A phenomenological approach shows a high coherence of warming patterns in dimictic aquatic systems across latitude

Wagner A, Hülsmann S, Paul L, Paul RJ, Petzoldt T, Sachse R, Schiller T, Zeis B, Benndorf J, Berendonk TU

Abstract

To predict the coherence in local responses to large-scale climatic forcing among aquatic systems, we developed a generalized approach to compare long-term data of dimictic water bodies based on phenomenologically defined hydrographic events. These climate-sensitive phases (inverse stratification, spring overturn, early thermal stratification, summer stagnation) were classified in a dual code (cold/warm) based on threshold temperatures. Accounting for a latitudinal gradient in seasonal timing of phases derived from gradients in cumulative irradiation (2.2days per degree latitude), we found a high spatial and temporal coherence in warm-cold patterns for six lakes (84%) and the Baltic Sea (78%), even when using the same thresholds for all sites. Similarity to CW-codes for the North Sea still was up to 72%. The approach allows prediction of phase-specific warming trends and resulting instantaneous or time-delayed ecological responses. Exemplarily, we show that warming during early thermal stratification controls food-web-mediated effects on key species during summer.