Agricultural Headwater Streams Research Articles

The effect of planting grass filter strips on fishes in agricultural headwater streams

Abstract Grass filter strips (i.e., herbaceous riparian buffers) are a widely used conservation practice in the United States to mitigate the impacts of agriculture on streams and rivers. Yet, only a limited amount of information is available on the effects of grass filter strips on stream fishes and instream habitat conditions and long‐term assessments are rare. The objective of this research was to document the long‐term effects of planting grass filter strips adjacent to channelized agricultural headwater streams on fish community structure and instream habitat. Fishes, instream habitat and riparian characteristics were sampled from three channelized agricultural headwater streams without grass filter strips, three channelized agricultural headwater streams with grass filter strips and two unchannelized streams having forested riparian habitats in central Ohio from spring of 2006 to autumn of 2015. Fish species richness, abundance, darter species richness, per cent darters, sunfish species richness, minnow species richness and per cent headwater fishes did not differ among riparian habitat types. Darter species richness and per cent darters differed annually. Mean water depth, velocity, wetted width, instream habitat diversity, dominant grain size and per cent instream wood did not differ among riparian habitat types. Mean water depth, water velocity, wetted width and instream habitat diversity exhibited annual differences. Additionally, mean riparian width, woody vegetation density and per cent canopy cover were greater in forested streams than the unplanted and grass filter strip streams. Riparian characteristics did not differ annually. The lack of fish community responses 10–13 years after planting grass filter strips is probably due to the lack of corresponding changes in hydrological characteristics, substrate and instream wood amounts. These results suggest that grass filter strips should not be implemented alone but in conjunction with conservation practices that will improve physical habitat quality and water quality.

Recognizing Agricultural Headwaters as Critical Ecosystems.

Agricultural headwaters are positioned at the interface between terrestrial and aquatic ecosystems and, therefore, at the margins of scientific disciplines. They are deemed devoid of biodiversity and too polluted by ecologists, overlooked by hydrologists, and are perceived as a nuisance by landowners and water authorities. While agricultural streams are widespread and represent a major habitat in terms of stream length, they remain understudied and thereby undervalued. Agricultural headwater streams are significantly modified and polluted but at the same time are the critical linkages among land, air, and water ecosystems. They exhibit the largest variation in streamflow, water quality, and greenhouse gas emission with cascading effects on the entire stream networks, yet they are underrepresented in monitoring, remediation, and restoration. Therefore, we call for more intense efforts to characterize and understand the inherent variability and sensitivity of these ecosystems to global change drivers through scientific and regulatory monitoring and to improve their ecosystem conditions and functions through purposeful and evidence-based remediation.

Identifying the environmental variables that predict crayfish assemblage structure in agricultural headwater streams

Identifying the environmental variables that predict crayfish assemblage structure in agricultural headwater streams

Phosphorus supply and floodplain design govern phosphorus reduction capacity in remediated agricultural streams

Abstract. Agricultural headwater streams are important pathways for diffuse sediment and nutrient losses, requiring mitigation strategies beyond in-field measures to intercept the transport of pollutants to downstream freshwater resources. As such, floodplains can be constructed along existing agricultural streams and ditches to improve fluvial stability and promote deposition of sediments and particulate phosphorus. In this study, we evaluated 10 remediated agricultural streams in Sweden for their capacity to reduce sediment and particulate phosphorus export and investigated the interplay between fluvial processes and phosphorus dynamics. Remediated streams with different floodplain designs (either on one side or both sides of the channel, with different width and elevation) were paired with upstream trapezoidal channels as controls. We used sedimentation plates to determine seasonal patterns in sediment deposition on channel beds and floodplains and monthly water quality monitoring. This was combined with continuous flow discharge measurements to examine suspended sediment and particulate phosphorus dynamics and reduction along reaches. Remediated streams with floodplains on both sides of the channel reduced particulate phosphorus concentrations and loads (−54 µg L−1, −0.21 kg ha−1 yr−1) along reaches, whereas increases occurred along streams with one-sided floodplains (27 µg L−1, 0.09 kg ha−1 yr−1) and control streams (46.6 µg L−1). Sediment deposition in remediated streams was five times higher on channel beds than on floodplains and there was no evident lateral distribution of sediments from channel to floodplains. There was no effect from sediment deposition on particulate phosphorus reduction, suggesting that bank stabilization was the key determinant for phosphorus mitigation in remediated streams, which can be realized with two-sided but not one-sided floodplains. Further, the overall narrow floodplain widths likely restricted reach-scale sediment deposition and its impact on P reductions. To fully understand remediated streams' potential for reductions in both nitrogen and different phosphorus species and to avoid pollution swapping effects, there is a need to further investigate how floodplain design can be optimized to achieve a holistic solution towards improved stream water quality.

Importance of Dense Aquatic Vegetation in Seasonal Phosphate and Particle Transport in an Agricultural Headwater Stream

AbstractAgricultural headwater streams and ditches commonly host dense stands of aquatic vegetation that grow and decay over seasons and exert physical and biological controls on the transport of nutrients from cropland to larger rivers. This study examined changes in the transport of phosphorus (P) in an agricultural drainage ditch in the Maumee River Basin (Ohio, USA) by conducting constant rate injections of a novel tracer mixture (conservative salt [Cl as NaCl], dissolved P [KH2PO4], and a fluorescent fine particle) in spring, summer, and fall. We quantified transport behavior for solutes and particles using a traditional transient storage modeling framework consisting of mobile and immobile storage zones connected by a first‐order exchange rate constant. Transient storage was greatest during the spring, when thicker vegetation caused more pooling and flow stagnation, and decreased through fall, as vegetation thinned. Soluble P uptake lengths were 8.7 times longer in fall than spring, likely due to declines in biological uptake rates with colder temperatures and immobile zone storage with thinning vegetation. Particle capture lengths also decreased by a factor of 4.3 from fall to spring. With the increasing eutrophication of Lake Erie and waterbodies around the world that lie downstream from agricultural landscapes, it is beneficial to understand nutrient transport across watersheds, including small agricultural streams. This study highlights the physical and biological roles that aquatic vegetation plays in small agricultural streams by creating seasonally variable immobile zones that slow the flow of nutrients, providing surface area for biofilms, and capturing particles that bind nutrients.

Using stable water isotopes to evaluate water flow and nonpoint source pollutant contributions in three southern Ontario agricultural headwater streams

AbstractImproved understanding of catchment‐scale hydrology and nutrient transport in agricultural catchments is needed. Here, the annual young water fraction is determined for three southern Ontario headwater catchments using the stable water isotope δ18O; the dominant event contributions for three streams were also determined using a two‐component isotopic hydrograph separation. On an annual average, Nissouri Creek (loamy soils, high tile drainage) sees the lowest amount of event water, followed by Big Creek (clay soils, high tile drainage) and then North Creek (clay soils, low tile drainage). Event hydrograph separations show Nissouri Creek has significantly higher median amounts of pre‐event water than Big and North Creek during events. These results in indicate soil type may be more influential than tile drainage presence with respect to the contribution of event contributions to total stream flow; specifically, higher contributions of event water occurred in the two clay soil catchments, despite differing tile presence. Antecedent moisture and event characteristics did not uniformly explain the observed pre‐event water contributions for events across all sites, with pre‐event contributions only weakly and positively correlated with discharge at event commencement and negatively with isotopic variability in stream water. The total contribution of pre‐event water during events was significantly correlated with mean event turbidity and the flow‐weighted mean concentrations of dissolved organic carbon, total phosphorus and total dissolved phosphorus, but not total nitrogen or nitrate. The correlations we observed between pre‐event water and water chemistry were stronger than between event size and water chemistry, suggesting isotopic information adds insight that simple hydrometrics are unable to provide. Our results show that event water is responsible for phosphorus loss at the watershed scale, indicating actions targeting fast pathways or targeting pools of phosphorus available to those fast pathways can aid in reducing phosphorus transported by event water.

Evaluating the Effects of Riparian Habitat Type on Nutrient Concentrations in Agricultural Headwater Streams

ABSTRACTGrass filter strips and other riparian buffer types are frequently used in agricultural watersheds to reduce nonpoint source pollution, but there is only limited information available on their long‐term watershed scale effects. Our research questions were as follows: (1) does nutrient concentrations differ among agricultural headwater streams with different riparian habitat types? and (2) does the effect of riparian habitat type differ annually or seasonally? Eight streams in central Ohio were selected based on watershed size, proportion of watershed agricultural land use, and riparian habitat type. Three streams possessed unplanted riparian habitats, three streams had grass filter strips, and two streams had forested riparian habitats. Water samples for the measurement of ammonia, nitrate + nitrite, total nitrogen, dissolved reactive phosphorus, total phosphorus, and dissolved organic carbon concentrations were collected weekly from two sites in each stream from March 2007 to February 2017. Mean nitrogen and phosphorus concentrations did not differ (p > 0.05) among riparian habitat types. Mean dissolved organic carbon concentrations exhibited different (p < 0.05) annual and seasonal trends among riparian habitat types. Our results indicated that planting grass filter strips alone may not decrease nitrogen and phosphorus concentrations and highlighted the influence of riparian woody vegetation on the temporal dynamics of dissolved organic carbon concentrations.

Total and dissolved phosphorus losses from agricultural headwater streams during extreme runoff events

Eutrophication continues to be a concerning global water quality issue. Managing and mitigating harmful algal blooms demands clear information on the conditions promoting large phosphorus losses from contributing watersheds. Of particular concern is the amount and form of phosphorus loading to receiving water bodies during extreme runoff events, which are expected to increase in frequency due to climate change. Five years (2015 to 2020) of water quantity and quality data from 11 agricultural watersheds in the lower Great Lakes basin were analyzed and used to model total and dissolved phosphorus losses. This study aimed to assess temporal dynamics in phosphorus concentrations and losses over runoff events covering a wide range of hydrologic conditions and to quantify their relative importance on annual phosphorus losses. Event concentration-discharge relationships for total and dissolved phosphorus were hysteretic and had contrasting dominant patterns across watersheds. The proportion of annual phosphorus losses during events was highly variable between watersheds, accounting for 47–94 %. Extreme events were particularly impactful: as few as three events per year were found to be responsible for nearly half of total phosphorus (20–50 %) and total dissolved phosphorus (14–44 %) losses. Variability in total and dissolved phosphorus losses and concentrations over a wide range of flow conditions suggests that event magnitude is an important control on the relative mobility of particulate and dissolved phosphorus fractions. This study showed that insights into nutrient dynamics and phosphorus budgets in the lower Great Lakes basin and agriculture dominated environments more broadly can be gained by assessing event nutrient losses with respect to flow conditions and patterns in concentration-discharge relationships.

Nutrient retention in agricultural headwater stream: artificial manipulation of main-channel morphology and hydrologic condition.

To make up for the deficiency of transient storage and nutrient retention capacity of some headwater streams, some effective artificial measures have been developed to improve the stream ecosystem functions. But few studies have focused on the effects of artificial manipulation on nutrient retention in hydrologic and non-hydrologic processes of streams. In response, we selected an agricultural headwater stream in the Banqiao River tributary of Chaohu Lake Basin, artificially altered the flow pattern in the main-channel by introducing barriers which were composed of soil, coarse sand, and stones, and used the tracer experiment and OTIS (one-dimensional transport with inflow and storage) model to assess the transient storage potential of stream and the nutrient retention of hydrologic and non-hydrologic processes. Compared with the control, the retention capacity of ammonium (NH4+) and phosphate (PO43-) and the transient storage potential were increased after introducing barriers. In addition, the total retention (TR), hydrologic retention (HR), and non-hydrologic retention (NHR) of NH4+ and PO43- were significantly increased after manipulation.

Stream network variation in dissolved oxygen: Metabolism proxies and biogeochemical controls

An explosion in high frequency dissolved oxygen (DO) observations at river network scales is creating new opportunities to understand dynamic signals in streams and rivers. Among the most informative metrics obtained from DO time series is stream metabolism—comprising gross primary production (GPP) and ecosystem respiration (ER)—but its estimation is non-trivial. There is thus interest in simpler metrics that can capture spatiotemporal patterns in stream metabolism and their consequences for critical ecosystem processes. Using hourly DO time series from 43 agricultural headwater streams reaches (Strahler order 1–5) across five watersheds and two years, we tested the hypothesis that simple DO metrics are useful proxies of stream metabolism, capturing key features of its spatiotemporal variation, and predicting attendant patterns in dissolved organic matter quality and catchment nitrogen processing via denitrification. Our results suggest the diel DO range scaled by stream depth is an excellent proxy for GPP throughout the network, accurately describing its spatial and temporal patterns. In contrast, we found that DO metrics were less successful as proxies for ER, with the maximum daily DO deficit scaled by depth being a good proxy for ER only in higher order streams. We also observed that DO metrics were strongly related to variation in dissolved organic matter quality and denitrification far better than GPP or ER. Finally, we found that DO metrics, GPP, and to a lesser extent ER, had power-law relationships with watershed area (scaling exponents, β = 0.2–0.5), implying increasing downstream metabolic activity. However, because lower order streams occupy ∼75% of network benthic area, total network GPP and ER (g O2 d−1) were disproportionately provided by lower order streams, consistent with recent theoretical modeling. These findings reveal the rich inference space that simple DO metrics can provide, and support their use as proxies for stream metabolism and for inferring network patterns of biogeochemical function.

Periphytic algal biomass as a bioindicator of phosphorus concentrations in agricultural headwater streams of southern Ontario

Algal blooms in Lake Erie have worsened in recent decades and are driven by diffuse export of phosphorus (P) from a large stream network that drains predominately agricultural land. Given the diffuse nature of nonpoint source pollution, best management practices (BMPs) must target areas where P levels are high. This requires long-term watershed-wide monitoring programs that do not currently exist in many jurisdictions. Instead of conventional nutrient analyses that can be costly and time-consuming, we propose the use of periphyton biomass as a bioindicator of trophic status in low-order streams, where agricultural runoff first enters watercourses. We carried out 2-week in-stream bioassays to measure periphytic algal biomass (CHLperi) in 19 low-order streams in southern Ontario across an agricultural gradient (8 % to 89 %). CHLperi was significantly related to total P (TP) concentration (r2 = 0.46; p = 0.0015) but was not significantly related to soluble reactive P (SRP). A relationship between TP and turbidity (r2 = 0.52; p = 0.0007) is consistent with previous observations of increasing SRP uptake in streams draining agriculturally-dominated landscapes. Stream temperature (°C) was correlated with the proportion of agricultural land (R = 0.55; p = 0.019) and may reflect the warming effects of the sun in unshaded agricultural streams. This method involving substrate rods (Peristix) is cost-effective, requires very little training, and yielded data that were significantly related to TP concentrations in agricultural streams. We recommend that environmental agencies and landowners use this bioassay to identify areas for implementing BMPs to reduce P export from the Lake Erie watershed.

Occurrence of pesticides and their transformation products in headwater streams: Contamination status and effect of ponds on contaminant concentrations

In France, more than 90% of monitored watercourses are contaminated with pesticides. This high contamination level increases at the head of agricultural watersheds, where dilution capacities are low and transport from treated lands is direct. Ponds, numerous around headwater streams, could provide additional protection against pesticide pollution. Because of their long hydraulic residence time and large water volumes, they mitigate pesticide concentrations between upstream and downstream rivers. However, pesticide transformation products may also be responsible for the degradation of environments, owing to their presence at high concentrations and their persistence, but related data are scarce, particularly because of their high level of molecular diversity. We first reported on the state of water contamination in agricultural headwater streams, based on high frequency water sampling. Analysis of 67 molecules (HPLC-ESI-MS/MS) showed pesticides and pesticide transformation product mixtures of up to 29 different compounds in one sample. Regardless of the sampling location, transformation products represented at least 50% of the detected compounds. Then, we demonstrated the capacity of a pond to reduce contaminant concentrations in downstream rivers for 90% of the detected compounds. Upstream from this pond, environmental quality or ecotoxicological standards were exceeded during sampling, with pesticide and transformation product sum concentrations of up to 27 μg/L. Downstream from the study pond, few exceedances were observed, with a maximum total concentration of 2.2 μg/L, reflecting significant water quality improvement.

Riparian forests can mitigate warming and ecological degradation of agricultural headwater streams

Abstract Riparian forests are commonly advocated as a key management option to mitigate the effects of agriculture on headwater stream biodiversity and ecosystem functions. However, the benefits of riparian forests might be reduced by uninterrupted catchment‐scale pollution. We studied the effects of riparian land use on multiple ecological endpoints in headwater streams in an agricultural landscape. We studied stream habitat characteristics, water temperature and algal accrual, and macrophyte, benthic macroinvertebrate and fish communities in 11 paired forested and open agricultural headwater stream reaches that differed in their extent of riparian forest cover but had similar water quality. Hydromorphological habitat quality was higher in forested reaches than in open reaches. Riparian forest had a strong effect on the summer water temperature regime, with maximum and mean water temperatures and temperature variation in forested reaches substantially lower than in open reaches. Macrophyte communities differed between forested and open reaches. The mean abundance of bryophytes was higher in forested reaches but the difference to open reaches was only marginally significant, whereas graminoids were significantly more abundant in open reaches. Within‐stream dissimilarity of benthic macroinvertebrate community structure was significantly related to the difference in riparian land use between reach pairs. The relative DNA sequence abundance of pollution‐sensitive Ephemeroptera, Plecoptera, and Trichoptera species tended to be higher in forested reaches than in open reaches. Finally, fish densities were not significantly different between forested and open reaches, although densities were higher in forested reaches. This unequivocal evidence for the ecological benefits of forested riparian reaches in agricultural headwater streams suggests that riparian forest can partly mitigate the adverse impacts of agricultural diffuse pollution on biota. The strong effect of forests on stream water temperature suggest that riparian forest could also mitigate harmful effects on headwater stream biodiversity and ecosystem functions of the predicted more frequent high summer temperatures.

Influence of Physical and Chemical Characteristics of Sediment on Macroinvertebrate Communities in Agricultural Headwater Streams

Agricultural land use leads to changes in physical and chemical characteristics of sediment that influence macroinvertebrate community diversity and abundance in streams. To the best of our knowledge the joint influence of sediment’s physical and chemical characteristics on stream macroinvertebrates has not been assessed. We measured sediment’s physical and chemical characteristics and sampled macroinvertebrates in eight agricultural headwater streams in Indiana, Michigan, and Ohio, United States, in 2017 and 2018 to determine the physical and chemical conditions of the sediment, to evaluate the relationships between physical and chemical characteristics of the sediment, and the relationship of macroinvertebrate communities with the sediment’s physical and chemical characteristics. Sediments within most sites were dominated by sand or silt. pH was suitable for macroinvertebrates and nitrate, herbicide, and trace metal concentrations were below concentration levels anticipated to affect macroinvertebrate survival. Linear mixed effect model analysis results indicated that a physical gradient of percent small gravel and percent silt was positively correlated (p < 0.05) with a chemical gradient of potassium concentrations, magnesium concentrations, and percent total nitrogen in the sediments. Our linear mixed effect model analysis results also indicated that Invertebrate Community Index scores were negatively correlated (p < 0.05) with a chemical gradient of simazine and calcium concentrations and were negatively correlated (p < 0.05) with physical gradient of grain size diversity and percent sand. Our results suggest that watershed management plans need to address physical and chemical degradation of sediment to improve macroinvertebrate biotic integrity within agricultural headwater streams in the Midwestern United States.

Conservation implications of fish-habitat relationships in channelized agricultural headwater streams.

Channelized agricultural headwater streams are common throughout agricultural watersheds in the midwestern United States. Understanding fish-habitat relationships within these streams will provide information that can assist with developing conservation and restoration strategies for these degraded streams. From spring 2006 to fall 2010, we collected fishes and measured riparian habitat, instream habitat, and water chemistry variables from seven sites in Cedar Creek, Indiana, and 14 sites in Upper Big Walnut Creek, Ohio. We found that fish community structure was more strongly correlated with instream habitat than riparian habitat or water chemistry in both watersheds. We also observed interrelationships among instream habitat, watershed size, and fish communities within both watersheds that suggest that the hydrological changes that occur with increasing watershed size are the underlying factor for fish community changes that occur with increasing watershed size. Our results suggest that conservation and restoration efforts within channelized agricultural headwater streams in the midwestern United States, where nutrients and herbicide concentrations are low, need to address physical habitat degradation to positively influence fish community structure.

Carbon dioxide dynamics in an agricultural headwater stream driven by hydrology and primary production

Abstract. Headwater streams are known to be hotspots for carbon dioxide (CO2) emissions to the atmosphere and are hence important components in landscape carbon balances. However, surprisingly little is known about stream CO2 dynamics and emissions in agricultural settings, a land use type that globally covers ca. 40 % of the continental area. Here we present hourly measured in situ stream CO2 concentration data from a 11.3 km2 temperate agricultural headwater catchment covering more than 1 year (in total 339 d excluding periods of ice and snow cover). The stream CO2 concentrations during the entire study period were generally high (median 3.44 mg C L−1, corresponding to partial pressures (pCO2) of 4778 µatm) but were also highly variable (IQR = 3.26 mg C L−1). The CO2 concentration dynamics covered a variety of different timescales from seasonal to hourly, with an interplay of hydrological and biological controls. The hydrological control was strong (although with both positive and negative influences dependent on season), and CO2 concentrations changed rapidly in response to rainfall and snowmelt events. However, during growing-season base flow and receding flow conditions, aquatic primary production seemed to control the stream CO2 dynamics, resulting in elevated diel patterns. During the dry summer period, rapid rewetting following precipitation events generated high CO2 pulses exceeding the overall median level of stream CO2 (up to 3 times higher) observed during the whole study period. This finding highlights the importance of stream intermittency and its effect on stream CO2 dynamics. Given the observed high levels of CO2 and its temporally variable nature, agricultural streams clearly need more attention in order to understand and incorporate these considerable dynamics in large-scale extrapolations.

Injury frequency and severity in crayfish communities as indicators of physical habitat quality and water quality within agricultural headwater streams.

Crayfishes (Decapoda) are common inhabitants of agricultural headwater streams in the Midwestern USA that have been impacted by physical habitat degradation and contamination by agricultural pollutants. The frequency and severity of injuries within crayfish communities are indicators of crayfish aggression, which is influenced by physical, chemical, and biotic factors. Previous studies have not evaluated the relationships of the frequency and severity of crayfish injuries with physical habitat quality, water quality, and biotic factors within agricultural headwater streams. Understanding these relationships will assist with determining if crayfish injury variables can serve as an indicator of physical habitat quality or water quality in these small degraded streams. We sampled crayfishes, documented the frequency and type of injuries, and measured instream habitat and water chemistry in 2014 and 2015 within 12 agricultural headwater streams in Indiana, Michigan, and Ohio. We documented five native crayfish species from 1641 adult captures. The most abundant species were Faxonius rusticus, Faxonius immunis, and Faxonius propinquus. Linear mixed effect model analyses indicated that four crayfish injury response variables were positively correlated (p < 0.05) with crayfish density, physical habitat quality, and water velocity diversity and that crayfish injury response variables were more strongly correlated with crayfish density than physical habitat quality or water quality. Our results indicate that response variables describing the severity and frequency of crayfish injuries can be effective indicators of physical habitat quality in agricultural headwater streams.

Characteristics of the Dissolved Nitrous Oxide (N2O) Concentrations and Influencing Factors in a Representative Agricultural Headwater Stream in the Upper Reach of the Yangtze River

Headwater streams around agricultural farmlands can act as important sinks of active nitrogen (N) and potential sources of indirect nitrous oxide (N2O) emissions, as well as aggravating agricultural non-point source N pollution. In this study, the dynamic characteristics of the dissolved N2O concentration in an agricultural headwater stream in the hilly area of purple soil in the upper reach of the Yangtze River were observed during the period Dec. 2014-Oct. 2015 by measuring the headspace gaseous N2O concentration using headspace equilibration-gas chromatography, and the dissolved N2O concentration was calculated according to the related parameters. Simultaneously, the physical and chemical parameters of the stream water were also monitored to understand the factors that affect the dissolved N2O concentration. The results showed that the dissolved N2O concentration in the agricultural headwater stream ranged from 0.26 to 1.28 μg·L-1 with an annual mean value of 0.57 μg·L-1, with nitrate (NO3--N, with an annual mean concentration of 1.45 mg·L-1) as the predominant reactive N form. The seasonal mean concentrations of the dissolved N2O in winter, spring, summer, and autumn were 0.63, 0.45, 0.53, and 0.64 μg·L-1, respectively, without significant seasonal variations. The annual dynamics of the dissolved N2O concentration were primarily governed by the concentration of NO3--N in the stream water, with denitrification being the main process producing N2O. The saturation levels of the dissolved N2O in the stream water showed oversaturation, with an annual mean value of 203.9% (109.7%-546.5%), with a seasonal pattern in which the saturation levels in the summer and autumn were higher than those in the winter and spring, indicating that the agricultural headwater stream can release indirect N2O emissions throughout the year. The temporal variations in the saturation levels of the dissolved N2O were mainly controlled by the water temperature and the NO3--N concentration of the stream water. During April-October, the concentration of dissolved N2O in the stream fluctuated obviously as a result of heavy rainfall, which resulted in an increase of the concentration of NO3-N in the stream water in the short term after the rain, which promoted denitrification and then increased the dissolved N2O level correspondingly.

Event-transport of beta-d-glucuronidase in an agricultural headwater stream: Assessment of seasonal patterns by on-line enzymatic activity measurements and environmental isotopes

Understanding the fate of fecal pollution in the landscape is required for microbial risk analysis. The aim of this study was to assess the patterns and dynamics of beta-d-glucuronidase (GLUC), which has been suggested as a surrogate for fecal pollution monitoring, in a stream draining an agricultural headwater catchment. Automated enzymatic on-site measurements of stream water and sediments were made over two years (2014–2016) to quantify the sources and pathways of GLUC in a stream. The event water fraction of streamflow was estimated by stable isotopes. Samples from field sediments on a hillslope, streambed sediment and stream water were analyzed for GLUC and with a standard E. coli assay. The results showed ten times higher GLUC and E. coli concentrations during the summer than during the winter for all compartments (field and streambed sediments and stream water). The E. coli concentrations in the streambed sediment were approximately 100 times those of the field sediments. Of the total GLUC load in the study period, 39% were transported during hydrological events (increased streamflow due to rainfall or snowmelt); of these, 44% were transported when the stream contained no recent rainwater. The results suggested that a large proportion of the GLUC and E. coli in the stream water stemmed from resuspended streambed sediments. Moreover, the results strongly indicated the existence of remnant populations of GLUC-active organisms in the catchment.

[Indirect Nitrous Oxide Emissions from an Agricultural Headwater Stream During the Rainy Season in the Upper Reach of the Yangtze River].

Agricultural headwater streams have a close hydrologic connection with adjacent farmland ecosystems. Based on the aggravation of agricultural nonpoint source of nitrogen (N) pollution, these streams can become an important sink of N and source of indirect nitrous oxide (N2O) emission. In this study, indirect N2O emissions from an agricultural headwater stream in the hilly area of purple soil in the upper reach of the Yangtze River were measured in situ using the closed static chamber-GC technique during the rainy season (June to September 2015). The results show that the headwater stream is a source of indirect N2O emissions, with a mean emission rate of 12.8 μg·(m2·h)-1, which is close to the direct N2O emission level from local farmland during the same season. The indirect N2O emission factor (EF5r=0.01%) determined in this study is much lower than the default value proposed by the Intergovernmental Panel on Climate Change (0.25%; IPCC, 2006) for the estimation of indirect agricultural N2O emissions and far lower than the recalculated mean value (0.20%) based on available global data. However, based on the limited number of studies on EF5r and the high spatial variations among them, more in situ observations are needed and vital to generate more accurate EF5r values and reduce the uncertainty of indirect N2O estimations calculated based on the EF5r. The indirect N2O fluxes are positively correlated with the NO3--N concentrations of the stream. Thus, denitrification is the main process of N2O production. Moreover, the indirect N2O fluxes could be notably promoted by the rapid increase of the NO3--N concentrations that were driven by rainfall>9 mm during days with continuous rain.