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.

Human modification of water and nutrient flows has resulted in widespread degradation of aquatic ecosystems. The resulting global water crisis causes millions of deaths and trillions of USD in economic damages annually. Semiarid regions have been disproportionately affected because of high relative water demand and pollution. Many proven water management strategies are not fully implemented, partially because of a lack of public engagement with freshwater ecosystems. In this context, we organized a large citizen science initiative to quantify nutrient status and cultivate connection in the semiarid watershed of Utah Lake (USA). Working with community members, we collected samples from ~200 locations throughout the 7,640 km2 watershed on a single day in the spring, summer, and fall of 2018. We calculated ecohydrological metrics for nutrients, major ions, and carbon. For most solutes, concentration and leverage (influence on flux) were highest in lowland reaches draining directly to the lake, coincident with urban and agricultural sources. Solute sources were relatively persistent through time for most parameters despite substantial hydrological variation. Carbon, nitrogen, and phosphorus species showed critical source area behavior, with 10–17% of the sites accounting for most of the flux. Unlike temperate watersheds, where spatial variability often decreases with watershed size, longitudinal variability showed an hourglass shape: high variability among headwaters, low variability in mid-order reaches, and high variability in tailwaters. This unexpected pattern was attributable to the distribution of human activity and hydrological complexity associated with return flows, losing river reaches, and diversions in the tailwaters. We conclude that participatory science has great potential to reveal ecohydrological patterns and rehabilitate individual and community relationships with local ecosystems. In this way, such projects represent an opportunity to both understand and improve water quality in diverse socioecological contexts.

Extended AbstractThe introduction of the invasive perennial grass Phragmites autralis in the 1980s has dramatically impacted the ecosystem of Utah Lake. This invasive species has choked out native plants, reducing biodiversity and decreasing the aesthetic value of the lake. State legislators have thus allocated significant funding for its elimination. The current method of removal involves aerial application of glyphosate-based herbicides followed by mowing, leaving the roots in the sediment. However, studies have shown that P. australis plants sequester trace metals in their roots. Thus, management in this fashion only recycles the contaminants into the lake, even potentially worsening the water quality by introducing herbicides to the system. While it is important to control proliferation of P. autralis for ecosystem stability, its removal must be done holistically and thoughtfully. We hypothesize that trace metal concentration in sediments and water in locations where herbicide has been applied will be increasingly higher with time due to the slow decomposition of plant biomass relative to locations where Aqua Neat has not been applied, thereby reducing water quality.P. australis, sediment and water samples will be collected from eight sites selected at random surrounding Utah Lake, including both treated and untreated areas for a period of 5 months. Sediment core samples (0-90cm) taken from each location will be divided into 15 cm increments and each increment composited for their respective location. Five replicate samples will be taken at each site. All samples will be prepped for acid microwave digestion, filtered and analyzed for trace metal content using the ICP-OES. Samples will be sent to the Utility Testing Lab in Salt Lake City for herbicide concentration determination. To understand the behavior of trace elements in each respective site, parameters such as temperature, pH, organic matter (OM), electrical conductivity, redox potential, dissolved oxygen, particle size distribution, total nitrogen, and total phosphate will be determined.Rapid proliferation of the invasive P. australis is not only a local issue in Utah Lake, it is a continent-wide environmental concern as this species has invaded wetlands and most disturbed habitats across North America. Its aggressive distribution has been attributed to its ability to grow in soils with a wide range of pH, salinity, soil textures and in extreme environmental conditions. Although various approaches have been used to address the threats produced by this invasive plant, the manner in which the unintended consequences of P. australis control is addressed in Utah may also have implications for its management in other regions of North America. The proposed work will also advance the science of understanding geochemistry-ecological feedbacks in ecosystems, particularly involving exotic invasions. Many of these feedbacks are unknown and underappreciated. By demonstrating the potentially far-reaching consequences of invasive plant control efforts to geochemica...