DISSOLVED GAS ANALYSIS FOR ASSESSING THE FATE OF NITRATE IN WETLANDS1

ABSTRACT: Dissolved gas analysis permits direct detection of ground water denitrification, a technique we used in this study to assess the fate of nitrate in a riparian wetland. Dissolved argon (Ar) and dinitrogen (N₂) were measured in transects of nested piezometers installed at different depths within upwelling regions of a riparian wetland. The method uses the Ar content in the water as a natural inert tracer for assessing background content of N₂ from the previous air/water equilibrium. Within the wetland under study, anoxic to suboxic ground water became more oxic in piezometers close to the aquifer layer, indicating upwelling of oxic ground water. Assessment of loss of nitrate and Ar in ground water within an upwelling zone indicated that shallow piezometers had significant N₂ loss through degassing. Most of the measured nitrate‐nitrogen (NO₃^?‐N) loss of 205 μM in a piezometer nest could be accounted for by total N₂‐N produced (169 μM N), calculated from changes in dissolved N₂ and estimated N₂ degassed. Degassing due to methane (CH₄) production was also detected in some shallow piezometers within nests. This technique for analysis of dissolved gases in ground water can be applied to detect small changes in N gas concentration and aids in assessing the fate of nitrate along a ground water flow path.     

Integration of SWAT and HSPF for Simulation of Sediment Sources in Legacy Sediment‐Impacted Agricultural Watersheds

A total maximum daily load for the Chesapeake Bay requires reduction in pollutant load from sources within the Bay watersheds. The Conestoga River watershed has been identified as a major source of sediment load to the Bay. Upland loads of sediment from agriculture are a concern; however, a large proportion of the sediment load in the Conestoga River has been linked to scour of legacy sediment associated with historic millpond sites. Clarifying this distinction and identifying specific segments associated with upland vs. channel sources has important implications for future management. In order to address this important question, we combined the strengths of two widely accepted watershed management models — Soil and Water Assessment Tool (SWAT) for upland agricultural processes, and Hydrologic Simulation Program FORTRAN (HSPF) for instream fate and transport — to create a novel linked modeling system to predict sediment loading from critical sources in the watershed including upland and channel sources, and to aid in targeted implementation of management practices. The model indicates approximately 66% of the total sediment load is derived from instream sources, in agreement with other studies in the region and can be used to support identification of these channel source segments vs. upland source segments, further improving targeted management. The innovated linked SWAT‐HSPF model implemented in this study is useful for other watersheds where both upland agriculture and instream processes are important sources of sediment load.     

Effects of initial resources on the development of strains during a stressful training situation: Some counterintuitive results

Resource theories of occupational stress argue that employees' personal and environmental resources protect them from too much distress or strain during stressful work experiences. We examined four resources (emotional stability, previous experience, low drain on pre‐existing resources, and workgroup quality) available to soldiers at the beginning of a stressful 3‐month training experience as predictors of the trajectories of their strains over that period of time. Based on conservation of resources theory and the job demands–resources model, we predicted that the trends of strains would be more favorable (would increase more slowly or decline more quickly) if participants started the training with greater resources. The resources, primarily emotional stability and lack of pre‐existing resource drain, tended to be negatively related to strains, consistent with the idea that they can reduce strains. Significant interactions predicting trends were found predicting two of the three strains (post‐traumatic stress symptoms and depression, but not reports of physical health). Contrary to expectations, however, the three resources that significantly predicted trends over time (emotional stability, previous experience, and low pre‐existing resource drain) were associated with worsening rather than improving strains. Copyright © 2014 John Wiley & Sons, Ltd.     

Spatial and seasonal variation in δ15N and δ13C values in a mesopredator shark, Squalus suckleyi, revealed through multitissue analyses

We used variance decomposition to explore the importance of body size, sex, location, and sampling period as predictors of intrapopulation variation in δ¹⁵N and δ¹³C values in spiny dogfish Squalus suckleyi from the Puget Sound–Strait of Georgia basin. Isotopes in two tissues with long (dorsal white muscle) and short (liver) isotopic turnover rates (~1 year and ~3–4 months, respectively) were sampled to evaluate whether the relative importance of each variable differed depending on the time span over which diet information was integrated. Significant spatial variation was observed in both muscle and liver isotopic composition, whereby location uniquely explained 25 and 17 % of the total variance, respectively. The remaining variables explained considerably less variation in both tissue types. Furthermore, evidence of seasonal isotopic shifts in δ¹⁵N and δ¹³C values was apparent, but differed widely in direction and magnitude among groups. These findings suggest that members of spiny dogfish schools may share a common feeding history, possibly by spending extended time periods (weeks to months) foraging in a spatially fixed region. Another explanation is that individuals may move and feed in aggregations that exist for extended periods. These complex group-level patterns suggest that even for large-bodied, motile predators such as sharks, population-level diet estimates derived from averaging isotope ratios of individuals collected from only a few locations may poorly reflect the true population mean.     

The metabolic costs of building ant colonies from variably sized subunits

Ant colonies are superorganisms with emergent traits that, for some species, reflect the combined activity of physically distinct worker castes. Although larger castes have high production costs, they are thought to save their colonies energy by efficiently performing specialized tasks. However, because workers are generally idle until sensing specific stimuli, their maintenance costs may be an important component of colony-level investment. I used metabolic scaling to examine the maintenance costs of dimorphic major and minor Pheidole castes across levels of colony organization (e.g., individual, group, and colony). Majors from three species had lower mass-specific metabolic rates than minors because of allometries at both individual and group levels and subsequently lived longer when starved. Thus, large major castes may offset their production costs in both their idle and active states. The slope scaling metabolic rate from incipient to reproductive colonies of Pheidole dentata (∼colony mass^0.89) fell between the slopes for minor groups (∼group mass^1.04) and major groups (∼group mass^0.79) and appears to reflect developmental shifts in subunit mass and number and their offsetting effects on per capita energy demands. These results highlight how metabolic scaling may help visualize the energetic correlates of emergent behavior and unravel the mechanisms governing colony organization.     

Assessing the impact of petrol stations on their immediate surroundings

This paper describes a novel methodology for evaluating the extent to which petrol stations affect their surroundings. The method is based on the fact that the ratio of the concentrations of aliphatic and aromatic hydrocarbon pollutants in the air of the petrol stations and their surroundings (basically determined by vapor emissions from unburned gasoline) differs from the ratio found in urban air, which is mainly influenced by traffic emissions. Bearing this in mind, the spatial limit of influence of petrol stations in any direction would be the first point, moving away from the station, where the ratio becomes equal to the urban background ratio. Application of the methodology involves multipoint measuring campaigns of the air at the studied petrol station and built-up area in general and processing the data with software capable of providing isoconcentration contours. The procedure should help local authorities in terms of land management, so that a “belt” can be established around petrol stations where housing or vulnerable populations and activities such as those in schools, hospitals and community centers should be restricted.