Question

The photosynthetic reaction and its reverse reaction, respiration, demonstrate how closely linked oxygen-breathing animals are to CO2-breathing plants. In fact, the respiration reaction shows that plants "respire" (that is, take in O2 similarly to humans) once the sun goes down. This mechanism helps explain large "fish kills" that can happen overnight in freshwater lakes adjacent to farmland that is heavily fertilized.

Scenario - You are a forest ranger in Lake Winnipissaukee National Park, which is surrounded by farmland. You wake up one morning to find the park's lake covered with dead fish floating at the surface. You recently measured a high concentration of NO3, a nutrient that can spur plant growth, and you suspect that this is linked to the fish kill.

Your Task: Explain how the nutrients draining into the lake from the fertilized farmland began a cycle of rapid plant growth that eventually led to the deaths of all the fish.

Answer 1

Excess nutrients draining into the lake from fertilized farmlands increases rapid growth of plants within the lake. This results in dense growth of aquatic plants. These plants absorb oxygen for respiration and the quanity of dissolved oxygen (D.O.) in lakewater decreases. Other aquatic animals inside the lake such as different types of fishes also need oxygen for survival. This increases the biochemical oxygen demand (B.O.D.). Due to gradual decrease in dissolved oxygen these fishes can't breathe and that eventually leads to the deaths of fishes.