Abstract. Species' responses to seasonal environmental variation can influence trophic interactions and food web structure within an ecosystem. However, our ability to predict how species' interactions will vary spatially and temporally in response to seasonal variation unfortunately remains inadequate within most ecosystems. Fish assemblages in the Tonle Sap Lake (TSL) of Cambodia-a dynamic flood-pulse ecosystem -were studied for five years (2010-2014) using stable isotope and Bayesian statistical approaches to explore both within-and among-species isotopic niche variation associated with seasonal flooding. Roughly 600 individual fish specimens were collected during 19 sampling events within the lake. We found that fishes within the same species tended to have a broader isotopic niche during the wet season, likely reflecting assimilation of resources from either a wider range of isotopically distinct prey items or a variety of habitats, or both. Furthermore, among-species isotopic niches tended to overlap and range more broadly during the wet season, suggesting that floodplain inundation promotes exploitation of more diverse and similar resources by different species in the fish community. Our study highlights that the flood-pulse dynamic that is typical of tropical aquatic ecosystems may be an essential element supporting freshwater fish community structure and the fish diversity that underpins the TSL food web. This flow regime is currently threatened by regional dam development, which may in turn impact the natural function and structure of the fishery food web.

On‐road vehicle nitrogen oxide (NOx) sources currently dominate the U.S. anthropogenic emission budgets, yet vehicle NOx emissions have uncertain contributions to oxidized nitrogen (N) deposition patterns. Isotopic signatures serve as a potentially valuable observational tool to trace source contributions to NOx chemistry and N deposition, yet in situ emission signatures are underconstrained. We characterize the spatiotemporal variability of vehicle NOx emission isotopic signatures (δ15N‐NOx) representative of U.S. vehicle fleet‐integrated emission plumes. A novel combination of on‐road mobile and stationary urban measurements is performed using a field and laboratory‐verified technique for actively capturing NOx in solution to quantify δ15N‐NOx at hourly resolution. On‐road δ15N‐NOx upwind of Providence, RI, ranged from −7 to −3‰. Simultaneous urban background δ15N‐NOx observations showed comparable range and variations with on‐road measurements, suggesting that vehicles dominate NOx emissions in the Providence area. On‐road spatial δ15N‐NOx variations of −9 to −2‰ were observed under various driving conditions in six urban metropolitan areas and rural interstate highways during summer and autumn in the U.S. Northeast and Midwest. Although isotopic signatures were insensitive to on‐road driving mode variations, statistically significant correlations were found between δ15N‐NOx and NOx emission factor extremes associated with heavy diesel emitter contributions. Overall, these results constrain an isotopic signature of fleet‐integrated roadway NOx emission plumes, which have important implications for distinguishing vehicle NOx from other sources and tracking emission contributions to NOx chemistry and N deposition.

Nitrogen oxides (NO x = NO + NO 2 ) are a family of atmospheric trace gases that have great impact on the environment. NO x concentrations directly influence the oxidizing capacity of the atmosphere through interactions with ozone and hydroxyl radicals. The main sink of NO x is the formation and deposition of nitric acid, a component of acid rain and a bioavailable nutrient. NO x is emitted from a mixture of natural and anthropogenic sources, which vary in space and time. The collocation of multiple sources and the short lifetime of NO x make it challenging to quantitatively constrain the influence of different emission sources and their impacts on the environment. Nitrogen isotopes of NO x have been suggested to vary amongst different sources, representing a potentially powerful tool to understand the sources and transport of NO x . However, previous methods of collecting atmospheric NO x integrate over long (week to month) time spans and are not validated for the efficient collection of NO x in relevant, diverse field conditions. We report on a new, highly efficient field-based system that collects atmospheric NO x for isotope analysis at a time resolution between 30 min and 2 hr. This method collects gaseous NO x in solution as nitrate with 100% efficiency under a variety of conditions. Protocols are presented for collecting air in urban settings under both stationary and mobile conditions. We detail the advantages and limitations of the method and demonstrate its application in the field. Data from several deployments are shown to 1) evaluate field-based collection efficiency by comparisons with in situ NO x concentration measurements, 2) test the stability of stored solutions before processing, 3) quantify in situ reproducibility in a variety of urban settings, and 4) demonstrate the range of N isotopes of NO x detected in ambient urban air and on heavily traveled roadways.