Megafire affects stream sediment flux and dissolved organic matter reactivity, but land use dominates nutrient dynamics in semiarid watersheds.

Climate change is causing larger wildfires and more extreme precipitation events in many regions. As these ecological disturbances increasingly coincide, they alter lateral fluxes of sediment, organic matter, and nutrients. Here, we report the stream chemistry response of watersheds in a semiarid region of Utah (USA) that were affected by a megafire followed by an extreme precipitation event in October 2018. We analyzed daily to hourly water samples at 10 stream locations from before the storm event until three weeks after its conclusion for suspended sediment, solute and nutrient concentrations, water isotopes, and dissolved organic matter concentration, optical properties, and reactivity. The megafire caused a ~2,000-fold increase in sediment flux and a ~6,000-fold increase in particulate carbon and nitrogen flux over the course of the storm. Unexpectedly, dissolved organic carbon (DOC) concentration was 2.1-fold higher in burned watersheds, despite the decreased organic matter from the fire. DOC from burned watersheds was 1.3-fold more biodegradable and 2.0-fold more photodegradable than in unburned watersheds based on 28-day dark and light incubations. Regardless of burn status, nutrient concentrations were higher in watersheds with greater urban and agricultural land use. Likewise, human land use had a greater effect than megafire on apparent hydrological residence time, with rapid stormwater signals in urban and agricultural areas but a gradual stormwater pulse in areas without direct human influence. These findings highlight how megafires and intense rainfall increase short-term particulate flux and alter organic matter concentration and characteristics. However, in contrast with previous research, which has largely focused on burned-unburned comparisons in pristine watersheds, we found that direct human influence exerted a primary control on nutrient status. Reducing anthropogenic nutrient sources could therefore increase socioecological resilience of surface water networks to changing wildfire regimes.

Diné citizen science: Phytoremediation of uranium and arsenic in the Navajo Nation.

Mid-20th century mining in Naabeehó Bináhásdzo (Navajo Nation) polluted soil and groundwater with uranium and arsenic. The Diné and other indigenous residents of this region use groundwater for drinking, livestock, and irrigation, creating a serious environmental health risk. Currently, many individuals and communities on the Navajo Nation must purchase and transport treated water from hours away. Sunflowers (Helianthus annuus) preferentially take up uranium and arsenic, potentially representing a tool to remove these contaminants through on-site, low-cost phytoremediation. This study reports the results of a collaboration among researchers, high school students, teachers, and tribal leaders to analyze water chemistry and perform a phytoremediation experiment. In 2018 and 2019, we compiled existing data from the Navajo Nation Environmental Protection Agency (NNEPA) and collected samples from surface and groundwater. We then used sunflower seedlings grown in local soil to assess whether phytoremediation could be effective at removing arsenic and uranium. For the NNEPA-sampled wells, 9.5% exceeded the maximum contaminant level for uranium (30 µg per liter) and 16% for arsenic (10 µg per liter). For the new samples, uranium was highest in surface pools, suggesting leaching from local soil. Unlike studies from humid regions, sunflowers did not decrease uranium and arsenic in soil water. Instead, there was no change in arsenic concentration and an increase in uranium concentration in both planted and control treatments, attributable to weathering of uranium-bearing minerals in the desert soil. Because much of global uranium mining occurs in arid and semiarid regions, the ineffectiveness of phytoremediation on the Navajo Nation emphasizes the importance of prevention and conventional remediation. More generally, the participatory science approach created meaningful relationships and an important collaboration between a tribal chapter and a university, providing both cultural and scientific experiential learning opportunities for Diné high school students, undergraduate researchers, and senior personnel.

Tundra wildfire triggers sustained lateral nutrient loss in Alaskan Arctic.

Climate change is creating widespread ecosystem disturbance across the permafrost zone, including a rapid increase in the extent and severity of tundra wildfire. The expansion of this previously rare disturbance has unknown consequences for lateral nutrient flux from terrestrial to aquatic environments. Lateral loss of nutrients could reduce carbon uptake and slow recovery of already nutrient-limited tundra ecosystems. To investigate the effects of tundra wildfire on lateral nutrient export, we analyzed water chemistry in and around the 10-year-old  Anaktuvuk River fire scar in northern Alaska. We collected water samples from 21 burned and 21 unburned watersheds during snowmelt, at peak growing season, and after plant senescence in 2017 and 2018. After a decade of ecosystem recovery, aboveground biomass had recovered in burned watersheds, but overall carbon and nitrogen remained ~20% lower, and the active layer remained ~10% deeper. Despite lower organic matter stocks, dissolved organic nutrients were substantially elevated in burned watersheds, with higher flow-weighted concentrations of organic carbon (25% higher), organic nitrogen (59% higher), organic phosphorus (65% higher), and organic sulfur (47% higher). Geochemical proxies indicated greater interaction with mineral soils in watersheds with surface subsidence, but optical analysis and isotopes suggested that recent plant growth, not mineral soil, was the main source of organic nutrients in burned watersheds. Burned and unburned watersheds had similar δ15 N-NO3 - , indicating that exported nitrogen was of preburn origin (i.e., not recently fixed). Lateral nitrogen flux from burned watersheds was 2- to 10-fold higher than rates of background nitrogen fixation and atmospheric deposition estimated in this area. These findings indicate that wildfire in Arctic tundra can destabilize nitrogen, phosphorus, and sulfur previously stored in permafrost via plant uptake and leaching. This plant-mediated nutrient loss could exacerbate terrestrial nutrient limitation after disturbance or serve as an important nutrient release mechanism during succession.

Stream Microbial Community Structured by Trace Elements, Headwater Dispersal, and Large Reservoirs in Sub-Alpine and Urban Ecosystems.

Stream bacterioplankton communities, a crucial component of aquatic ecosystems and surface water quality, are shaped by environmental selection (i.e., changes in taxa abundance associated with more or less favorable abiotic conditions) and passive dispersal (i.e., organisms' abundance and distribution is a function of the movement of the water). These processes are a function of hydrologic conditions such as residence time and water chemistry, which are mediated by human infrastructure. To quantify the role of environmental conditions, dispersal, and human infrastructure (dams) on stream bacterioplankton, we measured bacterioplankton community composition in rivers from sub-alpine to urban environments in three watersheds (Utah, United States) across three seasons. Of the 53 environmental parameters measured (including physicochemical parameters, solute concentrations, and catchment characteristics), trace element concentrations explained the most variability in bacterioplankton community composition using Redundancy Analysis ordination. Trace elements may correlate with bacterioplankton due to the commonality in source of water and microorganisms, and/or environmental selection creating more or less favorable conditions for bacteria. Bacterioplankton community diversity decreased downstream along parts of the stream continuum but was disrupted where large reservoirs increased water residence time by orders of magnitude, potentially indicating a shift in the relative importance of environmental selection and dispersal at these sites. Reservoirs also had substantial effects on community composition, dissimilarity (Bray-Curtis distance) and species interactions as indicated by co-occurrence networks. Communities downstream of reservoirs were enriched with anaerobic Sporichthyaceae, methanotrophic Methylococcaceae, and iron-transforming Acidimicrobiales, suggesting alternative metabolic pathways became active in the hypolimnion of large reservoirs. Our results identify that human activity affects river microbial communities, with potential impacts on water quality through modified biogeochemical cycling.

Mercury and dissolved organic matter dynamics during snowmelt runoff in a montane watershed, Provo River, Utah, USA.

The mechanisms of Hg and dissolved organic matter (DOM) transport from watersheds to streams remain unclear, especially in snowmelt dominated montane systems. We characterized total Hg concentrations and DOM characteristics during snowmelt by weekly and/or monthly sampling at three locations in the upper Provo River, northern Utah, over two water years (2016 and 2017). Total Hg concentrations increased from <1 ng/L during baseflow to >7 ng/L during the snowmelt period (April-June), with decreasing concentrations from upstream to downstream. Filtered THg concentrations accounted for 65-75% of the unfiltered concentrations, suggesting that Hg is primarily transported in the dissolved phase. Annual THg loading in the upper Provo River was approximately 1 kg/yr, with ~90% of the flux occurring during the snowmelt period. Filtered THg concentrations were strongly correlated with in-situ fluorescence DOM (fDOM) measurements, allowing for the development of high-resolution proxy THg concentrations. Further, DOM characteristics, evaluated using excitation-emission matrices (EEMs) and parallel factor analysis (PARAFAC), identified the dominance of soil humic and fulvic acid fractions of DOM during runoff. Total Hg concentrations were low in snowpack but elevated in ephemeral streams during snowmelt runoff, indicating that Hg originated from shallow soil water rather than snow. Concentration-discharge relationships revealed clockwise hysteresis patterns, suggesting that Hg was flushed from soils on the rising limb of the hydrograph. Our results demonstrate that a majority of Hg transport occurs with a flux of DOM during the short snowmelt season as shallow soils are flushed by meltwater. The snowmelt-driven Hg pulse is a substantial source to downstream reservoirs and potentially contributes to a fish consumption advisory.

Sediment potentially controls in-lake phosphorus cycling and harmful cyanobacteria in shallow, eutrophic Utah Lake.

Lakes worldwide are impacted by eutrophication and harmful algal or cyanobacteria blooms (HABs) due to excessive nutrients, including legacy P released from sediments in shallow lakes. Utah Lake (northern Utah, USA) is a shallow lake with urban development primarily on the east side of the watershed, providing an opportunity to evaluate HABs in relation to a gradient of legacy sediment P. In this study, we investigated sediment composition and P concentrations in sediment, pore water, and the water column in relation to blooms of harmful cyanobacteria species. Sediments on the east side of the lake had P concentrations up to 1710 mg/kg, corresponding to elevated P concentrations in pore water (up to 10.8 mg/L) and overlying water column (up to 1.7 mg/L). Sediment P concentrations were positively correlated with Fe2O3, CaO, and organic matter abundance, and inversely correlated with SiO2, demonstrating the importance of sediment composition for P sorption and mineral precipitation. Although the sediment contained <3% Fe2O3 by weight, approximately half of the sediment P was associated with redox-sensitive Fe oxide/hydroxide minerals that could be released to the water column under reducing conditions. Cyanobacteria cell counts indicate that blooms of Aphanizomenon flos-aquae and Dolichospermum flosaquae species tend to occur on the east side of Utah Lake, corresponding to areas with elevated P concentrations in the sediment, pore water, and water column. Our findings suggest that shallow lake eutrophication may be a function of P in legacy sediments that contribute to observed HABs in specific locations of shallow lakes.

Relationships of surface water, pore water, and sediment chemistry in wetlands adjacent to Great Salt Lake, Utah, and potential impacts on plant community health.

We collected surface water, pore water, and sediment samples at five impounded wetlands adjacent to Great Salt Lake, Utah, during 2010 and 2011 in order to characterize pond chemistry and to compare chemistry with plant community health metrics. We also collected pore water and sediment samples along multiple transects at two sheet flow wetlands during 2011 to investigate a potential link between wetland chemistry and encroachment of invasive emergent plant species. Samples were analyzed for a suite of trace and major elements, nutrients, and relevant field parameters. The extensive sampling campaign provides a broad assessment of Great Salt Lake wetlands, including a range of conditions from reference to highly degraded. We used nonmetric multidimensional scaling (NMS) to characterize the wetland sites based on the multiple parameters measured in surface water, pore water, and sediment. NMS results showed that the impounded wetlands fall along a gradient of high salinity/low trace element concentrations to low salinity/high trace element concentrations, whereas the sheet flow wetlands have both elevated salinity and high trace element concentrations, reflecting either different sources of element loading or different biogeochemical/hydrological processes operating within the wetlands. Other geochemical distinctions were found among the wetlands, including Fe-reducing conditions at two sites and sulfate-reducing conditions at the remaining sites. Plant community health metrics in the impounded wetlands showed negative correlations with specific metal concentrations in sediment (THg, Cu, Zn, Cd, Sb, Pb, Ag, Tl), and negative correlations with nutrient concentrations in surface water (nitrite, phosphate, nitrate). In the sheet flow wetlands, invasive plant species were inversely correlated with pore water salinity. These results indicate that sediment and pore water chemistry play an important role in wetland plant community health, and that monitoring and remediation efforts should consider pore water and sediment chemistry in addition to surface water chemistry.

Dust-mediated loading of trace and major elements to Wasatch Mountain snowpack.

Depth-integrated snow columns were collected at 12 sites across the central Wasatch Mountains, Utah, during March and April 2010 to determine concentrations of trace elements, major anions and cations, and pH. Sample collection was conducted at or near maximum snow accumulation prior to the onset of melt, and included spring dust events driven by southerly pre-frontal winds. Snow samples were melted in the laboratory and subsampled for analyses on filtered (0.45 µm) and unfiltered fractions. All measured elements (Al, As, Ba, Ca, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Na, Ni, Pb, Sb, Sr, Ti, Tl, U, V, and Zn) and major anions (Cl, NO(3), and SO(4)) displayed significant increases in concentration (for example, factor of 2 to 5 increases for As, Cr, Hg, and Pb) between the six sites sampled in March (prior to dust events) and the six sites sampled in April (after dust events). Acid neutralizing capacity and pH were also elevated in April relative to March snowpack. Comparison of elemental concentration in the particulate (>0.45 µm; difference between unfiltered and filtered concentration) and soluble (<0.45 µm; filtered concentration) fractions shows that the concentration increase between March and April snowpack for the trace elements is primarily a result of association with dust particles >0.45 µm. The results suggest that the majority of trace element loading to the Wasatch snowpack occurs via dust deposition. The major elements were primarily loaded in the <0.45 µm fraction, suggesting deposition of soluble dust particles. The overall findings of this paper are similar to other studies regarding the role of dust on nutrient and trace element accumulation in soils and lake sediments, but to our knowledge this is the first study that compares trace element chemistry of seasonal snowpack before and after dust deposition events.