Background: Fire plays an important role in controlling the cycling and composition of organic matter and nutrients in terrestrial and aquatic ecosystems. In this study, we investigated the effects of wildfire severity, time since fire, and site-level characteristics on (1) concentration of multiple solutes (dissolved organic carbon, DOC; total dissolved nitrogen, TDN; dissolved organic nitrogen, DON; calcium, Ca 2+ ; magnesium, Mg 2+ ; potassium, K + ; sodium, Na + ; chloride, Cl − ; nitrate, NO 3 − ; ammonium, NH 4 + ; sulfate, SO 4 2− ; and phosphate, PO 4 3−), and (2) the molecular composition of stream-dissolved organic matter (DOM) across 12 streams sampled under baseflow conditions in Yosemite National Park, California, USA. Samples were collected from low-and high-severity burned stream reaches, as well as an unburned reference stream reach. Results: Fire severity, time since fire, and variability in site-level characteristics emerged as the strongest influences on streamwater chemistry. Results from mixed-effect models indicated that DOC and DON concentrations decreased with time since fire in high-severity burned stream reaches. In low-severity burned stream reaches, DOC concentrations increased, and DON concentrations slightly decreased with time since fire. We also found that declines in aromaticity (expressed as decreased SUVA 254) and mean molecular weight DOM (expressed as increased E 2 :E 3 ratios) with time since fire were associated with high-severity fires. Mixed-effect models also indicated that site-level characteristics played a role in solute responses. Aliphatic structures dominated streamwater DOM composition across fire-impacted catchments, but neither fire severity nor time since fire was a significant predictor of the proportion of aliphatic structures in streamwater DOM. North aspect exhibited the highest concentrations of Ca 2+ , K + , and Mg 2+ , whereas the north-northwest aspect exhibited the highest concentrations of Cl − and SO 4 2+. We also observed elevated Ca 2+ , K + , and Mg 2+ in burned (but not reference) stream reaches with pool-riffle versus step-pool bed morphology. Conclusions: Taken together, our findings suggest that the response of stream chemistry to wildfires in the Sierra Nevada, California, can persist for years, varying with both fire severity and site-specific characteristics. These impacts may have important implications for biogeochemical cycles and productivity in aquatic ecosystems in fire-adapted landscapes.

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Disturbance can play an important role in structuring stream food webs. Although floods have received the greatest attention as a disturbance agent in rivers, wildfire — which can strongly influence fluvial ecosystem structure and function — may also drive consumer trophic dynamics. We measured the relative effects of wildfire, hydrologic disturbance, ecosystem size, and canopy openness (as a proxy for in-stream productivity) on trophic position and reliance on aquatically-derived nutritional subsidies of riparian spiders of the family Tetragnathidae along two rivers on the west slope of the Sierra Nevada in California, USA. Ecosystem size received strong support as an environmental determinant of both trophic measures, with variability in flood magnitude emerging as an important mechanism linking ecosystem size and trophic responses. Piecewise linear regression revealed significant breakpoints in spider trophic position and reliance on aquatically-derived nutritional subsidies that were related to thresholds in fire extent within the catchment. These nonlinear relationships with wildfire may lend additional insight into the potential interactions among ecosystem size, productivity, and disturbance that determine stream–riparian food-web architecture.

River regulation can modify natural flow regimes with deleterious effects on aquatic communities. While the effects of flow manipulation on the physical environment and populations and assemblages of aquatic organisms have been described thoroughly, how and to what extent river regulation influences ecosystem processes like food web architecture is less studied. Emergent aquatic insect prey can provide an important food resource to riparian consumers like birds and bats with concomitant consequences for nutrient cycling through aquatic-terrestrial food webs, thus potentially increasing the spatial influence of river regulation into the riparian zone and beyond. We used naturally abundant stable isotopes of carbon and nitrogen to compare food web architecture (trophic position and reliance on an aquatic nutritional pathway) leading to birds and bats between a regulated river, the Tuolumne River downstream of Hetch Hetchy Reservoir, and an adjacent unregulated river, the Merced River, located in Yosemite National Park on the west slope of the Sierra Nevada, California, USA. We found that both birds and bats derived >50% of their nutrition from food webs originating in photosynthesis by algae. In addition, birds and bats occupied a similar trophic position to predatory fish in other systems. Both birds and bats seemed to rely more strongly on an aquatic nutritional pathway during the dryer year of our study period, underscoring the potential importance of emergent aquatic prey as a water subsidy in dry systems and in dry years. In the Tuolumne River, reservoir managers strive to simulate characteristics of the natural flow regime, including seasonal scouring flows and prolonged floodplain inundation. Although we found no conclusive evidence of an effect of river regulation on food web responses, our study suggests that nutrient cycling through aquatic-terrestrial food webs expands the potential influence of river regulation to organisms and ecosystems typically characterized as terrestrial.