Nitrate uptake across biomes and the influence of elemental stoichiometry: A new look at LINX II

Considering recent increases in anthropogenic N loading, it is essential to identify the controls on N removal and retention in aquatic ecosystems because the fate of N has consequences for water quality in streams and downstream ecosystems. Biological uptake of nitrate (NO3−) is a major pathway by which N is removed from these ecosystems. Here we used data from the second Lotic Intersite Nitrogen eXperiment (LINX II) in a multivariate analysis to identify the primary drivers of variation in NO3− uptake velocity among biomes. Across 69 study watersheds in North America, dissolved organic carbon:NO3− ratios and photosynthetically active radiation were identified as the two most important predictor variables in explaining NO3− uptake velocity. However, within a specific biome the predictor variables of NO3− uptake velocity varied and included various physical, chemical, and biological attributes. Our analysis demonstrates the broad control of elemental stoichiometry on NO3− uptake velocity as well as the importance of biome-specific predictors. Understanding this spatial variation has important implications for biome-specific watershed management and the downstream export of NO3−, as well as for development of spatially explicit global models that describe N dynamics in streams and rivers

Climate, snowmelt dynamics and atmospheric deposition interact to control dissolved organic carbon export from a northern forest stream over 26 years

Increasing concentrations of dissolved organic carbon (DOC) have been identified in many freshwater systems over the last three decades. Studies have generally nominated atmospheric deposition as the key driver of this trend, with changes in climatic factors also contributing. However, there is still much uncertainty concerning net effects of these drivers on DOC concentrations and export dynamics. Changes in climate and climate mediated snowfall dynamics in northern latitudes have not been widely considered as causal factors of changes in long-term DOC trends, despite their disproportionate role in annual DOC export. We leveraged long-term datasets (1988–2013) from a first-order forested tributary of Lake Superior to understand causal factors of changes in DOC concentrations and exports from the watershed, by simultaneously evaluating atmospheric deposition, temperature, snowmelt timing, and runoff. We observed increases in DOC concentrations of approximately 0.14 mg C l−1 yr−1 (mean = 8.12 mg C l−1) that were related with declines in sulfate deposition (0.03 mg SO24− l−1 yr−1). Path analysis revealed that DOC exports were driven by runoff related to snowmelt, with peak snow water equivalences generally being lower and less variable in the 21st century, compared with the 1980s and 1990s. Mean temperatures were negatively related (direct effects) to maximum snow water equivalences (−0.71), and in turn had negative effects on DOC concentrations (−0.58), the timing of maximum discharge (−0.89) and DOC exports (indirect effect, −0.41). Based on these trends, any future changes in climate that lessen the dominance of snowmelt on annual runoff dynamics—including an earlier peak discharge—would decrease annual DOC export in snowmelt dominated systems. Together, these findings further illustrate complex interactions between climate and atmospheric deposition in carbon cycle processes, and highlight the importance of long-term monitoring efforts for understanding the consequences of a changing climate

eDNA as a tool for identifying freshwater species in sustainable forestry: A critical review and potential future applications

Environmental DNA (eDNA) is an emerging biological monitoring tool that can aid in assessing the effects of forestry and forest manufacturing activities on biota. Monitoring taxa across broad spatial and temporal scales is necessary to ensure forest management and forest manufacturing activities meet their environmental goals of maintaining biodiversity. Our objectives are to describe potential applications of eDNA across the wood products supply chain extending from regenerating forests, harvesting, and wood transport, to manufacturing facilities, and to review the current state of the science in this context. To meet our second objective, we summarize the taxa examined with targeted (PCR, qPCR or ddPCR) or metagenomic eDNA methods (eDNA metabarcoding), evaluate how estimated species richness compares between traditional field sampling and eDNA metabarcoding approaches, and compare the geographical representation of prior eDNA studies in freshwater ecosystems to global wood baskets. Potential applications of eDNA include evaluating the effects of forestry and forest manufacturing activities on aquatic biota, delineating fish-bearing versus non fish-bearing reaches, evaluating effectiveness of constructed road crossings for freshwater organism passage, and determining the presence of at-risk species. Studies using targeted eDNA approaches focused on fish, amphibians, and invertebrates, while metagenomic studies focused on fish, invertebrates, and microorganisms. Rare, threatened, or endangered species received the least attention in targeted eDNA research, but are arguably of greatest interest to sustainable forestry and forest manufacturing that seek to preserve freshwater biodiversity. Ultimately, using eDNA methods will enable forestry and forest manufacturing managers to have data-driven prioritization for conservation actions for all freshwater species

Gradients of anthropogenic nutrient enrichment alter N Composition and DOM stoichiometry in freshwater ecosystems

Plain language summary Ammonium and nitrate in freshwaters have received considerable attention due to their clear ecological and health effects. A comprehensive assessment of N in freshwaters that includes DON is lacking. Including DON in studies of surface water chemistry is important because it can cause eutrophication and certain forms can be rapidly removed by microbial communities. Here, we document how elevated levels of TDN impact the concentrations and relative proportions of all three forms of dissolved N and the stoichiometry of DOM. Our results suggest that human activities fundamentally alter the composition of the dissolved nitrogen pool and the stoichiometry of DOM. Results also highlight feedbacks between the C and N cycles in freshwater ecosystems that are poorly studied.A comprehensive cross-biome assessment of major nitrogen (N) species that includes dissolved organic N (DON) is central to understanding interactions between inorganic nutrients and organic matter in running waters. Here, we synthesize stream water N chemistry across biomes and find that the composition of the dissolved N pool shifts from highly heterogeneous to primarily comprised of inorganic N, in tandem with dissolved organic matter (DOM) becoming more N-rich, in response to nutrient enrichment from human disturbances. We identify two critical thresholds of total dissolved N (TDN) concentrations where the proportions of organic and inorganic N shift. With low TDN concentrations (0–1.3 mg/L N), the dominant form of N is highly variable, and DON ranges from 0% to 100% of TDN. At TDN concentrations above 2.8 mg/L, inorganic N dominates the N pool and DON rarely exceeds 25% of TDN. This transition to inorganic N dominance coincides with a shift in the stoichiometry of the DOM pool, where DOM becomes progressively enriched in N and DON concentrations are less tightly associated with concentrations of dissolved organic carbon (DOC). This shift in DOM stoichiometry (defined as DOC:DON ratios) suggests that fundamental changes in the biogeochemical cycles of C and N in freshwater ecosystems are occurring across the globe as human activity alters inorganic N and DOM sources and availability. Alterations to DOM stoichiometry are likely to have important implications for both the fate of DOM and its role as a source of N as it is transported downstream to the coastal ocean

Detection of Heteroplasmic Mitochondrial DNA in Single Mitochondria

BACKGROUND: Mitochondrial DNA (mtDNA) genome mutations can lead to energy and respiratory-related disorders like myoclonic epilepsy with ragged red fiber disease (MERRF), mitochondrial myopathy, encephalopathy, lactic acidosis and stroke (MELAS) syndrome, and Leber's hereditary optic neuropathy (LHON). It is not well understood what effect the distribution of mutated mtDNA throughout the mitochondrial matrix has on the development of mitochondrial-based disorders. Insight into this complex sub-cellular heterogeneity may further our understanding of the development of mitochondria-related diseases. METHODOLOGY: This work describes a method for isolating individual mitochondria from single cells and performing molecular analysis on that single mitochondrion's DNA. An optical tweezer extracts a single mitochondrion from a lysed human HL-60 cell. Then a micron-sized femtopipette tip captures the mitochondrion for subsequent analysis. Multiple rounds of conventional DNA amplification and standard sequencing methods enable the detection of a heteroplasmic mixture in the mtDNA from a single mitochondrion. SIGNIFICANCE: Molecular analysis of mtDNA from the individually extracted mitochondrion demonstrates that a heteroplasmy is present in single mitochondria at various ratios consistent with the 50/50 heteroplasmy ratio found in single cells that contain multiple mitochondria

Seasonal variation in nutrient uptake in a 1st-order tributary of Lake Superior and implications for climate change

In-stream biogeochemical cycling can control the timing and form of nutrients exported from watersheds to downstream ecosystems, and seasonal changes in light availability, discharge, temperature, or nutrient inputs may affect nutrient transformation and retention. Without an understanding of how in-stream biogeochemical cycling varies seasonally in snow-dominated regions it is uncertain how climate change will affect nutrient export to downstream ecosystems. Further compounding this uncertainty, few studies have examined in-stream nutrient processing during winter. Long-term monitoring (30 years) of climate and snowpack at Calumet watershed, a first order tributary of Lake Superior, has documented trends of increasing winter temperatures and greater snowmelt contributions to early season runoff. Identifying environmental variables that drive nutrient uptake is important because these observed trends may shift the timing of nutrient pulses relative to water temperatures and light availability. We hypothesized that ammonium (NH4) uptake velocity, a measure of nutrient uptake efficiency, would be greater in spring and fall due to increased light availability and nutrient pulses contributed by snowmelt in spring and leaf litter in fall. To test this hypothesis, we measured nutrient uptake velocity of ammonium (NH4) at 2-4 week intervals for one year in Calumet watershed by releasing inorganic nutrients (NH4Cl, KH2PO4) and a conservative tracer (rhodamine WT) into the stream and quantifying changes in nutrient and tracer concentrations along the stream reach. Canopy cover, ambient NH4 concentrations, stream water temperature, periphyton biomass, and discharge were also measured to identify which environmental covariates affected NH4 uptake velocities. The lowest NH4 uptake velocities were observed in winter (2.33 mm min-1) and summer months (2.03-2.08 mm min-1). Spring NH4 uptake velocities were variable: the greatest uptake velocities were observed following snowmelt (4.93-6.56 mm min-1), and they declined in late April (1.9 mm min-1) and early May (2.07 mm min-1) but increased again in late May (5.68 mm min-1). In late fall, the NH4 uptake velocity (4.63 mm min-1) was comparable to the peak velocities observed in spring. Linear regression showed that stream temperature, ambient nutrient concentration, and canopy cover were not significant predictors of uptake velocity, but we are continuing to build our time series of measurements. Our results support our hypothesis that stream organisms take up nutrients more quickly in spring and fall relative to winter and summer seasons but we cannot yet identify which environmental conditions drive these patterns. Therefore, it is important to incorporate temporal variation when estimating in-stream nutrient cycling, particularly in snow-dominated ecosystems where climate change is expected to increase temperatures, which may alter snowpack dynamics

Nitrogen and phosphorus, but not carbon, are quickly taken up in streams: assessing variability in nutrient uptake across six Lake Superior tributaries

Background/Question/Methods An abundance of headwater streams along Lake Superior’s shores may comprise a major input of nutrients and account for nutrient sources that are currently unaccounted for in Lake Superior nutrient budgets. However, in-stream biogeochemical cycling can control how quickly nutrients are taken up or transformed, and consequently alter the timing and form of nutrients exported to the downstream lake. Our objective was to measure uptake of ammonium (NH4), soluble reactive phosphate (SRP), and dissolved organic carbon (DOC) in six streams to determine whether nutrients entering from terrestrial ecosystems may be modified or retained before being exported to Lake Superior. These streams represent regional land cover characteristics and exhibit a range in stream size (discharge: 3 to 756 L s-1). To measure nutrient uptake we released nutrients (NH4Cl and KH2PO4 or C12H22O11) and a conservative tracer (rhodamine WT) into the stream and quantified changes in concentrations along each stream reach. To account for variability among streams we measured canopy cover, periphyton biomass, nutrient concentrations, DOC quality, water temperature, discharge, and land cover, and analyzed whether these variables were correlated with nutrient uptake velocities. Results/Conclusions We found that NH4 and SRP uptake velocities (Vf) varied across the tributaries (2.1 - 5.7 mm min-1 and 1.5 - 8.4 mm min-1, respectively). NH4 uptake was significant in all but one stream, where background NH4 concentrations were elevated at least 2.5x relative to other streams. Three tributaries exhibited significant uptake of SRP. Only one site exhibited significant uptake of DOC (Vf = 0.6 mm min-1). This site also had the lowest DOC concentrations and specific ultra-violet absorbance (SUVA254) values, indicating lower C aromaticity. Neither NH4 Vf nor DOC Vfcorrelated with measured environmental variables, while SRP Vf was correlated with [NH4], [DOC], and canopy cover. On average, NH4 traveled 141 to 1667 m, SRP traveled 370 to 455m, and DOC traveled 2500 m before being taken up, but nutrients must travel a distance of ~2000 to 13000 m from our study reaches before reaching Lake Superior. Our results suggest that N and P may be transformed or retained in the stream before being exported to Lake Superior, but DOC may not be. To better understand nutrient export to Lake Superior future research should focus on identifying pathways of nutrient transformation, and the location/duration of retention in streams