Lake Ilmen in European Russia was studied from 2003 to 2020 to understand its health. Scientists used two indicators to measure its trophic status: pH levels and oxygen saturation, as well as total phosphorus concentration. The lake was mostly eutrophic, meaning it had high nutrient levels, except for a few years when it was mesotrophic. The phosphorus levels varied over the years, with some years being hypertrophic or mesotrophic. The study suggests reducing human activities that add phosphorus to the lake to keep it healthy.