Final answer:
The amount of biochemical oxygen demand (BOD) increases in the water when all the plants begin to die, leading to oxygen depletion that can harm aquatic life.
Step-by-step explanation:
When all the plants begin to die in a body of water, the amount of biochemical oxygen demand (BOD) in the water begins to increase. This happens because bacteria in the water decompose the dead organic matter, a process that consumes dissolved oxygen (O2), leading to decreased levels of oxygen in the water. The major source of dead organic matter in natural waters is typically sewage, but plant debris can also contribute to this, especially during massive die-offs. If the BOD is high, it indicates that the water body is polluted, potentially causing harm to aquatic life due to oxygen depletion. This creates a negative impact on species that rely on higher oxygen levels, such as trout, and may favor species that can tolerate lower levels, like carp and catfish.
Furthermore, excessive nutrients, especially nitrogen (N) and phosphorus (P), from decaying plants can lead to algal blooms. As algae die and decompose, they further consume oxygen, exacerbating hypoxic conditions. These conditions can lead to fish kills and disrupt the ecosystem. Thus, an increase in decomposing plant matter can have multiple detrimental effects on the water quality and aquatic life.