Trophic State Evolution of 45 Yellowstone Lakes over Two Decades: Field Data and a Longitudinal Study

From 1998 to 2024, we collected field samples at 45 selected lakes in Yellowstone National Park during the months of April through October. We estimated inflows, outflows, and Secchi depths for most lakes. We analyzed the samples for total phosphorous and chlorophyll-a. We used these data to classify the lake trophic states using the Carlson TSI (CTSI), Vollenweider (VW), and Larsen–Mercier (LM) models to assess how trophic states evolved over this 26-year period. This longitudinal dataset is unique because of its extensive 26-year time span gathered from difficult-to-access locations. We found that the data depended on lake size, lake elevation, and the month when data were collected. Most of the lakes exhibit mesotrophic conditions, with variations depending on the trophic state model used. The CTSI distribution shows median values typically between 40 and 55, while the VW and LM index distributions present a somewhat similar pattern but with fewer lakes categorized due to data requirements. We visualized temporal patterns using heatmaps and analyzed trends using the Mann–Kendall test to identify trends and if they were statistically significant. We found only four lakes with statistically significantly increasing trends and two with decreasing trends. Because of the difference in the months when data were collected, the increasing trends in three of the lakes are less certain. We found that, except for four lakes, the trophic states of Yellowstone lakes were maintained or improved over this ~20-year period. Only the trophic state of Nymph Lake clearly deteriorated. The remaining lakes had stable trophic states, with three having weak evidence of worsening conditions. This long-term dataset, which we publish for others’ use, provides an opportunity to better understand eutrophication processes and water quality dynamics in Yellowstone, providing critical information for park management and conservation efforts.