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As human activities alter environmental conditions, the emergence and spread of disease represents an increasing threat to wildlife. Studies that examine how host–pathogen relationships play out across seasons and latitudes can serve as proxies for understanding how natural and anthropogenic changes in climate may influence infection and disease dynamics. Amphibians are ideal host organisms for studying the impacts of climate on disease because they are ectothermic and threatened by chytridiomycosis, a recently emerged and globally important disease caused by fungal pathogens in the genusBatrachochytrium. Previous studies suggest that temperature affects the interaction between amphibians andBatrachochytriumpathogens. However, a clearer understanding of this host–pathogen–environment interaction is needed to predict how the risk of chytridiomycosis will vary in space and time. Here, we investigate how daily, seasonal, and latitudinal variations in temperature affect the incidence and impact ofBatrachochytrium dendrobatidis(Bd) infection in a broadly distributed host, the northern cricket frog (Acris crepitans), using a combination of field and laboratory studies. In a four‐year field study conducted at three latitudes, we found that daily maximum air temperature over a 15‐d period prior to sampling best predicted patterns ofBdinfection and that the lightest infection loads followed periods when these temperatures exceeded 25°C. In a laboratory exposure experiment, we found pathogen load and mortality to be greater at temperatures that mimic winter temperatures at the southern extent of this host's range than for scenarios that mimic temperature conditions experienced in other areas and seasons. Taken together, our findings suggest that changes in temperature across timescales and latitudes interact to influence the dynamics of infection and disease in temperate amphibians.

Freshwater salinization is an emerging global problem impacting safe drinking water, ecosystem health and biodiversity, infrastructure corrosion, and food production. Freshwater salinization originates from diverse anthropogenic and geologic sources including road salts, human-accelerated weathering, sewage, urban construction, fertilizer, mine drainage, resource extraction, water softeners, saltwater intrusion, and evaporative concentration of ions due to hydrologic alterations and climate change. The complex interrelationships between salt ions and chemical, biological, and geologic parameters and consequences on the natural, social, and built environment are called Freshwater Salinization Syndrome (FSS). Here, we provide a comprehensive overview of salinization issues (past, present, and future), and we investigate drivers and solutions. We analyze the expanding global magnitude and scope of FSS including its discovery in humid regions, connections to human-accelerated weathering and mobilization of ‘chemical cocktails.’ We also present data illustrating: (1) increasing trends in salt ion concentrations in some of the world’s major freshwaters, including critical drinking water supplies; (2) decreasing trends in nutrient concentrations in rivers due to regulations but increasing trends in salinization, which have been due to lack of adequate management and regulations; (3) regional trends in atmospheric deposition of salt ions and storage of salt ions in soils and groundwater, and (4) applications of specific conductance as a proxy for tracking sources and concentrations of groups of elements in freshwaters. We prioritize FSS research needs related to better understanding: (1) effects of saltwater intrusion on ecosystem processes, (2) potential health risks from groundwater contamination of home wells, (3) potential risks to clean and safe drinking water sources, (4) economic and safety impacts of infrastructure corrosion, (5) alteration of biodiversity and ecosystem functions, and (6) application of high-frequency sensors in state-of-the art monitoring and management. We evaluate management solutions using a watershed approach spanning air, land, and water to explore variations in sources, fate and transport of different salt ions (e.g.monitoring of atmospheric deposition of ions, stormwater management, groundwater remediation, and managing road runoff). We also identify tradeoffs in management approaches such as unanticipated retention and release of chemical cocktails from urban stormwater management best management practices (BMPs) and unintended consequences of alternative deicers on water quality. Overall, we show that FSS has direct and indirect effects on mobilization of diverse chemical cocktails of ions, metals, nutrients, organics, and radionuclides in freshwaters with mounting impacts. Our comprehensive review suggests what could happen if FSS were not managed into the future and evaluates strategies for reducing increasing risks to clean and safe drinking water, human health, costly infrastructure, biodiversity, and critical ecosystem services.

Anthropogenic increases in nitrogen (N) and phosphorus (P) concentrations can strongly influence the structure and function of ecosystems. Even though lotic ecosystems receive cumulative inputs of nutrients applied to and deposited on land, no comprehensive assessment has quantified nutrient‐enrichment effects within streams and rivers. We conducted a meta‐analysis of published studies that experimentally increased concentrations of N and/or P in streams and rivers to examine how enrichment alters ecosystem structure (state: primary producer and consumer biomass and abundance) and function (rate: primary production, leaf breakdown rates, metabolism) at multiple trophic levels (primary producer, microbial heterotroph, primary and secondary consumers, and integrated ecosystem). Our synthesis included 184 studies, 885 experiments, and 3497 biotic responses to nutrient enrichment. We documented widespread increases in organismal biomass and abundance (mean response = +48%) and rates of ecosystem processes (+54%) to enrichment across multiple trophic levels, with no large differences in responses among trophic levels or between autotrophic or heterotrophic food‐web pathways. Responses to nutrient enrichment varied with the nutrient added (N, P, or both) depending on rateversusstate variable and experiment type, and were greater in flume and whole‐stream experiments than in experiments using nutrient‐diffusing substrata. Generally, nutrient‐enrichment effects also increased with water temperature and light, and decreased under elevated ambient concentrations of inorganic N and/or P. Overall, increased concentrations of N and/or P altered multiple food‐web pathways and trophic levels in lotic ecosystems. Our results indicate that preservation or restoration of biodiversity and ecosystem functions of streams and rivers requires management of nutrient inputs and consideration of multiple trophic pathways.

Factors driving freshwater salinization syndrome (FSS) influence the severity of impacts and chances for recovery. We hypothesize that spread of FSS across ecosystems is a function of interactions among five state factors:human activities,geology,flowpaths,climate, andtime. (1)Human activitiesdrive pulsed or chronic inputs of salt ions and mobilization of chemical contaminants. (2)Geologydrives rates of erosion, weathering, ion exchange, and acidification‐alkalinization. (3)Flowpathsdrive salinization and contaminant mobilization along hydrologic cycles. (4)Climatedrives rising water temperatures, salt stress, and evaporative concentration of ions and saltwater intrusion. (5)Timeinfluences consequences, thresholds, and potentials for ecosystem recovery. We hypothesize that state factors advance FSS in distinct stages, which eventually contribute to failures in systems‐level functions (supporting drinking water, crops, biodiversity, infrastructure, etc.). We present future research directions for protecting freshwaters at risk based on five state factors and stages from diagnosis to prognosis to cure.