Jonathan M. O'Brien

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

Nitrous oxide emission from denitrification in stream and river networks

Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3−) concentrations, but that <1% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3−. We suggest that increased stream NO3− loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y−1 of anthropogenic N inputs to N2O in river networks, equivalent to 10% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.

Correlations between in situ denitrification activity and nir-gene abundances in pristine and impacted prairie streams.

Denitrification is a process that reduces nitrogen levels in headwaters and other streams. We compared nirS and nirK abundances with the absolute rate of denitrification, the longitudinal coefficient of denitrification (i.e., Kden, which represents optimal denitrification rates at given environmental conditions), and water quality in seven prairie streams to determine if nir-gene abundances explain denitrification activity. Previous work showed that absolute rates of denitrification correlate with nitrate levels; however, no correlation has been found for denitrification efficiency, which we hypothesise might be related to gene abundances. Water-column nitrate and soluble-reactive phosphorus levels significantly correlated with absolute rates of denitrification, but nir-gene abundances did not. However, nirS and nirK abundances significantly correlated with Kden, as well as phosphorus, although no correlation was found between Kden and nitrate. These data confirm that absolute denitrification rates are controlled by nitrate load, but intrinsic denitrification efficiency is linked to nirS and nirK gene abundances.

Saturation of NO3− uptake in prairie streams as a function of acute and chronic N exposure

Abstract We conducted a series of stepwise NO3− additions to investigate the response of NO3− uptake to short-term (acute) changes in N concentration in 3 prairie streams. Observed NO3− uptake rates increased with short-term elevations in NO3− concentration and were consistent with linear and Michaelis–Menten kinetics models. We compiled these data with uptake rates from additional published studies to calculate robust estimates of N uptake kinetics for prairie streams. Half-saturation coefficients based on compiled data were 6.7 µg/L for NH4+ and 67 µg/L for NO3−-N. This difference in half-saturation coefficients suggests that NH4+ is more efficiently assimilated than NO3−, indicating a preference for NH4+ as an N source. Similarly, ambient concentrations of NH4+ and NO3− were less than their respective half-saturation coefficients, and aerial uptake rates were generally <5% of the maximum, suggesting severe limitation of N uptake at ambient conditions. The observed pattern of uptake kinetics suggests that physiological constraints limit biotic N uptake in these low-N streams and contrasts with the pattern of uptake observed in streams with chronically elevated ambient NO3− concentrations.

Natural stressors in uncontaminated sediments of shallow freshwaters: The prevalence of sulfide, ammonia, and reduced iron

Potentially toxic levels of 3 naturally occurring chemical stressors (dissolved sulfide, ammonia, and iron) can appear in freshwater sediments, although their roles in shaping ecosystem structure (i.e., plant and animal communities) and function (e.g., biologically mediated elemental cycles) have received little study. The present critical review discusses the prevalence and ecological effects of potentially toxic concentrations of sulfide, ammonia, and iron in uncontaminated freshwater sediments, including a review of the literature as well as a case study presenting previously unpublished data on sediment porewaters from a diverse set of shallow (<2 m) freshwater ecosystems in southwest Michigan, USA. Measured concentrations are compared with surface water quality criteria established by the US Environmental Protection Agency (USEPA) and with acute and chronic toxic thresholds in the published literature, where available. Based on USEPA criteria for aquatic life for these 3 stressors, the benthic environment of almost every freshwater ecosystem sampled was theoretically stressful to some component of aquatic life in some area or at some time (i.e., in at least 1 sample), and 54% of samples exceeded more than 1 criterion simultaneously. Organismal tolerances to chemical stressors vary, so the observed concentrations are likely shaping benthic animal communities and influencing rates of ecosystem processes. Consideration of the role of natural chemical stressors is important in shaping freshwater benthic environments and in developing bioassessments, restoration goals, and remediation plans. Environ Toxicol Chem 2015;34:467-479.

The Lotic Intersite Nitrogen Experiments: an example of successful ecological research collaboration

Collaboration is an essential skill for modern ecologists because it brings together diverse expertise, viewpoints, and study systems. The Lotic Intersite Nitrogen eXperiments (LINX I and II), a 17-y research endeavor involving scores of early- to late-career stream ecologists, is an example of the benefits, challenges, and approaches of successful collaborative research in ecology. The scientific success of LINX reflected tangible attributes including clear scientific goals (hypothesis-driven research), coordinated research methods, a team of cooperative scientists, excellent leadership, extensive communication, and a philosophy of respect for input from all collaborators. Intangible aspects of the collaboration included camaraderie and strong team chemistry. LINX further benefited from being part of a discipline in which collaboration is a tradition, clear data-sharing and authorship guidelines, an approach that melded field experiments and modeling, and a shared collaborative goal in the form of a universal commitment to see the project and resulting data products through to completion.Abstract: Collaboration is an essential skill for modern ecologists because it brings together diverse expertise, viewpoints, and study systems. The Lotic Intersite Nitrogen eXperiments (LINX I and II), a 17-y research endeavor involving scores of early- to late-career stream ecologists, is an example of the benefits, challenges, and approaches of successful collaborative research in ecology. The scientific success of LINX reflected tangible attributes including clear scientific goals (hypothesis-driven research), coordinated research methods, a team of cooperative scientists, excellent leadership, extensive communication, and a philosophy of respect for input from all collaborators. Intangible aspects of the collaboration included camaraderie and strong team chemistry. LINX further benefited from being part of a discipline in which collaboration is a tradition, clear data-sharing and authorship guidelines, an approach that melded field experiments and modeling, and a shared collaborative goal in the form of a universal commitment to see the project and resulting data products through to completion.

Potential Denitrification Rates in an Agricultural Stream in Southern Illinois

ABSTRACT The influences of stream temperature, nitrate concentration, dissolved oxygen concentration, and substrata composition on potential denitrification rate (PDR) were investigated over one year in Big Creek, an agricultural watershed in southern Illinois. PDR ranged from 0 to 106 μg N g AFDM−1 hr−1 with higher rates in the early spring and late summer. The greatest PDRs were measured in April 2000 and April 2001, which both coincided with newly established periphyton communities. PDR did not correlate as expected with seasonally variable, ambient stream conditions such as temperature, dissolved oxygen or nitrate concentration; however, significant relationships did exist between PDR and dissolved oxygen within seasons. The coarse sediments from the headwater section of Big Creek had a significantly higher PDR (13.1 μg N g AFDM−1 hr−1) than the fine sediments (5.2 μg N g AFDM−1 hr−1) from the lower channelized reach, which may have been a function of lower C:N organic matter in the headwater reach. Based on the PDR measured in this stream, denitrification was not a significant sink of the stream nitrogen load.

Aquatic macrophytes alter productivity‐richness relationships in eutrophic stream food webs

Traditional productivity-diversity theory predicts that eutrophication will result in greater species richness due to increased resources at the bottom of the food web. However, few studies on the effects of increasing ecosystem productivity on biological communities have included responses at multiple trophic levels. We hypothesized that the effect of eutrophication on species richness would vary between different trophic levels due to shifts in community composition and trophic interactions. To investigate the mechanisms driving productivity-richness relationships, we constructed food webs for 18 streams across a eutrophication gradient on South Island, New Zealand, and measured productivity, water quality, and habitat characteristics in each stream. A principal components analysis yielded two orthogonal axes of eutrophication: one associated with macrophytes, benthic substrate, and gross primary productivity (GPP) and the other with catchment area, temperature, and algal standing stock (chlorophyll a). Surprisingly, the majority of community response variables, especially defended primary consumer abundance and biomass, were more strongly associated with the macrophyte/habitat axis. The lack of change in abundance and declines in biomass and average mass of predatory invertebrates and fish with increasing eutrophication, despite increases in prey abundance, suggest that energy was not being passed up the food chain to higher trophic levels. The predominance of defended producers (macrophytes) and consumers (snails and microcrustacea) in eutrophic streams likely served as trophic bottlenecks; both are largely inedible by higher trophic levels. Consequently, our results suggest that managers need to focus on preserving food web linkages and energy flow, as well as biodiversity, in eutrophic systems.