Ecological Memory Contributes to a Spatial Mismatch in Water Quality Trends Between the Surface and Bottom Waters of 382 Temperate Lakes.

Ecosystem states are often influenced by both concurrent and antecedent environmental drivers. However, the relative importance of antecedent conditions varies within and among ecosystems. Here, we analysed long-term depth-profile data from 382 temperate lakes across 10 countries to assess how differential changes in spring versus summer air temperature mediate summer water quality. We found that summer bottom-water conditions were more associated with spring air temperatures, while surface-water conditions were more associated with summer air temperatures. The relative influence of spring versus summer air temperature was mediated by lake morphometry, stratification and latitude. Across these lakes, summer air temperatures have increased more rapidly than spring air temperatures, potentially contributing to a growing thermal difference between surface and bottom waters (median = +0.5°C/decade). Consequently, our results demonstrate that predicting the ecological impacts of climate change may require considering spatial differences in ecological memory within ecosystems.