How do fertilizers such as nitrates and phosphates affect aquatic ecosystems?

Fertilizers containing nitrates and phosphates play a significant role in aquatic ecosystems primarily by contributing to eutrophication, a process that triggers algal blooms. When these nutrients enter water bodies, often due to runoff from agricultural fields, they promote the rapid growth of algae. This excessive algal growth can block sunlight, preventing aquatic plants from photosynthesizing and disrupting the ecosystem's balance.

Once the algae die, their decomposition is carried out by bacteria, a process that consumes substantial amounts of dissolved oxygen in the water. This reduction in oxygen levels can lead to hypoxic conditions, making it difficult for fish and other aquatic organisms to survive. Consequently, the overall health of the aquatic ecosystem is compromised, as the decreased oxygen levels can cause fish kills and a decline in biodiversity. Thus, the correct answer highlights the detrimental impact of fertilizers on aquatic ecosystems through their contribution to algal blooms and subsequent oxygen depletion.