Interactive Effects of Elevated Salinity and Heatwaves on Freshwater Zooplankton: From Community Ecology to Individual Physiology

Abstract

Freshwater ecosystems are being disturbed by multiple new and ongoing stressors that often occur simultaneously or asynchronously. Among those stressors, fluctuations in salinity levels and heatwaves that are becoming more frequent and intensified can individually and interactively impact freshwater organisms. However, we lack a good understanding of their joint impacts and how the temporal fluctuation of these stressors affects the impacts. To help fill this gap, I conducted field and lab experiments using freshwater zooplankton to investigate their interactive effects on communities, populations, and physiological conditions. My results show that the joint effects of elevated salinity and heatwaves vary at different scales: antagonism (i.e., the joint effect is less than the sum of each individual effect) at the community level, mainly driven by species compositional changes; no interaction at the population level, caused by evolutionary changes; synergism (i.e., the joint effect is greater than the sum of each individual effect) at the individual level, due to physiological responses. I also found that the interactions between stressors change with different time intervals between them, because of the legacy effect of the prior stressor or population and community recovery processes. My work contributes to the area of multiple-stressor interaction by showing the joint effects of elevated salinity and heatwave conditions at different levels of biological organization. The findings demonstrate that ecosystem management and restoration efforts that commonly just address the effect of a single stressor should consider prior stressor exposure, and the combined effects of two asynchronously occurring stressors can be more accurately predicted when accounting for the time intervals between the exposures. Moreover, the upscaling of lower-level (physiological and individual) results to the community or ecosystem level, which is sometimes used in the management of ecosystems, should be done with caution since there can be inconsistency in responses across ecological scales.