The rivers and streams of California's Sierra Nevada form a vital, interconnected freshwater network that supports hundreds of aquatic invertebrate species along with other organisms. These species rely on the ecosystem's rain- and snow-fed waters for habitat, migration, and survival. When the snow—which functions as a frozen reservoir—begins to melt in late spring, the stream water winds its way down the mountain and reaches rivers, reservoirs, cities, and farms across California.
But as the planet warms, scientists expect two things: less snow will fall in the Sierra Nevada, and any snow that does accumulate will melt earlier or faster. "What we would currently consider an exceptional drought year, like 2015 or 2021, might be normal by the end of the century," said Kyle Leathers, PhD '24 Environmental Science, Policy, and Management (ESPM).
Leathers, now a postdoctoral researcher with the US Geological Survey, spent years studying how Sierra Nevada streams may change under a hotter, drier climate as a graduate student in the lab of Associate Professor Albert Ruhi. Their study, published in the most recent issue of Ecological Monographs, has revealed that the ecological impacts of drought are far more complex and context-dependent than previously understood. Their findings also cast doubt on an assumption used to predict future climate impacts: that today’s communities in warm sites foreshadow what communities will look like in the cooler, upstream areas of a watershed once they are impacted by future climate change.
"Waiting to see what happens in response to climate change is costly and takes a lot of time, so researchers will often compare two relatively similar sites as a shortcut," explained Ruhi, the study's senior author. "They'll look at one site that has experienced warming and predict that the second site will experience similar changes under future warming. However, when studying rivers, this shortcut doesn't really work."
For their study, Leathers, Ruhi, and their collaborators focused on 60 locations along Bull Creek within the Kings River Experimental Watersheds, an area where roughly 70% of precipitation falls as snow and whose hydrologic conditions have been closely monitored by researchers since 2002. The researchers were able to draw on more than 20 years of long-term data and observations, including samples of stream invertebrates collected during some of California's wettest and driest years on record. Data on water temperature, a common factor studied by aquatic ecologists, were analyzed alongside measurements of water velocity and fine sediment levels to determine whether a single factor—or a combination of factors—associated with drought most strongly influences the composition of aquatic invertebrate communities.
"A lot of studies about climate impacts focus on either changes in water temperature or the complete drying of a stream," explained Leathers, the study's lead author. "But doing that might obscure other ways that streams and rivers are affected."
All three drought factors—not just water temperature—were found to influence the abundance of over one-third of invertebrate species sampled within the river. Interestingly, the authors noted that each factor had a different impact when examined across space or over time. Drought-linked increases in water temperature, for instance, led to greater species abundance across sites. But when tracked over time, the same factor was linked to decreases in species abundance. Similarly, levels of fine sediment had a significant impact on seven invertebrate species across space upstream and downstream, but only a temporary impact on one species over the years.
"We thought there would be some similarity, so it was surprising to find that they differed in every aspect," said Leathers. "What that means is that if you just look at how species respond across space and try to extrapolate those findings for the future, you will probably get inaccurate results."
By working with long-term data, the authors were able to track exactly how the composition of aquatic invertebrates has changed under specific drought conditions. Some sites on Bull Creek experienced higher levels of community turnover, where species more sensitive to climate effects were replaced by hardier organisms usually found at lower elevations. Other sites had considerable reordering in their communities, where species that were once rare became more abundant.
Ruhi said their findings demonstrate the value of long-term, carefully crafted monitoring programs when attempting to make climate predictions. "By having 20 years of data, we were able to capture so many extreme events and different conditions that people couldn't anticipate," he said. "Even if it is costly, collecting long-term data sets is incredibly important—the value only increases every time we add a new data point."
Additional co-authors include David Herbst, a research biologist with the Sierra Nevada Aquatic Research Laboratory and UC Santa Cruz; University of Arizona professor Michael Bogan; and Gabriela Jeliazkov, BS '23 Molecular Environmental Biology.
Read the full study in Ecological Monographs, and learn more about the Ruhi Lab's research.
