What type of organism present in huge numbers indicates that eutrophication has taken place?

Eutrophication is a process that occurs when water bodies become enriched with nutrients, particularly nitrogen and phosphorus. This leads to excessive growth of certain organisms, particularly algae. In the case of eutrophication, the organism that typically indicates this process has taken place is algae, especially blue-green algae (cyanobacteria).

The growth of these algae can form algal blooms, which can deplete oxygen in the water and produce harmful toxins, severely affecting aquatic life and water quality. Following the algal bloom, the subsequent decay process can lead to hypoxic conditions (low oxygen levels) that can cause fish kills and negatively impact other aquatic organisms. Therefore, the presence of large quantities of algae in a water body is a clear indicator of eutrophication.