The article discusses how scientists classify lakes based on their nutrient levels. They use different indicators like physical, chemical, and biological factors to determine if a lake is 'oligotrophic' (low nutrients) or 'eutrophic' (high nutrients). They created a Trophic State Index (TSI) to rank lakes on a scale from 0 to 100. By studying 29 Italian lakes, they found that some were 'oligotrophic', some were 'eutrophic', and others were 'mesotrophic' (moderate nutrients). This helps us understand the health of lakes and how human activities can affect them.