The strength of top-down (TD) and bottom-up (BU) interactions are traditionally evaluated through manipulative experiments, but how trophic control varies through time in response to natural environmental fluctuations and across larger spatio-temporal scales is an open question. In this study, we used monthly time series data from 13 lakes (ranging from oligotrophic to eutrophic) to quantify how interactions between phytoplankton (chl-a) and herbivorous zooplankton varied across lakes and through time within lakes. We analyzed the data using empirical dynamic modeling (EDM) to infer how chl-a and zooplankton growth rates vary in response to past chl-a and zooplankton abundances. This methodology produces estimates of TD and BU interaction strengths at each timepoint. We explored how these interaction strengths varied with nutrient enrichment, temperature, and other abiotic and biotic variables.
Results/Conclusions
Across all lakes, trophic control strength was associated with nutrient levels. In mesotrophic lakes, zooplankton had negative effects on phytoplankton, but in oligotrophic lakes, zooplankton stimulated phytoplankton growth. This suggests that nutrient recycling by grazers may be important for phytoplankton growth in nutrient-limited systems. Within individual lakes, we found that trophic control strength varied seasonally. The results were consistent with a “seasonal reset” hypothesis, in which TD and BU interaction strengths are both greatest in magnitude at the beginning of the growing season and decline as the season progresses due to successional processes and changing abiotic conditions. Overall, our results indicate that trophic control is not static, but varies seasonally and with abiotic conditions - dynamics that only become evident when observing changes over large spatial and temporal scales.