Most research on the effects of environmental change in freshwaters has
focused on incremental changes in average conditions, rather than fluctuations
or extreme events such as heatwaves, cold snaps, droughts, floods
or wildfires, which may have even more profound consequences. Such
events are commonly predicted to increase in frequency, intensity and duration
with global climate change, with many systems being exposed to
conditions with no recent historical precedent. We propose a mechanistic
framework for predicting potential impacts of environmental fluctuations
on running-water ecosystems by scaling up effects of fluctuations from individuals
to entire ecosystems. This framework requires integration of four key
components: effects of the environment on individual metabolism, metabolic
and biomechanical constraints on fluctuating species interactions,
assembly dynamics of local food webs, and mapping the dynamics of the
meta-community onto ecosystem function. We illustrate the framework by
developing a mathematical model of environmental fluctuations on dynamically
assembling food webs. We highlight (currently limited) empirical
evidence for emerging insights and theoretical predictions. For example,
widely supported predictions about the effects of environmental fluctuations
are: high vulnerability of species with high per capita metabolic demands
such as large-bodied ones at the top of food webs; simplification of food
web network structure and impaired energetic transfer efficiency; and
reduced resilience and top-down relative to bottom-up regulation of food
web and ecosystem processes. We conclude by identifying key questions
and challenges that need to be addressed to develop more accurate and predictive
bio-assessments of the effects of fluctuations, and implications of
fluctuations for management practices in an increasingly uncertain world.