Salinization and sedimentation drive contrasting assembly mechanisms of planktonic and sediment-bound bacterial communities in agricultural streams.

Agriculture is the most dominant land use globally and is projected to increase in the future to support a growing human population but also threatens ecosystem structure and services. Bacteria mediate numerous biogeochemical pathways within ecosystems. Therefore, identifying linkages between stressors associated with agricultural land use and responses of bacterial diversity is an important step in understanding and improving resource management. Here, we use the Mississippi Alluvial Plain (MAP) ecoregion, a highly modified agroecosystem, as a case study to better understand agriculturally associated drivers of stream bacterial diversity and assembly mechanisms. In the MAP, we found that planktonic bacterial communities were strongly influenced by salinity. Tolerant taxa increased with increasing ion concentrations, likely driving homogenous selection which accounted for ~90% of assembly processes. Sediment bacterial phylogenetic diversity increased with increasing agricultural land use and was influenced by sediment particle size, with assembly mechanisms shifting from homogenous to variable selection as differences in median particle size increased. Within individual streams, sediment heterogeneity was correlated with bacterial diversity and a subsidy-stress relationship along the particle size gradient was observed. Planktonic and sediment communities within the same stream also diverged as sediment particle size decreased. Nutrients including carbon, nitrogen, and phosphorus, which tend to be elevated in agroecosystems, were also associated with detectable shifts in bacterial community structure. Collectively, our results establish that two understudied variables, salinity and sediment texture, are the primary drivers of bacterial diversity within the studied agroecosystem, whereas nutrients are secondary drivers. Although numerous macrobiological communities respond negatively, we observed increasing bacterial diversity in response to agricultural stressors including salinization and sedimentation. Elevated taxonomic and phylogenetic bacterial diversity likely increases the probability of detecting community responses to stressors. Thus, bacteria community responses may be more reliable for establishing water quality goals within highly modified agroecosystems that have experienced shifting baselines.

Stream algal biomass response to experimental phosphorus and nitrogen gradients: A case for dual nutrient management in agricultural watersheds.

Watershed managers generally focus on P reduction strategies to combat freshwater eutrophication despite evidence that N co-limits primary production. Our objective was to test the role of P in limiting stream periphyton biomass within the Buffalo River watershed in Arkansas by conducting a 31-d streamside mesocosm experiment. To represent potentially different starting states, cobbles were transplanted from two different tributary streams and initially exposed to a range of P (0, 0.012, 0.025, 0.05, 0.1, and 0.2 mg L-1 P) to assess benthic ash-free dry mass (AFDM) and chlorophyll-a (chl a) and responses during a P only enrichment period. Later, the experiment was continued under a N/P (10:1 molar ratio) enrichment gradient to examine co-limitation. Mean AFDM was higher on Day 31 of the N+P enrichment compared with Day 17 of the P-only enrichment (p < .001). Overall differences in AFDM and chl a were observed between cobbles from different stream sites. Phosphorus enrichment stimulated benthic chl a biomass, but enrichment effects were greater when streams were enriched with N+P (p < .001). Chlorophyll-a increased (4.4-57.9 mg m-2 ) with increasing P concentrations (p < .001) after P enrichment but was threefold greater after N+P enrichment, increasing from 13.3 to 171.1 mg m-2 across the enrichment gradient. Results support the need to consider both N and P limitation in freshwater systems and demonstrate that potential increases in nutrient concentrations may influence accumulation of algae on cobble substrates from the Buffalo River watershed.

Mulch-Derived Organic Carbon Stimulates High Denitrification Fluxes from Agricultural Ditch Sediments.

Reactive N is an essential input for healthy, vibrant crop production, yet excess N is often transported off field via agricultural ditches to downstream receiving ecosystems, where it can cause negative impacts to human health, biodiversity loss, as well as eutrophication and resultant hypoxia. Denitrification, the transformation of reactive N to unreactive N gas, within agricultural ditches has potential to reduce impacts to downstream ecosystems but requires substantial organic C substrates. We used a flow-through intact core experiment to test the effects of low-cost management options including a common agricultural amendment, gypsum, and an overlying hardwood mulch layer on promoting denitrification within agricultural ditch sediments. We found significantly higher denitrification potentials in mulch (11.2 mg N-N m h) and mulch-gypsum cores (9.2 mg N-N m h) than in gypsum (1.3 mg N-N m h) or control cores (0.6 mg N-N m h). Higher denitrification rates corresponded with high dissolved organic C (DOC) fluxes within the mulch and mulch-gypsum treatments (72.8-115.2 mg m h) and were ultimately able to remove 65 to 69% of N loads. Results indicate DOC from overlying mulch additions to agricultural ditches significantly increase denitrification in intact cores and suggest that the addition of DOC sources in agricultural ditches may contribute a simple, low-cost option to reduce reactive N export and improve ecological outcomes within aquatic agroecosystems.

Diatom Assemblage Changes in Agricultural Alluvial Plain Streams and Application for Nutrient Management.

In large, alluvial floodplains dominated by agriculture, small streams have the potential to experience nutrient enrichment affecting algal assemblage structure and metabolism. Nutrient enrichment is largely driven by application of nutrients and altered hydrologic regimes. To inform stressor-response-based nutrient reduction goals for agricultural alluvial plain streams, diatom assemblages were sampled from 25 streams located within the Mississippi Alluvial Plain (MAP) with various land management practices and associated P and N inputs. From August through September 2015, epidendric diatom assemblage samples were collected from instream woody debris. Field nutrient gradients were skewed toward higher concentrations, and ranges of previously reported diatom assemblage response thresholds indicative of oligotrophic conditions were not well represented. Ordination analysis identified a gradient in species composition associated with increasing P and decreasing dissolved oxygen. A significant shift in diatom assemblage structure occurred when total P concentrations in the MAP streams exceeded 0.12 mg L-. Phosphorus-enriched systems were represented by a distinct set of indicator species, lower abundances of ubiquitous species, greater abundances of highly tolerant species, and greater abundances of high-P indicator species. No relationships were observed among diatom assemblage measures or traits with increasing N. Current results do not address potential criteria for identifying high-quality, oligotrophic streams. However, measures of diatom assemblage structure have potential for helping set benchmarks to reduce nutrient impacts and monitor effects of agricultural best management practices on MAP streams.

Environmental flows in the context of unconventional natural gas development in the Marcellus Shale.

Quantitative flow-ecology relationships are needed to evaluate how water withdrawals for unconventional natural gas development may impact aquatic ecosystems. Addressing this need, we studied current patterns of hydrologic alteration in the Marcellus Shale region and related the estimated flow alteration to fish community measures. We then used these empirical flow-ecology relationships to evaluate alternative surface water withdrawals and environmental flow rules. Reduced high-flow magnitude, dampened rates of change, and increased low-flow magnitudes were apparent regionally, but changes in many of the flow metrics likely to be sensitive to withdrawals also showed substantial regional variation. Fish community measures were significantly related to flow alteration, including declines in species richness with diminished annual runoff, winter low-flow, and summer median-flow. In addition, the relative abundance of intolerant taxa decreased with reduced winter high-flow and increased flow constancy, while fluvial specialist species decreased with reduced winter and annual flows. Stream size strongly mediated both the impact of withdrawal scenarios and the protection afforded by environmental flow standards. Under the most intense withdrawal scenario, 75% of reference headwaters and creeks (drainage areas <99 km2 ) experienced at least 78% reduction in summer flow, whereas little change was predicted for larger rivers. Moreover, the least intense withdrawal scenario still reduced summer flows by at least 21% for 50% of headwaters and creeks. The observed 90th quantile flow-ecology relationships indicate that such alteration could reduce species richness by 23% or more. Seasonally varying environmental flow standards and high fixed minimum flows protected the most streams from hydrologic alteration, but common minimum flow standards left numerous locations vulnerable to substantial flow alteration. This study clarifies how additional water demands in the region may adversely affect freshwater biological integrity. The results make clear that policies to limit or prevent water withdrawals from smaller streams can reduce the risk of ecosystem impairment.

Seasonal Differences in Relationships between Nitrate Concentration and Denitrification Rates in Ditch Sediments Vegetated with Rice Cutgrass.

Increased application of nitrogen (N) fertilizers in agricultural systems contributes to significant environmental impacts, including eutrophication of fresh and coastal waters. Rice cutgrass [ (L.) Sw.] can significantly enhance denitrification potential in agricultural ditch sediments and potentially reduce N export from agricultural watersheds, but relationships with known drivers are not well understood. To address this, we examined effects of nitrate (NO) availability on dinitrogen gas (N) and NO fluxes seasonally. Net denitrification rates were measured as positive N fluxes from vegetated intact sediment cores using membrane inlet mass spectrometry (MIMS). We developed Michaelis-Menten models for N fluxes across NO gradients in the spring, summer, and fall seasons. Summer N models exhibited the highest (maximum amount of net N flux) and (concentration of NO in the overlying water at which the net N flux is half of ), with a maximum production of N of ∼20 mg N m h. Maximum percentage NO retention occurred at 1 mg NO L in the overlying water in all seasons, except summer where maximum retention persisted from 1 to 5 mg NO L. Denitrification rates were strongly correlated with NO uptake by vegetated sediments in spring ( = 0.94, < 0.0001) and summer ( = 0.97, < 0.0001), but low NO uptake in fall and winter resulted in virtually no net denitrification during these seasons. Our results indicate that vegetated ditch sediments may act as effective NO sinks during the growing season. Ditch sediments vegetated with cutgrass not only immobilized a significant fraction of NO entering them but also permanently removed as much as 30 to 40% of the immobilized NO through microbial denitrification.

Contrasting Nutrient Mitigation and Denitrification Potential of Agricultural Drainage Environments with Different Emergent Aquatic Macrophytes.

Remediation of excess nitrogen (N) in agricultural runoff can be enhanced by establishing wetland vegetation, but the role of denitrification in N removal is not well understood in drainage ditches. We quantified differences in N retention during experimental runoff events followed by stagnant periods in mesocosms planted in three different vegetation treatments: unvegetated, cutgrass [ (L.) Sw.], and common cattail ( L.). We also quantified denitrification rates using membrane inlet mass spectrometry from intact cores extracted from each mesocosm treatment. All treatments retained 60% or more of NO-N loads during the 6-h experimental runoff event, but mesocosms planted with cutgrass had significantly higher (68%) retention than the cattail (60%) or unvegetated (61%) treatments. After the runoff event, mesocosms planted in cattail reduced NO-N concentrations by >95% within 24 h and cutgrass achieved similar reductions within 48 h, whereas reductions in the unvegetated mesocosms were significantly less (65%). Cores from cutgrass mesocosms had significantly higher average denitrification rates (5.93 mg m h), accounting for as much as 56% of the immobilized NO-N within 48 h, whereas denitrification rates were minimal in cores from the unvegetated (-0.19 mg m h) and cattail (0.2 mg m h) mesocosms. Our findings have implications for mitigating excess NO-N in agricultural runoff. While vegetated treatments removed excess NO-N from the water column at similar and significantly higher rates than unvegetated treatments, the high denitrification rates observed for cutgrass highlight the potential for permanent removal of excess N from agricultural runoff in vegetated ditches and wetlands.

Thoracolumbar interfascial plane (TLIP) block: a pilot study in volunteers.

PURPOSE: Regional anesthesia has been shown to improve outcomes in several recent studies. The transversus abdominis plane (TAP) block provides anesthesia to the abdominal wall by introducing local anesthetic to the ventral rami of the thoracolumbar nerves. This work quantifies the area of anesthesia obtained after performing the novel thoracolumbar interfascial plane block (analogous to the TAP block but intended for the back) which targets the sensory component of the dorsal rami of the thoracolumbar nerves. METHODS: Ten participants underwent bilateral ultrasound-guided injections of 0.2% ropivacaine 20 mL into the fascial plane between the multifidus and longissimus muscles. After five and 20 min, respectively, the area of anesthesia was plotted on the participant's back. Anesthesia was defined as loss of point discrimination to pinprick. RESULTS: Participants reported a mean (SD) area of anesthesia surrounding the needle injection site of 137.4 (71.0) cm(2) and 217.0 (84.7) cm(2) at five and 20 min after injection, respectively. The mean (SD) cephalad and caudal spread of local anesthetic from the site of injection was 6.5 (1.8) cm and 3.9 (1.2) cm, respectively. There were no complications or adverse events reported. CONCLUSION: This report shows that a reproducible area of anesthesia can be obtained by ultrasound-guided injection of local anesthetic in the fascial plane between the multifidus and longissimus muscles of the thoracolumbar spine. The area of anesthesia consistently covered the midline and had a predictable spread. This project was registered with clinicaltrials.gov (NCT02297191).

Influence of drought and total phosphorus on diel pH in wadeable streams: implications for ecological risk assessment of ionizable contaminants.

Climatological influences on site-specific ecohydrology are particularly germane in semiarid regions where instream flows are strongly influenced by effluent discharges. Because many traditional and emerging aquatic contaminants, such as pharmaceuticals, are ionizable, we examined diel surface water pH patterns (i.e., change in pH over a 24-h period) at 23 wadeable streams in central Texas, USA, representing a gradient of nutrient enrichment during consecutive summers of 2006 and 2007. The years of our study were characterized by decidedly different instream flows, which likely affected production:respiration dynamics and led to distinctions in diel pH patterns between 2006 and 2007. Site-specific ambient water quality criteria for NH(3) and the aquatic toxicity of the model weak base pharmaceutical sertraline were predicted using continuous water quality monitoring data from the sites. Drought conditions of 2006 significantly increased (p<0.05) diel pH changes compared to high instream flows of 2007,and the magnitude of diel pH variability was most pronounced at nutrient-enriched sites in 2006. Differences in diel pH change patterns between 2006 and 2007 affected predictions of the environmental fate and effects for model weak base pharmaceuticals and NH(3). Overall, site-specific diel pH was more variable at some sites than the difference in mean surface water pH between the 2 summers. Diel pH variability affected regulatory criteria, because 20% of the study sites in 2006 experienced greater than 5-fold differences in National Ambient Water Quality Criteria for NH(3) over 24-h periods. Our study emphasizes the potential uncertainty that diel pH variability may introduce in site-specific assessments and provides recommendations for environmental assessment of ionizable contaminants.