Explore the vast range of titles available.

Estimates of nitrate loading to the Arctic Ocean are limited by the lack of field observations within deltas partly due to logistical constraints. To overcome this limitation, we use a remote sensing framework to estimate retention of nitrate in Arctic deltas. We achieve this by coupling hydrological and biogeochemical process models at the network scale for five major Arctic deltas. Binary masks of delta channels were used to simulate flow direction and magnitude through networks. Models were parameterized using historical and seasonal observations. Simulated nitrate retention ranged from 2.9% to 15% of the incoming load. Retention rates were largest during winter but smallest during spring conditions when increased discharges export large nitrate masses to the coast. Under future climate scenarios, retention rates fall by ∼1%–10%. Arctic deltas have an important effect on the magnitude of nitrate entering Arctic seas and the inclusion of processing in deltas can improve flux estimates. Plain Language Summary The Arctic Ocean is the smallest and shallowest of the world's oceans but receives the largest riverine input per basin volume. Thus, the flux of nitrate from land to sea through rivers in the circumpolar Arctic can have large impacts on coastal life and global biogeochemical cycles. To assess the impact of terrestrially derived nitrate on Arctic Ocean chemistry under current and future scenarios, accurate accounting of nitrate loading is needed. Deltas act as filters that reduce nitrate loads, but current estimates of flux exclude the effects of Arctic deltas on nitrate export. We use a novel approach based on satellite imagery to estimate the potential impact of deltas on nitrate retention on five Arctic deltas, representing a range of morphologies and sizes. Our analysis shows seasonal and morphological differences in nitrate processing rates in each delta with retention rates ranging from 2.9% to 15% of incoming load and most retention occurring during winter months. Under future climate scenarios, the efficiency of nitrate retention decreases by up to 10%. Our models suggest that Arctic deltas alter the magnitude of nitrate entering the ocean and future predictions of loading or current earth system models could be improved by incorporating their effect. Key Points Remotely sensed data provide a useful tool for revealing patterns of nitrate retention in Arctic deltas Channel networks of Arctic deltas are most efficient at retaining nitrate during spring and winter Hotspots of nitrate removal cluster according to network structure and discharge on Arctic deltas

Urban streams are often severely impaired due to channelization, high loads of nutrients and contaminants, and altered land cover in the watershed. Physical restoration of stream channels is widely used to offset the effects of urbanization on streams, with the goal of improving ecosystem structure and function. However, these efforts are rarely guided by strategic analysis of the factors that mediate the responsiveness of stream ecosystems to restoration. Given that ecological gradients from headwater streams to mainstem rivers are ubiquitous, we posited that location within a river network could mediate the benefits of channel restoration. We studied existing stream restorations in Milwaukee, Wisconsin, to determine (1) whether restorations improve ecosystem function (e.g., nutrient uptake, whole-stream metabolism) and (2) how ecosystem responses vary by position in the urban river network.We quantified a suite of physicochemical and biological metrics in six pairs of contiguous restored and concrete channel reaches, spanning gradients in baseflow discharge (19–196 L/s) and river network position (i.e., headwater to mainstem). Hydrology differed dramatically between the restored and concrete reaches; water velocity was reduced 2- to 13-fold while water residence time was 50–5,000% greater in adjacent restored reaches. Restored reaches had shorter nutrient uptake lengths for ammonium, nitrate, and phosphate, as well as higher whole-stream metabolism. Furthermore, the majority of reaches were autotrophic (i.e., gross primary production > ecosystem respiration), which is not common in stream ecosystems. The difference in ecosystem functioning between restored and unrestored reaches was generally largest in headwaters and declined to equivalence in mainstem restorations. Our results suggest that headwater sites offer higher return on investment compared to larger downstream channels, where ecosystem responsiveness is low. If this pattern proves to be general, the scaling of ecosystem responses with river size could be integrated into planning guidelines for urban stream restorations to enhance the societal and ecological benefits of these expensive interventions.

Soils are currently leaching out dissolved organic matter (DOM) at an increasing pace due to climate and land use change or recovery from acidification. The implications for stream biogeochemistry and food webs remain largely unknown, notably the metabolic balance (biotic CO2 emissions) and carbon cycling between autotrophs and bacteria. We increased by 12% the flux of DOM in a stream for three weeks to mimic a pulse of natural DOM supply from soils rich in organic matter. We were able to track its fate into the food web through the use of a before and after control impact experimental design and the addition of DOM with a distinctive δ13C signature. We used whole‐stream metabolism to quantify carbon fluxes. Both photosynthesis and heterotrophic respiration increased rapidly following C addition, but this was short lived, likely due to nutrient limitations. Carbon exchange between autotrophs and bacteria in the control stream accounted for about 49% of bacterial production and 37% of net primary production, under stable flow conditions. Net primary production relied partly (19% in the control) on natural allochthonous dissolved organic carbon via the CO2 produced by bacterial respiration, intermingling the green and brown webs. The preferential uptake of labile carbon by bacteria and excess bacterial CO2 relative to nutrients (N, P) for autotrophs shifted the reciprocal carbon exchange between bacteria and autotrophs to a predominantly one‐way carbon flow from bacteria to autotrophs, increasing the C:N:P molar ratios of autotrophs, the latter likely to become less palatable to consumers. The bacterial response to sucrose addition shifted the metabolic balance toward heterotrophy increasing biotic CO2 emissions (+125%), shortened the average distance travelled by a molecule of organic matter (−40%), and thus provided less organic matter of lower quality for downstream ecosystems. Even a small increase in labile dissolved organic matter supply due to climate and land use change could significantly alter in‐stream carbon cycling, with large effects on stream food web and biogeochemistry in small streams draining catchments with soils rich in organic carbon.

Calcium carbonate (CaCO₃) deposition is common in aquatic ecosystems and may reduce phosphorus availability via coprecipitation of phosphate, an impact with broad implications for ecosystem processes. To determine if CaCO₃ deposition in streams increases phosphorus (P) retention in minerals while reducing P availability to organisms, we studied paired streams (with and without active CaCO₃ deposition) subjected to experimental shading and monitored changes in ecosystem attributes (e.g., periphyton biomass content, nutrient spiraling, periphyton nutrient limitation, and leaf litter decomposition). Shading reduced rates of CaCO₃ deposition by over 50%, suggesting that a substantial proportion of CaCO₃ deposition is supported by photosynthetically induced changes in alkalinity. Shading-induced reductions in CaCO₃ deposition led to increases in epilithon biomass P content (P < 0.05) and periphyton growth (F2,12 = 5.79, P < 0.05). Reductions in CaCO₃ deposition also relieved P limitation of periphyton growth (F3,16 = 59.32, P < 0.001), extended P uptake lengths at least an order of magnitude, and reduced both P mass transfer velocity and areal uptake rates by over 80% (F2,3 = 22.62, P < 0.05 and F2,3 = 13.19, P < 0.05, respectively). Finally, while shading caused reductions in leaf litter decomposition in the non-CaCO₃ depositing stream (F5,7 = 22.45, P < 0.001), shading had no effect on leaf litter decomposition in the stream with active CaCO₃ deposition. These results indicate that CaCO₃ deposition can regulate P bioavailability and retention in streams and may drive streams to be P limited, as has been suggested in lake and wetland ecosystems.

Interactions among multiple nutrients uptake certainly have a great effect on their retention in headwater streams, yet little research has been made to explore the quantitative characteristics of their interactions, especially in mesotrophic streams. In response, we conducted an identical series of instantaneous nutrient addition experiments, using ammonium nitrogen (NH 4 -N) and phosphate phosphorus (PO 4 -P) alone or together, in two mesotrophic agricultural headwater streams in Chaohu Lake Basin, China, to quantify the relationships between nutrient concentrations and uptake rates, and examine how NH 4 -N and PO 4 -P interact to affect their individual uptake. Both the Michaelis-Menten (M-M) equation and response surface model were utilized to analyze coupled NH 4 -N and PO 4 -P uptake patterns across a range of nutrient concentrations, by fitting the kinetic processes of NH 4 -N and PO 4 -P uptake in single- and dual-nutrient additions. The capacity of both NH 4 -N and PO 4 -P uptake was increased in different degrees in dual-nutrient additions. Response surface models could quantitatively characterize the three-dimensional dynamic evolution trend of NH 4 -N or PO 4 -P uptake rates at different concentrations. The influence of PO 4 -P additions on NH 4 -N uptake was generally greater than that of NH 4 -N on PO 4 -P uptake in the five tracer tests. In addition, results of correlation analysis indicated that water temperature might be the main factor affecting the coupling of N and P uptake in mesotrophic streams and followed by hydrological factors (e.g., discharge) and channel geomorphology.

Assessments of biotic nutrient limitation in aquatic ecosystems typically rely on concentrations and ratios of potentially limiting nutrients. While successful in lakes, this approach has been less effective in streams, which often are dominated by benthic autotrophs (e.g., algae and vascular plants). We compared water-column nutrient concentration (C) to 3 alternative metrics and assessed their ability to predict autotrophic nutrient limitation in streams. One metric, nutrient flux (F) is the product of nutrient concentration and stream discharge. The other 2 metrics, autotrophic uptake length (Sw,a) and autotrophic uptake velocity (Vf,a), are derived from nutrient-spiraling theory and incorporate nutrient uptake. To evaluate the ability of each metric to predict nutrient limitation, we analyzed nutrient diffusing substrata (NDS) data from the Lotic Intersite Nitrogen eXperiment (LINX) (phases I and II; n = 80 streams across North America). We calculated NDS response ratios (RRs) to quantify the strength of autotrophic nutrient limitation in each stream, and we regressed RRs against each of the 4 metrics for N and P. For the LINX I analysis, Sw,a was the best predictor of autotrophic N limitation. F and Vf,a also performed well, and C was a poor predictor. In contrast, all 4 metrics were poor predictors of N limitation for the LINX II analysis when evaluated for individual streams, by land use, and within regions. None of the metrics were able to predict P limitation for either LINX study. Accuracy could be enhanced by developing new methods to quantify autotrophic nutrient uptake and limitation in stream reaches, but consistency with nutrient-spiraling theory and improved predictability of autotrophic N limitation make Sw,a a potentially useful metric for evaluating N limitation in streams.

The number of anthropogenic compounds that occur in aquatic ecosystems today is in the thousands, many at trace concentrations. One group of compounds that has captured the interest of both the scientific community and the general public is pharmaceutical and personal care products (PPCPs), for example, hormones, chemotherapy drugs, antihistamines, stimulants, antimicrobials and various cosmetic additives. Toxicology of some PPCPs is currently understood, but their effect on ecological structure and function of aquatic ecosystems is largely unknown. We review sources and fates of these compounds in aquatic ecosystems and discuss how methods developed to study aquatic ecosystem ecology can contribute to our understanding of the influence of PPCPs on aquatic ecosystems. We argue that aquatic ecology has a well-developed tool kit for measuring the transformation, fate, and transport of solutes using assays and experiments and that these methods could be employed to investigate how PPCPs impact ecological function. We discuss the details of these approaches and conclude that application of existing ecological methods to the study of this issue could substantially improve our understanding of the effect of these compounds in aquatic ecosystems.

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.

Given recent focus on large rivers as conduits for excess nutrients to coastal zones, their role in processing and retaining nutrients has been overlooked and understudied. Empirical measurements of nutrient uptake in large rivers are lacking, despite a substantial body of knowledge on nutrient transport and removal in smaller streams. Researchers interested in nutrient transport by rivers (discharge > 10 000 L/s) are left to extrapolate riverine nutrient demand using a modeling framework or a mass balance approach. To begin to fill this knowledge gap, we present data using a pulse method to measure inorganic nitrogen (N) transport and removal in the Upper Snake River, Wyoming, USA (seventh order, discharge 12 000 L/s). We found that the Upper Snake had surprisingly high biotic demand relative to smaller streams in the same river network for both ammonium (NH₄⁺) and nitrate (NO₃⁻). Placed in the context of a meta-analysis of previously published nutrient uptake studies, these data suggest that large rivers may have similar biotic demand for N as smaller tributaries. We also found that demand for different forms of inorganic N (NH₄⁺ vs. NO₃⁻) scaled differently with stream size. Data from rivers like the Upper Snake and larger are essential for effective water quality management at the scale of river networks. Empirical measurements of solute dynamics in large rivers are needed to understand the role of whole river networks (as opposed to stream reaches) in patterns of nutrient export at regional and continental scales.

Fire can transform the boreal forest landscape, thereby leading to potential changes in the loading of organic matter and nutrients to receiving streams and in the retention or transformation of these inputs within the drainage network. We used the Tracer Additions for Spiraling Curve Characterization (TASCC) method to conduct 17 nutrient-addition experiments (9 single additions of NO3 – and 8 combined additions of NH4 + and PO4 3–) in 5 boreal headwater streams underlain by continuous permafrost and draining watersheds with a range of burn histories (4–>100 y since last burn) in the Nizhnyaya Tunguska River watershed in Central Siberia. Hydrology, ambient nutrient concentration, and the ratio of dissolved organic C (DOC) to nutrients drove rates of nutrient uptake in the streams. Nutrients were taken up with greater efficiency and magnitude under conditions with high flow and reduced diffusive boundary layer (DBL), regardless of watershed burn history. Ambient molar ratio of DOC∶PO4 3– explained some variation in ambient uptake velocity (υ f ) for NH4 + and PO4 3–. We also observed tight coupling between ambient rates of NH4 + and PO4 3– uptake across the watershed burn-history gradient. These data suggest that fire-driven changes in stream chemistry may alter N and P retention and subsequent export of materials to downstream receiving waters. Climate change is likely to enhance the frequency and intensity of boreal forest fires and alter the extent of permafrost. Therefore, understanding the interactions among C, N, and P in these Arctic systems has important implications for global biogeochemical cycling.