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Abstract. Wetlands are known to be effective sinks for nitrate. Wetland restoration and construction have gained traction as viable conservation measures to improve water quality in intensively managed agricultural landscapes. In addition to reducing nitrate in situ, wetlands may have impacts on water chemistry and temperature dynamics that extend beyond the confines of the wetlands themselves. Nonsaturating nitrate concentrations (NO 3 − ), enhanced organic carbon effluxes, and altered temperature dynamics in streams downstream of wetlands could all affect denitrification rates within a stream network, potentially extending water quality benefits beyond wetland boundaries. We investigated the effect of wetlands on water chemistry, water temperature, and benthic denitrification rates in downstream agricultural ditches through a field measurement campaign over the open water season. We found that although ditches located downstream of wetlands had lower NO 3 − and higher DOC, ditch denitrification rate was not significantly altered by the presence of upstream wetlands. Rather, wetlands indirectly affected denitrification within ditches by strongly influencing the stoichiometry of the two limiting resources, NO 3 − and organic carbon. Peak denitrification rates in ditches were observed when DOC and NO 3 − supplies were approximately balanced, that is, at DOC: NO 3 − ratios that were near the microbial requirement for denitrification. NO 3 − limitation occurred primarily at sites with >3% wetland cover, and in the fall season at all sites, and DOC limitation occurred primarily at sites with <1% wetland cover. Temperature was found to be a secondary control that was important only when NO 3 − and DOC availabilities were balanced. Our results suggest that wetland restoration and construction targeting nitrate reduction within intensively agriculturally managed basins should be implemented in a way that promotes balanced resource availability throughout fluvial networks. Wetlands are an important regulator of resource availability and thus could be used to create conditions that maximize denitrification in NO 3 − -enriched watersheds.

BackgroundSkeletal and eye abnormalities in amphibians are not well understood, and they appear to be increasing while global populations decline. Here, we present the first study of amphibian abnormalities in Alaska.ObjectiveIn this study we investigated the relationship between anthropogenic influences and the probability of skeletal and eye abnormalities in Alaskan wood frogs (Rana sylvatica).MethodsFrom 2000 to 2006, we examined 9,269 metamorphic wood frogs from 86 breeding sites on five National Wildlife Refuges: Arctic, Innoko, Kenai, Tetlin, and Yukon Delta. Using road proximity as a proxy for human development, we tested relationships between skeletal and eye abnormalities and anthropogenic effects. We also examined a subsample of 458 frogs for the trematode parasite Ribeiroia ondatrae, a known cause of amphibian limb abnormalities.ResultsPrevalence of skeletal and eye abnormalities at Alaskan refuges ranged from 1.5% to 7.9% and were as high as 20% at individual breeding sites. Proximity to roads increased the risk of skeletal abnormalities (p = 0.004) but not eye abnormalities. The only significant predictor of eye abnormalities was year sampled (p = 0.006). R. ondatrae was not detected in any Alaskan wood frogs.ConclusionsAbnormality prevalence at road-accessible sites in the Kenai and Tetlin refuges is among the highest reported in the published literature. Proximity to roads is positively correlated with risk of skeletal abnormalities in Alaskan wood frogs.

The repeated occurrence of abnormal amphibians in nature points to ecological imbalance, yet identifying causes of these abnormalities has proved complex. Multiple studies have linked amphibian abnormalities to chemically contaminated areas, but inference about causal mechanisms is lacking. Here we use a high incidence of abnormalities in Alaskan wood frogs to strengthen inference about the mechanism for these abnormalities. We suggest that limb abnormalities are caused by a combination of multiple stressors. Specifically, toxicants lead to increased predation, resulting in more injuries to developing limbs and subsequent developmental malformations. We evaluated a variety of putative causes of frog abnormalities at 21 wetlands on the Kenai National Wildlife Refuge, south‐central Alaska, USA, between 2004 and 2006. Variables investigated were organic and inorganic contaminants, parasite infection, abundance of predatory invertebrates, UVB, and temperature. Logistic regression and model comparison using the Akaike information criterion (AIC) identified dragonflies and both organic and inorganic contaminants as predictors of the frequency of skeletal abnormalities. We suggest that both predators and contaminants alter ecosystem dynamics to increase the frequency of amphibian abnormalities in contaminated habitat. Future experiments should test the causal mechanisms by which toxicants and predators may interact to cause amphibian limb abnormalities.