What is a key byproduct of cultural eutrophication that can negatively affect aquatic ecosystems?

Oxygen depletion is a significant byproduct of cultural eutrophication, primarily caused by the rapid growth of algae in aquatic systems due to excess nutrient runoff, particularly nitrogen and phosphorus from agricultural fertilizers and wastewater. As these algae flourish, they eventually die and decompose, a process that consumes a considerable amount of dissolved oxygen in the water.

As oxygen levels drop, it can lead to hypoxic (low oxygen) conditions or anoxic (no oxygen) conditions, creating a hostile environment for aquatic life. Many fish and other aquatic organisms require adequate oxygen levels to survive; thus, oxygen depletion can result in fish kills and a decline in biodiversity. This disruption can have cascading effects throughout the food web, ultimately altering the balance of the ecosystem and impacting overall water quality.

The other options do not directly relate to the processes associated with cultural eutrophication in the same manner. Increased salinity, heavy metal accumulation, and thermal pollution involve different mechanisms and sources of pollution, whereas oxygen depletion is directly tied to the nutrient overload that characterizes cultural eutrophication.