Long-term trends in study duration of lakes and reservoirs over recent decades

Owing to increased awareness of the value of long-term ecological studies, there have been numerous calls for long-term data. Natural lakes and reservoirs provide ecosystem services and serve as sentinels of change but differ in origin, water movement, and location. We conducted a meta-analysis of study duration for lake and reservoir studies from 1975 to 2015 (6362 studies were initially identified, and 801 met study criteria). Study duration increased among longer studies (> 0.5 quantile) with increases of 20 years for the highest (0.9) quantile, and there were no changes among short-term studies. Increased study duration was inversely related to number of focal areas or metrics (e.g., phytoplankton, zooplankton, vertebrates) with few focal areas or metrics (< 2 metrics) indicating few focal areas for long-term studies. Surprisingly, despite continued construction of reservoirs, the ratio of studies of lakes to reservoirs was 5 to 1 with no change over time, with the dearth of data on reservoirs making it difficult to characterize long-term trends. Increased duration in high quantiles concurrent with static duration of short-term studies suggests limnologists have fundamentally altered the way they approach questions that are influenced by study duration, but also that gaps remain in multifaceted studies and in reservoir studies.