Long-Term Trends in Productivity Across Intermountain West Lakes Provide No Evidence of Widespread Eutrophication

Eutrophication represents a major threat to freshwater systems and climate change is expected to drive further increases in freshwater primary productivity. However, long-term in situ data is available for very few lakes and makes identifying trends and drivers of eutrophication challenging. Using remote sensing data, we conducted a retrospective analysis of long-term trends in trophic status among lakes greater than 10 ha across the Intermountain West, a region with understudied water quality trends and limited long-term data sets. We found that most lakes (55%) were not exhibiting shifts in trophic status from 1984 to 2019. Our results also show that increases in eutrophication were rare (3% of lakes) during this period, and that lakes becoming increasingly oligotrophic were more common (17% of lakes). Lakes that were not trending occupied a wide range of lake and landscape characteristics, whereas lakes that were becoming more oligotrophic tended to have larger residence times and were located in catchments with greater clay content and more development. Our results highlight that while there are well-established narratives that climate change can lead to more eutrophication of lakes, this is not broadly observed in our data set, where we found more lakes in the Intermountain West becoming more oligotrophic than lakes becoming eutrophic. Lakes are often classified by their productivity. Low productivity lakes are generally deep with low nutrient levels and low amounts of algae. Whereas lakes with high levels of productivity support more plant growth and have higher amounts of algae. The accumulation of nutrients in freshwater systems often results in increases in productivity and can lead to the development of algal blooms. Algal blooms are a major concern due to their threat to ecosystem health, recreation, and drinking water sources. Yet the lack of long-term field data across large scales has resulted in a limited understanding of (a) what are the long-term trends in lake productivity and how does this relate to trends in algal blooms, and (b) what factors are driving productivity trends and the development of algal blooms across regions. Therefore, there is a pressing need to effectively monitor and understand these trends in order to inform management actions that address their frequency and intensity. Here, we use data obtained from satellite imagery from 1984 to 2019 to document lake productivity trends in 1,169 lakes across the Intermountain West. We show that substantial increases in productivity over this time period were rare, and that the majority of lakes have not undergone widespread change. Remote sensing imagery captures long-term trends in lake productivity across the Intermountain West The majority of lakes observed in this data set were not exhibiting shifts in trophic status from 1984 to 2019 The incorporation of fine-scale lake climate data from new deep learning data sets results in substantial improvement to model accuracy