Stream Metabolism Research Articles

Abstract. Hydrological disturbances following storm events influence the structure and functioning of headwater streams. However, understanding how these disturbances impact critical processes such as stream metabolism is challenging. We assessed the effect of storm events on the resistance and resilience of gross primary production (GPP) and ecosystem respiration (ER) in a heterotrophic headwater stream. We hypothesized that stream metabolism will show low resistance to storm events because GPP and ER will be either stimulated by inputs of limited resources (small storms) or suppressed by biofilm damage (large storms). We also expected resilience to decrease with the size of the storm event. To test these hypotheses, we hydrologically characterized 53 individual storm events during 4.5 years (period October 2018–February 2023) and estimated metabolic rates prior, during, and after each event. Individual storm events had different duration (from 4 to 32 d), and exhibited contrasting changes in discharge (discharge from 0.6 to 872.4 L s−1). Due to data and model constraints, we were able to estimate metabolic rates for 35 of these events, for which GPP and ER averaged 1.7 ± 1.8 and −13.4 ± 7 g O2 m−2 d−1, respectively. The two processes showed low resistance to storm events, with magnitudes increasing in 69 % and 86 % of the cases for GPP and ER, respectively. The relationship between hydrological parameters and changes in GPP was not statistically significant, while a positive relation with the magnitude of the storm event was found for ER (R2 > 0.37). Similarly, recovery times were positively related to the size of the event only for ER (R2 > 0.46). Yet recovery times were always lower than 6 d, suggesting that the positive effect of resource inputs on stream metabolic activity was limited over time. Our findings support the idea that storm events stimulate metabolic activity in headwater streams, especially ER, and highlight how changes in hydrological regimes could impact stream functioning and its role in global biogeochemical cycles.

Abstract Both local and global climate phenomena shape the hydrologic regimes of watersheds. For aridland rivers in the southwestern United States, peak flows occur during two distinct periods: spring snowmelt and summer monsoons. Although discharge (Q) is a primary driver of variation in the production and consumption of instream organic matter, or stream metabolism, few connection have been made regarding how climate impacts ecosystem processes through changes in flow and related disturbances. We considered how variation in disturbance variables, specifically Q and associated changes in turbidity, affected gross primary production during spring snowmelt and summer monsoons in the Rio Grande. Nine years of continuous environmental data (Q, turbidity, light) and climate indices (i.e., El Niño‐Southern Oscillation and Monsoon Index) were used to explain the variation in gross primary production estimates. We found that relationships were sensitive to the timescale of disturbance: at the seasonal scale, high snowmelt Q decreased spring mean gross primary production, while at the daily scale, high turbidity, and to a lesser extent Q, reduced gross primary production during summer. Also, mean Q and turbidity disturbances were uncoupled in spring and inversely related in summer. We conclude that long‐term datasets are essential to uncover emergent relationships between broad‐scale climate patterns and ecosystem processes and are necessary to better understand how hydroclimatic variability drives ecosystem processes at varying time scales in rivers across Earth.

Abstract Anthropogenic activities in catchments, such as urban and agricultural land use, negatively impact the biogeochemical cycles of carbon, nitrogen and phosphorus in streams by increasing concentrations of these nutrients and altering the composition of dissolved organic matter (DOM). In tropical climates with high temperatures and intense precipitation, streams are particularly vulnerable to high loading from the catchment. The combination of high nutrient loading from the catchment and high processing rates at high temperatures can lead to even higher concentrations and more severe impacts on biogeochemical cycles. However, studies linking human activity to changes in nutrient and DOM composition, and the resulting impacts on stream functions, are still scarce in tropical streams. This study addressed this gap by examining the relationships between land use and water column chromophoric DOM (CDOM), nitrogen and phosphorus across seasons in an Afrotropical watershed. In addition, the effects of nutrient enrichment and changes in DOM composition on stream metabolism were investigated. The results showed that urban land use had the most substantial influence on nutrient concentrations and DOM composition in the studied streams. Streams with a high proportion of urban land use in their riparian zone had high nutrient concentrations and a pronounced autochthonous DOM signal. In contrast, streams with more forest cover in their riparian zone had lower nutrient concentrations and a more allochthonous DOM signal in their water column. Stream metabolism increased with nutrient concentrations and autochthonous organic matter contribution, and these changes were more pronounced in the dry season, pointing to the combined effects of high nutrient loading and processing rates on carbon biogeochemistry. These results confirm that changes in nutrient loading and organic matter composition caused by human activities and seasonal changes will likely impact river ecosystem processes, with implications for food webs and tropical biogeochemical cycles.

Stream metabolism is traditionally defined as the combined metabolism of all aerobic organisms in a stream. Its component processes of oxygenic photosynthesis and aerobic respiration create and consume dissolved oxygen (DO) and therefore can be measured using time series of DO concentration, solar radiation, and water temperature, in conjunction with a model of DO dynamics that includes photosynthesis, respiration, and oxygen exchange with the atmosphere. A complication is that stream communities typically exhibit pronounced longitudinal heterogeneity in habitat type (e.g., shaded versus unshaded reaches) and species composition and abundance. The influence of a given stream reach and associated community on DO concentration propagates downstream with the current, gradually being replaced, over a transition zone, by the influence of the next downstream reach. Knowing the approximate length of this transition zone is important when estimating stream metabolism with one-station DO monitoring, since it indicates which stream reach and associated community the metabolism estimates apply to. We propose new methods for estimating the transition-zone length and for estimating the proportions of DO at a given location in a stream reach that entered the reach from upstream, from photosynthesis within the reach, and from atmospheric uptake within the reach. We also propose methods for estimating the residence-time distribution of DO present at a given stream location, and the corresponding distribution of upstream distances at which the DO entered the stream. Both distributions are approximately exponential. Thus, habitat immediately upstream of the sonde has the greatest influence on metabolism estimates with one-station monitoring, and it is therefore important to place the sonde so this habitat is representative of the study reach.

Human-induced environmental changes are reshaping animal behavioural traits, yet their ecological consequences remain poorly understood. We demonstrate that among-population variation in the behavioural traits of two freshwater crayfish species profoundly affects key ecosystem functions in streams. Crayfish movement behaviour was strongly linked to increased water column metabolism in both natural streams and controlled mesocosm experiments. Movement also influenced nutrient cycling, highlighting the role of bioturbation in ecosystem dynamics. In contrast, boldness negatively impacted leaf litter breakdown, suggesting that less bold individuals rely more on leaf litter as refuge and food. Notably, within-species behavioural differences often outweighed species identity in determining ecological impacts. Our findings reveal that shifts in animal movement behaviour can drive fundamental ecological processes and emphasise the overlooked importance of within-species trait variation. These results advance our understanding of how behavioural diversity influences ecosystem functions and underscore the need to incorporate intraspecific variation into ecological frameworks.

Abstract Increases to summer Arctic rainfall and tundra thermal degradation are altering hydrological cycling in coastal watersheds with implications for carbon (C) cycling and transport of C to the atmosphere and coast. Arctic riverine research has focused on large rivers; however, small streams contribute significantly to vertical and longitudinal carbon dioxide (CO2) fluxes. Despite the well‐established connection between hydrology and biogeochemistry, the impact of extreme rainfall events on Arctic aquatic C cycling remains a knowledge gap. This study characterized how hydrology, biogeochemistry, and geomorphology control the supply of CO2 to low order streams and their propensity to act as atmospheric CO2 sources. We characterize biogeochemical and hydrologic processes in unique reaches from a beaded stream and stream impacted by thermal erosion. Rainfall and its resulting increases to terrestrial‐aquatic connectivity drove the movement of CO2 and biodegradable dissolved organic C (BDOC) from soils into streams, however, BDOC mineralization only contributed a small portion of surface CO2 fluxes. Rain events likely stimulated stream benthic respiration, which led to CO2 contributions from net ecosystem production often exceeding surface CO2 fluxes and downstream CO2 transport. In addition, thermal degradation played a role in terrestrial‐aquatic connectivity of the streams. The erosion‐affected stream had inconsistent and smaller inputs of CO2, had weaker heterotrophic conditions, and smaller CO2 emissions. Understanding how hydrologic regime, influenced by late summer rain events and stream morphology, controls the transport of CO2 and metabolism in small tundra streams will help improve predictions of landscape scale CO2 emissions from these critically understudied systems.

Clear-cutting of forests with little or no regard for riparian buffers alters the local abiotic habitat of streams within and downstream of clear-cuts by increasing temperature, incident light, suspended sediments and resource inputs such as carbon and inorganic nutrients. It is also well documented that streams with narrow or non-existent riparian buffers affect local stream ecosystem processes. Here, we ask whether ecosystem processes can also be affected downstream of clear-cuts. We tested this in nine headwater streams that run through recently harvested clear-cuts (1-6 years ago) with varying buffer widths (<10 and≥15m) in northern Sweden. We compared biofilm (periphytic algal and bacterial mats) and whole stream metabolic rates in stream reaches situated upstream of the clear-cuts, in the clear-cuts and downstream of the clear-cuts. We found that biofilm gross primary productivity (GPP) in streams with thin buffers (<10m) increased, on average, by 54 % downstream of clear-cuts in July, but that the net effect on the whole ecosystem was still a decrease in ecosystem productivity due to high respiration rates. In September, the situation was different as there was a 50 % decrease in biofilm GPP downstream of clear-cuts, and the net effect was again a decrease in ecosystem productivity. Wide buffer zones (>15m) could mitigate these longitudinal changes for both biofilm and whole stream metabolism, except in one stream that was dominated by fine sediments. Importantly, the magnitude of downstream propagation in biofilm GPP was related to the magnitude of responses in the clear-cut, which in turn was driven by nutrient concentrations. To upscale the potential magnitude of clear-cutting in Sweden, we estimated that nearly 6 % (or 57,400km) of the total forested stream length is situated within and 100m downstream of clear-cuts that were harvested 1-6 years ago. Based on this study, we conclude that clear-cut effects on stream ecosystem processes are not only local, but can also be propagated to downstream recipient waters if riparian buffer width in the clear-cut is less than 15m.

Abstract We determine where stream carbon dioxide () comes from by developing a model for the joint estimation of stream metabolism, oxygen‐carbon (O‐C) stoichiometry, and fluxes of dissolved inorganic carbon (DIC), based on observations of stream oxygen () and concentrations. The model is based on a stream reach mass balance of , DIC, and total alkalinity, and it accounts for the carbonate system and the contribution of lateral flow. and mass balances are coupled through stoichiometric coefficients for photosynthesis and combined autotrophic and heterotrophic respiration. Under the assumption of constant alkalinity and circumneutral pH, the model simplifies and includes 8 parameters, which are estimated through a Bayesian hierarchical framework. The model accurately reproduced time series of and from three diverse sites across size and carbonate chemistry gradients. Results allow partitioning of the stream DIC budget, and thus the source of stream outgassing, into internal (in‐stream net ecosystem production) and external (lateral input of terrestrial DIC and atmospheric input) contributions. We observed that the estimated stoichiometric coefficients were typically different from 1—contrary to typical assumptions—leading to divergent estimates of stream sources depending on the measurement (i.e., vs. C). Parameter posterior distributions revealed the source of parameter uncertainty and the equifinality of some processes in reproducing stream dynamics, suggesting targeted variables to further investigate in order to better constrain stream C balance. The proposed model is a useful tool for incorporating the rapidly growing stream data sets into our understanding of terrestrial‐aquatic C linkages.

Abstract Stream metabolism is a key biogeochemical process in river networks, synthesizing the balance between gross primary production (GPP) and ecosystem respiration (ER). Globally, more rivers and streams are drying due to climate change and water abstraction for human uses and this can alter the organic carbon residence time leading to decoupled ER and terrestrial organic matter supply. Although the consequences of drying on CO2 emissions have been recently quantified, its effects on stream metabolism are still poorly studied. We addressed the long‐term effects of drying and rewetting events on stream metabolism by monitoring oxygen dynamics at 20 reaches across a drying river network, including perennial (PR) and nonperennial reaches (NPR) for one year. We also calculated several climatic and land use variables and characterized local abiotic conditions and biofilm and sediment communities at five sampling dates. ER was significantly higher in NPR than in PR reaches demonstrating in situ the effects of drying on stream metabolism. When analyzing the long‐term drivers of ER and GPP, we found a direct positive effect of drying on ER and a negative effect on GPP. Drying also altered microbial community composition with algal communities from NPRs being different from those in PRs. In the short‐term, the total oxygen consumption (respiration) during rewetting events was positively related to the duration of precedent nonflow period. Our results show that drying had an important effect on stream metabolism both in the short‐ and long term, supporting the need for including NPRs in global estimates of stream metabolism.

Abstract The amount and quality of dissolved organic carbon (DOC) exported from terrestrial to riverine ecosystems are critical factors influencing aquatic metabolism and ecosystem health in streams, rivers, and lakes. This study investigates the interplay between hydrologic conditions and DOC dynamics in an alpine catchment, focusing on how DOC concentration and quality shift during baseflow, snowmelt, and storm events. Such dynamics were explored in the Oberer Seebach basin (Austria) where sub‐daily DOC concentration data, along with high resolution excitation‐emission matrices and absorbance spectra, were used to characterize DOC concentration and quality. We quantitatively linked hydrologic pathways with DOC dynamics by advancing a framework that couples water age, which tracks the time water spends within the catchment, with the Reactivity Continuum model, which quantifies the evolution of DOC reactivity and ensuing concentration. Results show that simulating both water age and DOC reactivity effectively reproduces DOC concentrations and reveals a correlation between modeled reactivity and observed DOC quality indices. During snowmelt and storm events, rapid hydrologic pathways transport reactive DOC with a quality profile similar to that of freshly formed terrestrial DOC, while during baseflow, slower pathways carry less reactive DOC with a signature of preceding degradation processes. These findings shed light on the role of catchment hydrology in carbon cycling and on its implications for riverine ecosystem functioning.

ABSTRACTIncreasing drought frequency and severity from climate change are causing streamflow to become increasingly intermittent in many areas. This has implications for the spatio‐temporal characteristics of water quality regimes which need to be understood in terms of risks to the provision of clean water for public supplies and instream habitats. Recent advances in sensor technology allow reliable and accurate high‐resolution monitoring of a growing number of water quality parameters. Here, we continuously monitored a suite of water quality parameters over 3 years in an intermittent stream network in the eutrophic, lowland Demnitzer Millcreek catchment, Germany. We focused on the effects of wetland systems impacted by beaver dams on the diurnal, seasonal and inter‐annual variation in water quality dynamics at two sites, upstream and downstream of these wetlands. We then used the data to model stream metabolism. Dissolved oxygen and pH were higher upstream of the wetlands, while conductivity, turbidity, chlorophyll a and phosphorous concentrations were higher downstream. We found clear diurnal cycling of dissolved oxygen and pH at both sites. These dynamics were correlated with seasonal hydroclimatic changes and stream metabolism, becoming increasingly pronounced as temperatures increased and flows decreased in spring and summer. Upstream of the wetlands this corresponded to the stream rapidly becoming increasingly heterotrophic as modelled Gross Primary Production (GPP) was exceeded by Ecosystem Respiration (ER). Downstream, where GPP was lower, the stream was usually strongly heterotrophic and prone to increasingly hypoxic conditions (i.e., insufficient oxygen) before streamflow ceased in summer. This coincided with lower velocities and deeper channels in beaver impacted areas. Seasonal and inter‐annual variations in water quality were found to mainly correlate with hydroclimatic factors (particularly temperature) and their influence on streamflow. This study highlights that heterotrophy and hypoxia in lowland rivers in central Europe is an important seasonal feature of intermittent streams where agricultural landscapes continue leaching nutrients. These insights contribute to an evidence base for understanding how climate change will affect the quantity and quality of rural water resources in intermittent lowland streams with wetlands where the presence of beavers requires management responses.

Zoogeochemical impacts of freshwater mussels on stream metabolism are mediated by their ecophysiological and behavioral traits

Monitoring the seasonal and diurnal variations in headwater stream metabolic regimes can provide critical information for understanding how ecosystems will respond to future environmental changes. In East Fork Creek, a headwater stream in middle Tennessee, week-long field campaigns were set up each month from May 2022 to May 2023 to collect stream metabolism estimators. In a more extensive field campaign from July 2-5 in 2022, diel signals were observed for temperature, pH, turbidity, and concentrations of Ca, Mg, K, Se, Fe, Ba, chloride, nitrate, DIC, DO, DOC, and total algae. Gross Primary Productivity (GPP) and Ecosystem Respiration (ER) were calculated based on a Bayesian model using the dissolved oxygen (DO) time series approach. DO showed diurnal swings between oversaturation in daytime and undersaturation at night, with DO amplitudes being greatest in summer. GPP measurements have a clear seasonal variation, peaking in July and staying low in winter, and strong diel signals that couple with the daily light regime variation. ER does not vary seasonally except for a slight increase in Fall which might be caused by terrestrial organic inputs. The dominant control on GPP is light intensity and on ER is temperature. East Fork Creek shows a heterotrophic metabolic regime for 54 of 57 campaign days and therefore consumes O2 and emits CO2 to the atmosphere throughout the year. If carbon inputs are not a limiting factor, the positive temperature dependence of ER may cause increased CO2 emissions from headwater streams and more frequent hypoxia events in a warming climate.

Abstract Metabolism in stream ecosystems alters the fate of organic carbon (OC) received from surrounding landscapes, but our understanding of in‐stream metabolic processes in boreal ecosystems remains limited. Determining the factors that regulate OC metabolism will help predict how the C balance of boreal streams may respond to future environmental change. In this study, we addressed the question: what controls OC metabolism in boreal headwater streams draining catchments with discontinuous permafrost? We hypothesized that metabolism is collectively regulated by OC reactivity, phosphorus availability, and temperature, with discharge modulating each of these conditions. We tested these hypotheses using a combination of laboratory experiments and whole‐stream ecosystem metabolism measurements throughout the Caribou‐Poker Creeks Research Watershed (CPCRW) in Interior Alaska, USA. In the laboratory experiments, respiration and dissolved OC (DOC) removal were both co‐limited by the supply of reactive C and phosphorus, but temperature and residence time acted as stronger controls of DOC removal. Ecosystem respiration (ER) was largely predicted by discharge and site, with some variance explained by gross primary production (GPP) and temperature. Both ER and GPP varied inversely with watershed permafrost extent, with an inverse relationship between temperature and permafrost extent providing one plausible explanation. Our results provide some of the first evidence of a functional response to permafrost thaw in stream ecosystems and suggest the role of metabolism in landscape C cycling may increase as climate change progresses.

Abstract Dissolved oxygen (DO) is a critical water quality constituent that governs habitat suitability for aquatic biota, biogeochemical reactions and solubility of metals in streams. Recently introduced high‐frequency sensors have increased our ability to measure DO, but we still lack the capacity to understand and predict DO concentrations at high spatial resolutions or in unmonitored locations. Machine learning (ML) has been a commonly used approach for modelling DO, however, conventional ML models have no representation of the limnological processes governing DO dynamics. Here we implement and evaluate two process‐guided deep learning (PGDL) approaches for predicting daily minimum, mean and maximum DO concentrations in rivers from the Delaware River Basin, USA. In both cases, a multi‐task approach was taken in which the PGDL models predicted stream metabolism and gas exchange rates in addition to the DO concentrations themselves. Our results showed that for these sites, the PGDL approaches did not improve upon baseline predictions in temporal and spatially similar holdout experiments. One of the approaches did, however, improve predictions when applied to spatially dissimilar sites. Although this particular PGDL approach did not improve predictive accuracy in most cases, our results suggest that process guidance, perhaps a more constrained approach, could benefit a data‐driven DO model.

Land cover changes alter hydrologic (e.g., infiltration-runoff), biochemical (e.g., nutrient loads), and ecological processes (e.g., stream metabolism). We quantified differences in aquatic ecosystem respiration in two contrasting stream reaches from a forested watershed in Colorado (1st-order reach) and an agricultural watershed in Iowa (3rd-order reach). We conducted two rounds of experiments in each of these reaches, featuring four sets of continuous injections of Cl− as a conservative tracer, resazurin as a proxy for aerobic respiration, and one of the following nutrient treatments: (a) N, (b) N + C, (c) N + P, and (d) C + N + P. With those methods providing consistent information about solute transport, stream respiration, and nutrient processing at the same spatiotemporal scales, we sought to address: (1) Are respiration rates correlated with conservative transport metrics in forested or agricultural streams? and (2) Can short-term modifications of stoichiometric conditions (C:N:P ratios) override respiration patterns, or do long-term physicochemical conditions control those patterns? We found greater respiration in the reach located in the forested watershed but no correlations between respiration, discharge, and advective or transient storage timescales. All the experiments conducted in the agricultural stream featured a reaction-limited transformation of resazurin, suggesting the existence of nutrient or carbon limitations on respiration that our short-term nutrient treatments did not remove. In contrast, the forested stream was characterized by nearly balanced transformation and transient storage timescales. We also found that our short-lived nutrient treatments had minimal influence on the significantly different respiration patterns observed between reaches, which are most likely driven by the longer-term and highly contrasting ambient nutrient concentrations at each site. Our experimental results agree with large-scale analyses suggesting greater microbial respiration in headwater streams in the U.S. Western Mountains region than in second-to-third-order streams in the U.S. Temperate Plains region.

Abstract Headwater streams influence the carbon cycle, but their productivity estimation remains challenging. We propose the use of dissolved oxygen data (% saturation, DOsat) and on‐site photosynthetically active radiation (PAR) data to develop DOsat~PAR curves as an analogy to the well‐known photosynthesis–irradiance (P–E) curves. The premise of our research is that although these curves are simple, they provide detailed information of stream ecosystem productivity dynamics. We used data from two streams in the Oregon Coast Range to investigate daily gross primary productivity (GPP). We used properties of the light‐limited portion of the DOsat~PAR regression curves to produce a model to estimate GPP. We found that the slope of the DO–PAR relation varied widely between 1.6 × 10−4 and 0.045 and had strong correlations (r2 &gt; 0.78). The data from one of the two study sites (Oak Creek) was used for model development while the data from the other site (South Fork Mill Creek) was used for model validation. The model's ability to quantify the effects of a discrete storm event on stream productivity was tested by comparing GPP estimates calculated through a Bayesian framework (streamMetabolizer) and our raw data‐driven estimates of GPP which were based on the variability of the DOsat~PAR regression curves. The proposed methodology was successful in estimating GPP in headwaters. We foresee that this method may be used to assess disturbances and construct a baseline understanding of productivity dynamics in other headwater ecosystems that is independent of the methodological challenges of the current stream metabolism models.

Streams are significant contributors of greenhouse gases (GHG) to the atmosphere, and the increasing number of stressors degrading freshwaters may exacerbate this process, posing a threat to climatic stability. However, it is unclear whether the influence of multiple stressors on GHG concentrations in streams results from increases of in-situ metabolism (i.e., local processes) or from changes in upstream and terrestrial GHG production (i.e., distal processes). Here, we hypothesize that the mechanisms controlling multiple stressor effects vary between carbon dioxide (CO2) and methane (CH4), with the latter being more influenced by changes in local stream metabolism, and the former mainly responding to distal processes. To test this hypothesis, we measured stream metabolism and the concentrations of CO2 (pCO2) and CH4 (pCH4) in 50 stream sites that encompass gradients of nutrient enrichment, oxygen depletion, thermal stress, riparian degradation and discharge. Our results indicate that these stressors had additive effects on stream metabolism and GHG concentrations, with stressor interactions explaining limited variance. Nutrient enrichment was associated with higher stream heterotrophy and pCO2, whereas pCH4 increased with oxygen depletion and water temperature. Discharge was positively linked to primary production, respiration and heterotrophy but correlated negatively with pCO2. Our models indicate that CO2-equivalent concentrations can more than double in streams that experience high nutrient enrichment and oxygen depletion, compared to those with oligotrophic and oxic conditions. Structural equation models revealed that the effects of nutrient enrichment and discharge on pCO2 were related to distal processes rather than local metabolism. In contrast, pCH4 responses to nutrient enrichment, discharge and temperature were related to both local metabolism and distal processes. Collectively, our study illustrates potential climatic feedbacks resulting from freshwater degradation and provides insight into the processes mediating stressor impacts on the production of GHG in streams.

Abstract Natural Flood Management (NFM) aims to reduce flood hazard by working with nature and is gaining prominence worldwide. One particular NFM technique involves the use of channel‐spanning woody dams that maintain a clearance height above baseflow. These dams function by increasing channel roughness during high flows and by forcing excessive water onto the floodplain. Whether these dams provide additional benefits to nature remains unclear. While there are many existing studies on natural in‐stream wood structures, very few have documented the impact of NFM woody dams in particular. This study adopted a multidisciplinary approach and a Before–After Control–Impact (BACI) research design to assess whether NFM woody dams installed in a small upland catchment had driven changes in benthic macroinvertebrate assemblages and benthic metabolic activities through the geomorphic changes that they had created. Statistical results indicate that macroinvertebrate density, richness, and diversity did not show any difference between stream reaches with and without NFM woody dams. The metrics were generally not related to grain‐size parameters and volumes of sediments eroded or deposited. However, individual genera such as Baetis and Rhithrogena became more dominant in the control reach towards the end of the study period, likely due to the higher flow velocities and coarser sediments there resulting from the lack of flow resistance in the absence of NFM woody dams. Rates of benthic respiration (but not rates of photosynthesis) were consistently significantly higher in woody dam reaches than in control reaches, likely due to the presence of patches of finer sediments in the former.

Highlights Aerial image results highlight the spatial and temporal variability of duckweed coverage in the headwater stream. In situ sensors suggest algae has a shortened season compared to other agricultural streams. Diurnal nitrate variability changed with shifts in aquatic vegetation and dissolved oxygen. Coupling in situ and aerial results elucidated reasons for limitations in nitrate concentration predictions. Abstract. Fate and transport of nutrients in karst streams remains a pressing research need. The objective of this study was to couple high-frequency and remote imagery data to quantify spatial and temporal variability of aquatic vegetation and determine the associated impacts on in-stream nitrate removal in karst headwater streams. The study was conducted in a spring-fed karst stream in the Inner-Bluegrass region of Central Kentucky, USA. Ten Unmanned Aircraft System (UAS) campaigns were coupled with three-years of high frequency in situ data of water quality. Automated segmentation and image classification analysis was performed on UAS imagery, and spatial variability in floating aquatic macrophytes was quantified. Results demonstrated the utility of UAS images to capture spatiotemporal variability of floating aquatic macrophytes (duckweed) with high accuracy, but the UAS analysis poorly predicted algal biomass dynamics due to spectral interferences in the water column and shading by duckweed. Further, in situ water quality data was used to estimate stream metabolism using the Bayesian Single Station Estimator (BASE) model, with results demonstrating primary production and ecosystem respiration was driven by algal biomass. Stream metabolism displayed a shorter season (March-August) relative to other agricultural streams in the region (March-October), likely reflecting the hydraulic structures in the channel which reduce flow velocities in the stream reach and promote transition to duckweed cover during the summer. The impact of aquatic vegetation dynamics on nitrate was assessed using diurnal analysis and regression with dissolved oxygen. Results for March through November showed an average daily diurnal variation of 0.25 mgN/L, which was more than two-fold greater than winter months. During the growing season, maxima generally occurred between 6 a.m.-2 p.m., and minima occurred between 2 p.m.-12 a.m., but varied depending on prominent aquatic vegetation and dissolved oxygen dynamics. These results suggest shifting N removal mechanisms as aquatic vegetation changes throughout the year. Implications for numerical modeling of fluvial nitrate fluxes are illustrated through the evaluation of a previously developed AI model simulating nitrate concentrations at the watershed outlet. The findings highlight the importance of integrating novel datasets, such as those presented in this study, to evaluate and improve predictions of numerical models. Keywords: Aquatic vegetation, Water quality sensing, Karst agroecosystem, Nitrate, watershed.