Daniel von Schiller

When the river runs dry: human and ecological values of dry riverbeds

Temporary rivers and streams that naturally cease to flow and dry up can be found on every continent. Many other water courses that were once perennial now also have temporary flow regimes due to the effects of water extraction for human use or as a result of changes in land use and climate. The dry beds of these temporary rivers are an integral part of river landscapes. We discuss their importance in human culture and their unique diversity of aquatic, amphibious, and terrestrial biota. We also describe their role as seed and egg banks for aquatic biota, as dispersal corridors and temporal ecotones linking wet and dry phases, and as sites for the storage and processing of organic matter and nutrients. In light of these valuable functions, dry riverbeds need to be fully integrated into river management policies and monitoring programs. We also identify key knowledge gaps and suggest research questions concerning the values of dry riverbeds.

Agriculture has changed the amount and composition of dissolved organic matter in Central European headwater streams

Dissolved organic matter (DOM) is an important part of the global carbon cycle and significantly influences aquatic ecosystem functions. Recent studies suggest that its amount and composition in freshwaters may be altered by agricultural land use, whereby the influence of preceding in-stream production and processing is not clear. To assess the land use effect on DOM amount and composition for the export from terrestrial to freshwater systems at the land-water interface, we sampled headwater streams draining agricultural and near-pristine catchments (forested and wetland) in the North German plains. To account for spatial and seasonal variation, we conducted a screening of DOM amount (53 sites) and composition (42 sites), and conducted bi-weekly samplings to investigate seasonal variation at eight sites over one year. Concentrations of dissolved organic carbon (DOC) were significantly higher for agricultural and wetland catchments than for forested catchments. Moreover, DOC loads exhibited higher seasonal variation for agricultural and wetland catchments than for forested catchments, which was due to higher variation in discharge. Parallel Factor Analysis revealed that the composition of DOM in agricultural catchments was significantly different from the other studied catchment types, and was characterized by low redox state and high structural complexity. Moreover, a gradient from protein- to humic-like fluorescence significantly separated forested from agricultural and wetland catchments. The contribution of humic-like DOM was strongly and positively related to DOC concentration, suggesting a mechanistic coupling of both. The effects of land use on patterns of DOC concentration and DOM composition were consistent across seasons, implying that land use strongly regulates DOM export. Overall, this study clearly shows the seasonally independent importance of agricultural land use for the amount and composition of DOM fluxes from the terrestrial zone to surface waters. These altered fluxes may affect ecosystem metabolism and health of agricultural headwaters and downstream situated aquatic ecosystems.

Resazurin as a “smart” tracer for quantifying metabolically active transient storage in stream ecosystems

[1] We propose the experimental use of resazurin (Raz) and develop a metabolically active transient storage (MATS) model to include processes that may provide additional information on transient storage from a biogeochemical perspective in stream ecosystems. Raz is a phenoxazine compound that reduces irreversibly to resorufin (Rru) in the presence of aerobic bacteria. Raz was added as a stream tracer to a 128-m reach of the forested second-order Riera de Santa Fe del Montseny (Catalonia, NE Spain), along with a conservative tracer, NaCl. Raz was transformed to Rru at a rate of 0.81 h � 1 in the hyporheic zone and only at a rate of 9.9 � 10 � 4 h � 1 in the stream surface channel. Raz transformation and decay and Rru production and decay were both correlated with O2 consumption measured at wells. The ratio of Raz to Rru concentration at the bottom of the reach was moderately correlated with instantaneous rates of net ecosystem production (NEP) measured over the whole reach. Data for Raz, Rru, and chloride were well fitted with the MATS model. The results from this study suggest that Raz transformation to Rru can be used as a ‘‘smart’’ tracer to detect metabolic activity, specifically aerobic respiration, associated with transient storage zones in stream ecosystems. Therefore, the Raz-Rru system can provide an assessment of the amount of transient storage that is metabolically active, an assessment that complements the physical characterization of transient storage obtained from conventional hydrologic tracers. The use of both physical and metabolic parameters of transient storage obtained with these tracers may increase our understanding of the relevance of transient storage on stream biogeochemical processes at whole reach scale, as well as the contribution of the different transient storage compartments to these processes.

Occurrence and in-stream attenuation of wastewater-derived pharmaceuticals in Iberian rivers

A multitude of pharmaceuticals enter surface waters via discharges of wastewater treatment plants (WWTPs), and many raise environmental and health concerns. Chemical fate models predict their concentrations using estimates of mass loading, dilution and in-stream attenuation. However, current comprehension of the attenuation rates remains a limiting factor for predictive models. We assessed in-stream attenuation of 75 pharmaceuticals in 4 river segments, aiming to characterize in-stream attenuation variability among different pharmaceutical compounds, as well as among river segments differing in environmental conditions. Our study revealed that in-stream attenuation was highly variable among pharmaceuticals and river segments and that none of the considered pharmaceutical physicochemical and molecular properties proved to be relevant in determining the mean attenuation rates. Instead, the octanol-water partition coefficient (Kow) influenced the variability of rates among river segments, likely due to its effect on sorption to sediments and suspended particles, and therefore influencing the balance between the different attenuation mechanisms (biotransformation, photolysis, sorption, and volatilization). The magnitude of the measured attenuation rates urges scientists to consider them as important as dilution when aiming to predict concentrations in freshwater ecosystems.

Hydrological extremes modulate nutrient dynamics in mediterranean climate streams across different spatial scales

Hydrology is a key factor in the terrestrial landscape that connects upland and riparian ecosystems to streams. Hydrological connectivity through the catchment and along the stream network influences not only the timing and the magnitude of terrestrial inputs to stream ecosystems, but also regulates stream metabolism and biogeochemical cycling. In mediterranean climate regions (med-regions), there is a marked seasonal alternation of dry and wet periods, and streams experience a wide range of hydrological conditions from floods to droughts. Moreover, nutrient dynamics in med-streams are highly variable within the year and between years compared to temperate streams. In this review, we explore how seasonal changes in hydrological connectivity and hydrological extremes affect nutrient cycling and transport at different spatial scales in med-catchments. We show that during the dry period, the stream network contracts more severely in med-catchments than in temperate catchments. During the contraction phase, upland-stream disconnection decreases nutrient supply from terrestrial ecosystems, while stream fragmentation increases the spatial variation of stream nutrient concentrations. Moreover, the precipitation regime typical of med-regions (with irregular distribution of precipitation and episodic large rainfall events) decreases the ability of terrestrial and aquatic biota to retain nutrients and favours nutrient export to downstream ecosystems. Altogether, this hydrological setting confers to med-streams a characteristic temporal pattern in stream nutrient dynamics that is analogous to a particular biogeochemical heartbeat with higher amplitude, variation and unpredictability than that observed in temperate streams.

Carbon dioxide emissions from dry watercourses

Abstract Temporary watercourses that naturally cease to flow and run dry comprise a notable fraction of the world’s river networks, yet estimates of global carbon dioxide (CO2) emissions from watercourses do not consider emissions from these systems when they are dry. Using data from a sampling campaign in a Mediterranean river during the summer drought period, we demonstrate that the CO2 efflux from dry watercourses can be substantial, comparable to that from adjacent terrestrial soils and higher than from running or stagnant waters. With an up-scaling approach, we show that including emissions from dry watercourses could increase the estimate of CO2 emissions from watercourses in our study region by 0.6–15%. Moreover, our results tentatively illustrate that emissions from dry watercourses could be especially important in arid regions, increasing the estimate of global CO2 emissions from watercourses by 0.4–9%. Albeit relatively small, the contribution of dry watercourses could help to constrain the highly uncertain magnitude of the land carbon sink. We foresee that in many areas of the world, the expected increase in the extent of temporary watercourses associated with future global change will increase the relevance of CO2 emissions from dry watercourses.

Influence of nitrate and ammonium availability on uptake kinetics of stream biofilms

Abstract. Human activity has significantly increased dissolved inorganic N (DIN) availability and has modified the relative proportion of NO3− and NH4+ species in many streams. Understanding the relationship between DIN concentration and DIN uptake is crucial to predicting how streams will respond to increased DIN loading. Nonetheless, this relationship remains unclear because of the complex interactions governing DIN uptake. We aimed to evaluate how biofilms from 2 streams differing in background DIN concentration would respond to increases in availability and changes in speciation (NO3− or NH4+) of DIN. We measured DIN uptake by biofilms in artificial flumes in each stream, using separate 15N-NO3− and 15N-NH4+ additions in a graded series of increasing DIN concentrations. The ambient uptake rate (U) was higher for NO3− than for NH4+ in both streams, but only U for NH4+ differed between streams. Uptake efficiency (UN-specific) at ambient conditions was higher in the low-N than in the high-N stream for both DIN species. A Michaelis–Menten model of uptake kinetics best fit the relationship between uptake and concentration in the case of NH4+ (for both streams) but not in the case of NO3− (neither stream). Moreover, saturation of NH4+ uptake occurred at lower rates (lower Umax) in the low-N than in the high-N stream, but affinity for NH4+ was higher (lower Ks) in the low-N stream. Together, these results indicate that the response capacity of biofilm communities to short-term increases of DIN concentration is determined primarily by the ambient DIN concentrations under which they develop. Our study also shows that DIN uptake by benthic biofilms varies with DIN availability and with DIN speciation, which often is modified by human activities.

Nitrogen processing and the role of epilithic biofilms downstream of a wastewater treatment plant

Abstract.  We investigated how dissolved inorganic N (DIN) inputs from a wastewater treatment plant (WWTP) effluent are processed biogeochemically by the receiving stream. We examined longitudinal patterns of NH4+ and NO3− concentrations and their 15N signatures along a stream reach downstream of a WWTP. We compared the &dgr;15N signatures of epilithic biofilms with those of DIN to assess the role of stream biofilms in N processing. We analyzed the &dgr;15N signatures of biofilms coating light- and dark-side surfaces of cobbles separately to test whether light constrains functioning of biofilm communities. We sampled during 2 contrasting periods of the year (winter and summer) to explore whether changes in environmental conditions affected N biogeochemical processes. The study reach had a remarkable capacity for transformation and removal of DIN, but the magnitude and relevance of different biogeochemical pathways of N processing differed between seasons. In winter, assimilation and nitrification influenced downstream N fluxes. These processes were spatially segregated at the microhabitat scale, as indicated by a significant difference in the &dgr;15N signature of light- and dark-side biofilms, a result suggesting that nitrification was mostly associated with dark-side biofilms. In summer, N processing was intensified, and denitrification became an important N removal pathway. The &dgr;15N signatures of the light- and dark-side biofilms were similar, a result suggesting less spatial segregation of N cycling processes at this microhabitat scale. Collectively, our results highlight the capacity of WWTP-influenced streams to transform and remove WWTP-derived N inputs and indicate the active role of biofilms in these in-stream processes.

Global effects of agriculture on fluvial dissolved organic matter

Agricultural land covers approximately 40% of Earth’s land surface and affects hydromorphological, biogeochemical and ecological characteristics of fluvial networks. In the northern temperate region, agriculture also strongly affects the amount and molecular composition of dissolved organic matter (DOM), which constitutes the main vector of carbon transport from soils to fluvial networks and to the sea, and is involved in a large variety of biogeochemical processes. Here, we provide first evidence about the wider occurrence of agricultural impacts on the concentration and composition of fluvial DOM across climate zones of the northern and southern hemispheres. Both extensive and intensive farming altered fluvial DOM towards a more microbial and less plant-derived composition. Moreover, intensive farming significantly increased dissolved organic nitrogen (DON) concentrations. The DOM composition change and DON concentration increase differed among climate zones and could be related to the intensity of current and historical nitrogen fertilizer use. As a result of agriculture intensification, increased DON concentrations and a more microbial-like DOM composition likely will enhance the reactivity of catchment DOM emissions, thereby fuelling the biogeochemical processing in fluvial networks, and resulting in higher ecosystem productivity and CO2 outgassing.

A round-trip ticket: the importance of release processes for in-stream nutrient spiraling

Most nutrient-spiraling studies have focused on estimates of gross uptake (Ugross), which show that streams take up dissolved inorganic nutrients very efficiently. However, studies based on estimates of net uptake (Unet) emphasize that streams tend to be at biogeochemical steady state (i.e., Unet ≈ 0), at least on a time scale of hours. These findings suggest that streams can be highly reactive ecosystems but remain at short-term biogeochemical steady state if Ugross is counterbalanced by release (R), a process that remains widely unexplored. Here, we propose a novel approach to infer R by comparing Unet and Ugross estimated from ambient and plateau concentrations obtained from standard short-term nutrient additions along a reach. We used this approach to examine the temporal variation of R and its balance with Ugross in 2 streams with contrasting hydrological regime (i.e., perennial vs intermittent) during 2 years. We focused on the spiraling metrics of NH4+ and soluble reactive P (SRP), essential sources of N and P in stream ecosystems. R differed substantially between the 2 streams. The perennial stream had a higher proportion of dates with R > 0 and a 2× higher mean R than the intermittent stream for both nutrients. Despite these differences, the magnitude of R and Ugross tended to be similar for both nutrients within each stream, which lead to Unet ≈ 0 in most cases. A notable exception occurred for SRP in the intermittent stream, where R tended to be higher than Ugross during most of the winter period, probably because of desorption of P from stream sediments. Together, our findings shed light on the contribution of release processes to the dynamics of nutrient spiraling and support the idea that streams can be active ecosystems with high spiraling fluxes while simultaneously approaching short-term biogeochemical steady-state.