The type of organism present in huge numbers that indicates eutrophication has taken place is phytoplankton. Phytoplankton, including algae, bloom excessively in aquatic ecosystems when there is an overabundance of nutrients, particularly nitrogen and phosphorus. This algal bloom is a key sign of eutrophication and leads to ecological imbalances such as oxygen depletion and loss of biodiversity. The excessive growth of phytoplankton due to nutrient enrichment is a primary characteristic of eutrophication in water bodies.