Trophic control changes with season and nutrient loading in lakes

Experiments have revealed much about top‐down and bottom‐up control in ecosystems, but manipulative experiments are limited in spatial and temporal scale. To obtain a more nuanced understanding of trophic control over large scales, we explored long‐term time‐series data from 13 globally distributed lakes and used empirical dynamic modelling to quantify interaction strengths between zooplankton and phytoplankton over time within and across lakes. Across all lakes, top‐down effects were associated with nutrients, switching from negative in mesotrophic lakes to positive in oligotrophic lakes. This result suggests that zooplankton nutrient recycling exceeds grazing pressure in nutrient‐limited systems. Within individual lakes, results were consistent with a ‘seasonal reset’ hypothesis in which top‐down and bottom‐up interactions varied seasonally and were both strongest at the beginning of the growing season. Thus, trophic control is not static, but varies with abiotic conditions – dynamics that only become evident when observing changes over large spatial and temporal scales. We estimated time‐varying trophic interaction strength between zooplankton and phytoplankton in 13 globally‐distributed lakes over decadal timescales using empirical dynamic modelling. Trophic interactions were not static, but varied seasonally (within lakes) and with abiotic conditions (across lakes) ‐ nonlinear dynamics which are only evident when pairing long‐term observational data with cutting edge modelling techniques.