No environmental threat may be as ubiquitous as climate change. Climate change is especially concerning in the Sierra Nevada because warmer air temperatures reduce snowpack and increase the length of summer droughts, threatening cold-adapted taxa in isolated streams. Extended low flows may raise water temperatures due to reduced thermal buffering alongside rising air temperatures, but low flows may also affect ecosystems via reduced water velocity, increased deposition of fine sediment, and reduced dissolved oxygen. Detailed study of the mechanisms and context of low-flow effects on stream communities is needed to anticipate and appropriately react to climate change-enhanced droughts in mountain streams–and more broadly, to inform river ecosystem conservation and restoration (Palmer and Ruhi 2019). Discovering how low flows mechanistically affect Sierra Nevada stream ecosystems requires integrating results across approaches, scales, contexts, and end points. In this pursuit, we combine experimental, observational, and modeling methods to incorporate causation with realism and foresight. Our research seeks to further our understanding of low flow effects, but the approaches and methods used are broadly effective at exploring the effects of other stressors in flowing waters.
We have performed multiple experiments examining how ecosystem processes and communities may respond to the extended low flows expected by the end of the century. This work has taken place at the Sierra Nevada Aquatic Research Laboratory (SNARL), where we manipulated the discharge of nine flow-through, outdoor stream channels fed by an adjacent Sierra Nevada stream. Inducing low flow conditions three weeks or six weeks prior to the current seasonality increased water temperature by 4.6-7.5 degrees Celsius and resulted in reduced biofilm production relative to respiration. Over half of influential invertebrate taxa changed the timing of their life cycle and the dominant insect taxa decreased in body size but nearly doubled in abundance. Changes in both invertebrate community structure (composition) and functioning (production) were mostly fine-scale, and response diversity at the community level stabilized seasonally aggregated responses. Stable isotope analysis on the macroinvertebrate community, focusing on predatory macroinvertebrates, found that extended low flows often compressed predator niches, suggesting that low flows will likely shift invertebrate predator diets, and thus food-web structure. A followup experiment testing the combined effects of low flow and nonnative fish found that the effects on macroinvertebrates and algae were additive. Invasions may profoundly alter stream food webs, as nonnative fish presence nearly doubled benthic algal biomass and increased the abundance of small macroinvertebrates
Ongoing climate change threatens to drastically increase stream water temperature means and extremes—particularly in ecosystems located in high altitudes or latitudes. Models predicting future stream water temperatures may fall short due to assumptions that the relationship between water and air temperature is static. We deployed over 100 high-frequency temperature sensors in a Sierra Nevada watershed and used advanced time series models to understand the controls and scales of aquatic habitat vulnerability to warming. Air temperature had the greatest influence on water temperature in the late spring and summer, although upstream water temperatures were the main driver of local conditions. We found that 26% of coldwater habitat in the study watershed could be lost by the end of the century because of rising air temperatures. This highlights that mountain streams could lose coldwater species and rely on immigration of warmwater species to maintain biodiversity and ecosystem functions.
Stream low flows can alter communities via multiple environmental and biological mechanisms, but their relative importance is uncertain (Hawkins et al. 1997, Waddle and Holmquist 2013, Herbst et al. 2019). Further, it is unclear whether drought-induced community change across space and over time are realized through similar environmental and biological processes (Angert 2024). We sampled macroinvertebrates in a river network in California’s Sierra Nevada encompassing 60 sites combined with long-term data in four reaches to address these uncertainties. The effects of abiotic mechanisms like water temperature and fine sediment differed across space and time, and varied from taxa to taxa. With regards to biological mechanisms of community change, we found that community dissimilarity across space was driven by differences in fine sediment causing species turnover (i.e., sensitive species being replaced by tolerant ones), while temporal dissimilarity was driven by differences in temperature and water velocity causing reordering (i.e., shifts in relative abundance). Our results challenge the key assumption of ‘space-for-time’ substitution that underpins abundant research on climate change ecology.
