Changing environmental conditions and increased water consumption have transformed many historically perennial stream systems into intermittent systems. Multiple drying and wetting events throughout the year might impact many stream processes including nitrification and denitrification, key components of the nitrogen (N) cycle. During summer 2007, an experimental stream was used to dry and then rewet stream sediments to determine the effects of desiccation and rewetting of stream sediment on nitrification and denitrification potentials. Mean (±SE) nitrification and denitrification rates in sediment not dried (controls) were 0.431 ± 0.017 lg NO 3 --N/cm 2 /h and 0.016 ± 0.002 lg N 2 O-N/cm 2 /h, respectively. As sediment samples dried, nitrification rates decreased. Rates in sediments dried less than 7 d recovered to levels equal or greater than those in the controls within 1 d of being rewetted. Denitrification rates were not affected by 1 d of drying, but samples dried greater than 1 d experienced reduced rates of denitrification. Denitrification in sediments dried 7 d or less recovered by day seven of being rewetted. Nitrification and denitrification processes failed to fully recover in sediments dried more than 7 d. These results demonstrate that alterations in stream's hydrology can significantly affect N-cycle processes.

Highlights Despite little to no dissolved nutrient supply in surface water, harmful algal blooms are sustained throughout the 2020 growing season. Sediment phosphorus release was high in a lake that has annual harmful algal blooms, and it is an important piece of the watershed management puzzle. Thresholds and hierarchical structure with individual physicochemical properties and pigment fluorescence at this lake explain a large portion of microcystin variability. Abstract. Harmful algal blooms (HABs) in freshwaters are a global concern, and research has focused on the nutrient drivers of cyanobacterial growth and toxin production. We explored the importance of nutrients on sustained cyanobacterial HABs producing measurable microcystin at Lake Fayetteville, Arkansas, USA. The specific objectives were to (1) quantify sediment phosphorus (P) flux and estimate potential equilibrium P concentrations (EPC0) in July 2020, (2) assess water quality conditions in the lake from March through September 2020, and (3) evaluate physicochemical thresholds (or change points, CPs) and hierarchical structure with total microcystin concentrations. The sediments were a potential P source under both oxic and anoxic conditions, and the SRP concentrations in the lake water were continuously less than the EPC0 estimated for bottom sediment (~0.03 mg L-1); sediments are likely a potential P source for cyanobacterial HABs at Lake Fayetteville. The physicochemical changes at Lake Fayetteville over the 2020 growing season were typical of small, hypereutrophic reservoirs, with low biomass in winter when nutrient supply was greatest and the greatest cyanobacterial growth and microcystin toxin as nutrient supply diminished into the growing season. Microcystin concentrations were elevated above 1 µg L-1 from mid-June through mid-August 2020, and most physicochemical parameters in this study showed thresholds or change points with microcystin. Hierarchical structure existed with total microcystin concentrations, showing the potential importance of cyanobacterial biomass, N supply, and total P on elevated microcystin. Nutrients and algal pigment raw fluorescence explained 83% of the variation in total microcystin concentrations at Lake Fayetteville during the 2020 growing season. Nutrients (both N and P) from external and internal sources are likely important drivers of these blooms and toxicity at Lake Fayetteville. Keywords: Harmful Algal Blooms, Nutrient Drivers, Sediment Phosphorus Release, Water Quality.

HighlightsA HOBO water level logger was deployed to obtain a continuous stage record across sites.SonTek-IQ acoustic Doppler instruments were deployed to measure discharge during high-flow events.Rating curves were developed using linear regression, LOESS regression, and Manning’s equation.The proposed method provides an opportunity to collect continuous flow records across multiple, small-scale watersheds.Abstract. Discharge monitoring stations are often costly and difficult to install, operate, and maintain, especially in small streams. The purpose of this study was to evaluate a low-cost method for remotely monitoring streamflow in small-scale watersheds to provide continuous discharge measurements across multiple sites and flow conditions. Within the Upper Poteau River Watershed (UPRW) in Arkansas, 12 sites were selected at bridge crossings near the outflow of HUC-12 or HUC-14 subwatersheds. A HOBO water level logger was deployed at each site to obtain continuous stage records, and HOBO barometric pressure transducers were installed within 16 km of each sample site to account for fluctuations in atmospheric pressure. SonTek-IQ acoustic Doppler instruments were deployed to measure discharge during high-flow events, and roving discharge monitoring stations were installed at each site to allow easy rotation of the SonTek-IQ instruments among sites between flood events. Once the roving discharge monitoring stations were installed at each site, one or more SonTek-IQ instruments could be rotated among sites to capture high-flow discharge measurements; therefore, a SonTek-IQ instrument was not required for every site of interest. The high-flow data captured during SonTek-IQ deployment, and baseflow discharge measurements collected on a monthly basis, were used to develop rating curves with a combination of simple linear regression, LOESS regression, and Manning’s equation. The rating curves well represented the measured flows, with Nash-Sutcliffe efficiencies ranging between 0.87 and 0.98. This method provides an opportunity to collect continuous records of flow across multiple, remote, small-scale watersheds, and in conjunction with constituent concentrations and load estimations, can be used to calibrate and validate watershed models. Keywords: Rating curves, Small-scale watersheds, Streamflow monitoring.