead zones.
Step-by-step explanation:
Plants and fish influence the pH of a lake through their roles in the ecosystem's nitrogen and phosphorus cycles. Excess nitrogen and phosphorus, which are important limiting nutrients, can lead to the proliferation of algae and phytoplankton. This phenomenon, known as an algal bloom, can have several detrimental effects on the aquatic environment.
When algal blooms occur, they can become so dense that they significantly reduce light penetration. This causes lakes and ponds to become aphotic zones, negatively affecting photosynthetic plants that require light to survive. Additionally, as algae die and decompose, they deplete the water's oxygen, resulting in severe oxygen shortages that can kill fish and other aquatic organisms, a process called eutrophication. This can lead to the creation of 'dead zones' where aquatic life struggles to survive, as seen in places like Lake Erie and the Gulf of Mexico.