Large-Scale Trends for Stream Benthic Respiration

Organic Matter Budgets for Streams: A Synthesis

Ecosystem metabolism is an important determinant of trophic structure, nutrient cycling, and other critical ecosystem processes in streams. Whereas watershed- and local-scale controls on stream metabolism have been independently investigated, little is known about how controls exerted at different scales interact to determine stream metabolic rates, particularly in urban streams and across seasons. To address this knowledge gap, we measured ecosystem metabolism in four urban and four reference streams in northern Kentucky, USA, with paired closed and open riparian canopies, during each of the four seasons. Gross primary production (GPP), ecosystem respiration, and net ecosystem production (NEP) were all best predicted by models with season as a main effect, but interactions between season, canopy, and watershed varied for each response. Urban streams exhibited higher GPP during most seasons, likely due to elevated nutrient loads. Open canopy reaches in both urban and forested streams, supported higher rates of GPP than the closed canopy which reaches during the summer and fall, when the overhead vegetation shaded the closed reaches. The effect of canopy cover on GPP was similar among urban and forested streams. The combination of watershed and local-scale controls resulted in urban streams that alternated between net heterotrophy (NEP <0) and net autotrophy (NEP >0) at the reach-scale during seasons with dense canopy cover. This finding has management relevance because net production can lead to accumulation of algal biomass and associated issues like nighttime hypoxia. Our study suggests that although watershed urbanization fundamentally alters ecosystem function, the preservation and restoration of canopied riparian zones can provide an important management tool at the local scale, with the strongest impacts on stream metabolism during summer.

Commentary on the Subsampling Procedures Used for Rapid Bioassessments

Assemblage Organization of Stream Fishes: The Effect of Rivers on Adventitious Streams

As catchments become increasingly urban, the streams that drain them become increasingly degraded. Urban streams are typically characterized by high-magnitude storm flows, homogeneous habitats, disconnected riparian zones, and elevated nitrogen concentrations. To reverse the degradation of urban water quality, watershed managers and regulators are increasingly turning to stream restoration approaches. By reshaping the channel and reconnecting the surface waters with their riparian zone, practitioners intend to enhance the natural nutrient retention capacity of the restored stream ecosystem. Despite the exponential growth in stream restoration projects and expenditures, there has been no evaluation to date of the efficacy of urban stream restoration projects in enhancing nitrogen retention or in altering the underlying ecosystem metabolism that controls instream nitrogen consumption. In this study, we compared ecosystem metabolism and nitrate uptake kinetics in four stream restoration projects within urban watersheds to ecosystem functions measured in four unrestored urban stream segments and four streams draining minimally impacted forested watersheds in central North Carolina, U.S.A. All 12 sites were surveyed in June through August of 2006 and again in January through March of 2007. We anticipated that urban streams would have enhanced rates of ecosystem metabolism and nitrate uptake relative to forested streams due to the increases in nutrient loads and temperature associated with urbanization, and we predicted that restored streams would have further enhanced rates for these ecosystem functions by virtue of their increased habitat heterogeneity and water residence times. Contrary to our predictions we found that stream metabolism did not differ between stream types in either season and that nitrate uptake kinetics were not different between stream types in the winter. During the summer, restored stream reaches had substantially higher rates of nitrate uptake than unrestored or forested stream reaches; however, we found that variation in stream temperature and canopy cover explained 80% of the variation across streams in nitrate uptake. Because the riparian trees are removed during the first stage of natural channel design projects, the restored streams in this study had significantly less canopy cover and higher summer temperatures than the urban and forested streams with which they were compared.

This paper proposes a high-frequency process model for estimating Dissolved Oxygen (DO) and net ecosystem metabolism (NEM) in streams. The model was implemented by using STELLA to predict DO concentrations at one-minute intervals downstream of a 150-m headwater reach of the Abant Creek (Bolu, Turkey). NEM was also predicted at each interval by using a two-station method along the reach. DO, water temperature and other environmental variables used in the model were measured during 17 months between August 2015 and December 2016. The model was run for a day representing every month of the year. Model parameters were calibrated and validated according to mean absolute error (MAE) between measured and simulated values of DO and NEM. The results showed that the model appeared to be promising in terms of high-frequency estimations of DO.

The results of research on the pollutant retardation potential of permeable riverbed sediments in catchments with significant groundwater–surface water (GW‐SW) interaction are presented. The fraction of organic carbon and cation exchange capacity of fluvial sediments in various geomorphologic environments have been quantified. Sediments in selected reaches of the rivers Tern and Leith (UK), from the underlying Permian sandstone aquifers, and from along the length of the rivers Severn and Eden into which the Tern and Leith discharge have been investigated. Statistical analyses show significant variation in the geochemistry and pollutant retardation potential of sediments from different geomorphologic features, and between upland and lowland rivers. The sorption potential of fine‐grained sediments deposited in pools was greater than sand in runs and coarser deposits in riffles. Similarly, sediments in lowland rivers were found to have a greater retardation potential than those in upland rivers. There was generally greater retardation potential in fluvial sediments of all types than in the underlying aquifers, and in lowland rivers the fluvial sediment retardation potential greatly dominated that of the aquifer. The findings demonstrate the potential for pollutant retardation processes in riverbed sediments of sandstone catchments, and suggest that consideration of retardation processes at the groundwater–surface water interface should be included into environmental risk‐assessment studies, in order to better assess and manage the effects of contaminated groundwater discharges to rivers, particularly in lowland catchments. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Stream metabolism is a fundamental ecosystem function that includes gross primary productivity (GPP) and ecosystem respiration (ER). These processes measure the energy supply and consumption in the aquatic system and are the basis for the green and brown food webs. Because of their sensitivity to environmental stressors, they are important in aquatic ecosystem management as functional indicators of the ecosystem&amp;#8217;s health. They also have the benefit of being an integrative indicator of ecosystem change since they are influenced by multiple factors at different scales. Although the drivers of metabolism in river systems are known, there is great variation in the factors controlling stream metabolism within and between individual river systems due to natural and anthropogenic drivers. Additionally, limiting factors can vary from one system to another, leading to distinct metabolic regimes.In Uganda's mountainous regions, the interaction between natural factors such as elevation and human-induced disturbances, including deforestation, agriculture and urbanization can cause metabolic patterns to deviate from those predicted for headwater streams. Understanding the drivers of stream metabolism in these anthropogenically impacted ecosystems is therefore crucial for their sustainable management. Considering the increased impact of anthropogenic activities on headwater streams and the general lack of understanding of the drivers of metabolism in these systems, this study examined the drivers of metabolism in anthropogenically disturbed headwater montane streams in western Uganda.Over 7 months, metabolism and its hypothesised drivers were measured in 11 tropical stream reaches at high elevation. Stepwise regression was used to build models to understand the factors influencing GPP and ER at catchment and local scales. At large scales, stream order, catchment area, and percentage of agriculture and forest cover influenced GPP, while stream order, elevation and the percentage of urban land use influenced ER. Structural equation modelling showed that catchment factors influenced GPP through effects on local drivers such as stream width, ammonia and phosphorous concentrations in sediments, turbidity and canopy cover. On the other hand, the catchment drivers controlled ER through influence on discharge, temperature, phosphorus, and ammonia. Our results suggest that metabolism in mountainous streams is not only affected by anthropogenic activities, but elevation also plays an important role for the observed patterns. The high elevation and steep slopes initiate further sediment-related processes, erosion and sedimentation, influencing metabolism.

In a 6th-order section of the lowland River Spree, we analysed the retention of particulate organic matter (POM) by the following characteristic benthic structural elements: stable sands colonised by macrophytes or mussels, shallow sandy banks, and shifting sands. Our hypothesis was that the retention and storage of suspended particles and, therefore, the organic matter budget of a specific river reach is determined by the retentive properties of the various types of benthic structural elements in the channel. The relative storage capacity of each structural element for POM, chlorophyll-a, and protein was calculated. This storage capacity reflects the dynamics of processes that occur on the time scale of several months. Additionally, the relative trapping efficiencies of the structural elements were determined by means of tracer particles, which provide information about their retentive properties on shorter time scales. The investigated structural elements differed significantly with respect to the amount and composition of stored POM, and the time scale of retention. Biotic structures were the most effective retention features on longer time scales in the investigated lowland river. Macrophyte stands showed the highest relative storage capacity for organic matter, followed by stable sands that were colonised by mussels. On a shorter time scale during low discharge conditions, shallow sandy banks were similarly retentive. Shifting sands were least retentive, but the highest nutritional quality of the organic matter was found there. Those spatial patterns within a river reach are supposed to be an important factor controlling the microbial processing of POM in the benthic zone and the distribution of benthic invertebrates. Based on our results, it can be expected that changes in the size and composition of benthic structural elements within the channel (e.g. due to bank reinforcements, straightening measures, or impoundments) will influence the organic matter budget of a stream ecosystem considerably.

Particulate organic matter (benthic and sestonic) and stream metabolism were determined in four occasions between January and June 1992 in a second order Mediterranean stream in SW Spain (Arroyo de la Montesina). We aimed to describe contrasting hydrological periods (presence and absence of surface flow) in terms of particulate organic matter resources and dynamics. The relationship between benthic (BOM) and sestonic (SOM) organic matter was examined using the BOM: SOM ratio (= BOM. SOM -1 . d-1, where BOM is in g.m -2 , SOM is in g.m -3 , and d is the mean depth in metres). The Montesina stream appears to have two distinct phases related to the hydrological period: 1) Surface water flow, when the Montesina appears as a stream and is heterotrophic, with low productivity, low SOM, and high BOM: SOM ratio; and 2) No flowing water, when the Montesina appears as a series of superficially-isolated pools and is autotrophic, with higher productivity, higher SOM, and a low BOM: SOM ratio. Thus, the hydrological regime is important in controlling particulate organic matter dynamics in the Montesina stream, and probably in other temporary Mediterranean streams as well.

Dissolved organic carbon (DOC) is an abundant form of organic matter in stream ecosystems. Most research has focused on the watershed as the source of DOC in streams, but DOC also comes from leaching of organic matter stored in the stream channel. We used a whole-ecosystem experimental approach to assess the significance of leaching of organic matter in the channel as a source of DOC in a headwater stream. Inputs of leaf litter were excluded from a forested Appalachian headwater stream for 3 years. Stream-water concentration, export, and instream generation of DOC were reduced in the litter-excluded stream as compared with a nearby untreated reference stream. The proportion of high molecular weight (HMW) DOC (more than 10,000 daltons) in stream water was not altered by litter exclusion. Mean DOC concentration in stream water was directly related to benthic leaf-litter standing stock. Instream generation of DOC from leaf litter stored in the stream channel contributes approximately 30% of daily DOC exports in this forested headwater stream. This source of DOC is greatest during autumn and winter and least during spring and summer. It is higher during increasing discharge than during base flow. We conclude that elimination of litter inputs from a forested headwater stream has altered the biogeochemistry of DOC in this ecosystem.

Streams and rivers are increasingly recognized as vital components of the global carbon cycle, especially in the context of climate change. To comprehensively understand their impact, it is essential to move beyond the study of individual reaches and consider the entirety of fluvial networks, including their terrestrial interactions. This holistic perspective is crucial for integrating fluvial networks into Earth System models and accurately assessing their role in the global carbon cycle.In this context, we describe the Metabolic Regimes in Alpine Stream Networks Program (METALP, https://metalp.epfl.ch), an ecohydrological and biogeochemical monitoring study of high-mountain streams in the Swiss Alps, running since 2016. Employing a network of high-frequency sensors (10-min) paired with monthly grab sampling, METALP examines the hydrological, thermal, light, and carbon regimes of high-mountain streams. Initially focused on the metabolism of alpine streams, the project has evolved to explore long-term trends in ecosystem characteristics and functions, with a particular emphasis on understanding climate change impacts. This unique observatory has so far collected over 20 million usable data points, describing annual regimes of streamwater flow, temperature, sediment load, carbon fluxes, and ecosystem metabolism.We present insights into the hydrologic and biogeochemical consequences of glacier loss, along with findings on dissolved organic carbon, gas exchange and CO2 emissions, oxygen concentrations, and gross primary production. Building on these insights, we then delve into the unique challenges associated with long-term monitoring in high-mountain catchments. These include marked hydrologic variability, with flows ranging over several orders of magnitude, and the need for monitoring equipment to withstand high flows, sediment loads, and avalanches, and remain functional during low flow periods. Seasonal snow cover and the remoteness complicate sampling campaigns and sensor maintenance. Additionally, the oligotrophic nature of high-mountain streams, with low analyte concentrations, necessitates sensitive monitoring programs capable of detecting subtle changes. These challenges inherently lead to gaps in data, necessitating not only technical adaptations for monitoring under difficult conditions but also innovative modeling strategies for compensating data loss.Finally, the METALP network, along with river networks located in different climatic regions (i.e., the Krycklan catchment in Sweden, the StreamPULSE project, or the Arctic Great Rivers Observatory), provides a broader perspective, enabling us to understand biogeochemical patterns and dynamics across multiple streams. This approach is crucial for constructing a comprehensive picture of stream biogeochemistry and its response to climate change.

Ecosystem metabolism, that is, gross primary productivity (GPP) and ecosystem respiration (ER), controls organic carbon (OC) cycling in stream and river networks and is expected to vary predictably with network position. However, estimates of metabolism in small streams outnumber those from rivers such that there are limited empirical data comparing metabolism across a range of stream and river sizes. We measured metabolism in 14 rivers (discharge range 14–84 m3 s−1) in the Western and Midwestern United States (US). We estimated GPP, ER, and gas exchange rates using a Lagrangian, 2-station oxygen model solved in a Bayesian framework. GPP ranged from 0.6–22 g O2 m−2 d−1 and ER tracked GPP, suggesting that autotrophic production supports much of riverine ER in summer. Net ecosystem production, the balance between GPP and ER was 0 or greater in 4 rivers showing autotrophy on that day. River velocity and slope predicted gas exchange estimates from these 14 rivers in agreement with empirical models. Carbon turnover lengths (that is, the distance traveled before OC is mineralized to CO2) ranged from 38 to 1190 km, with the longest turnover lengths in high-sediment, arid-land rivers. We also compared estimated turnover lengths with the relative length of the river segment between major tributaries or lakes; the mean ratio of carbon turnover length to river length was 1.6, demonstrating that rivers can mineralize much of the OC load along their length at baseflow. Carbon mineralization velocities ranged from 0.05 to 0.81 m d−1, and were not different than measurements from small streams. Given high GPP relative to ER, combined with generally short OC spiraling lengths, rivers can be highly reactive with regard to OC cycling.

Leaky rivers: Implications of the loss of longitudinal fluvial disconnectivity in headwater streams

Headwater streams in Sweden are the transition between the forested landscape and river networks. Due to their low water residence time, the dissolved organic matter contained in headwater streams is not greatly modified, and is preserved in the form that it exits the landscape. Here, we used an HPLC-Orbitrap technique to study the chemical variability of this dissolved organic matter across an 87,000km² region of Sweden. Contained is the raw data and Matlab code used to generate figures and tables found in the article "Regional diversity of complex dissolved organic matter across forested hemiboreal headwater streams" in Scientific Reports, 2018.