Terrestrial sources of nitrogen (N), particularly N-fixing alder, may be important for sustaining production in headwater streams that typically lack substantial subsidies of marine-derived nutrients from spawning salmon yet support upstream-dispersing juvenile salmonids. However, other physiographic characteristics, such as watershed slope and topographic wetness, also control transport of nutrients to streams and may confound apparent linkages between alder and stream N. Seasonal patterns in precipitation and temperature may interact with watershed characteristics to modulate stream N availability. We empirically modeled the effect of alder cover and other watershed physiographic variables on stream N and contrasted these relationships over the growing season among 25 first-order streams from the lower Kenai Peninsula, Alaska. For each date, percent alder cover, mean topographic wetness, and mean slope were used as watershed predictors of NO x -N concentration (nitrate ? nitrite) and daily NO x -N yield using Generalized Additive Models (GAM) and compared using Akaike's Information Criterion (AIC c ). Alder cover was the only probable model and explained 75-96% of the variation in NO x -N concentration and 83-89% of the variation in daily NO x -N yield. The relationship between alder and both NO x -N concentration and daily NO x -N yield changed from constant inputs in May across the range of alder cover (linear fit) to increasing inputs in July and September (non-linear fits) implying that high-alder watersheds were Nsaturated. The strong linkage between alder and stream N coupled with the concurrent timing of maximum stream N from alder in the spring to salmon fry emergence indicates the potential importance of this subsidy to headwater stream ecosystems.

Abstract:Climate is changing fastest in high-latitude regions, focusing our research on understanding rates and drivers of changing temperature regimes in southcentral Alaska streams and implications for salmon populations. We collected continuous water and air temperature data during open-water periods from 2008 to 2012 in 48 nonglacial salmon streams across the Cook Inlet basin spanning a range of watershed characteristics. The most important predictors of maximum temperatures, expressed as mean July temperature, maximum weekly average temperature, and maximum weekly maximum temperature (MWMT), were mean elevation and wetland cover, while thermal sensitivity (slope of the stream-air temperature relationship) was best explained by mean elevation and area. Although maximum stream temperatures varied widely between years and across sites (8.4 to 23.7°C), MWMT at most sites exceeded established criterion for spawning and incubation (13°C), above which chronic and sublethal effects become likely, every year of the study, which suggests salmon are already experiencing thermal stress. Projections of MWMT over the next ϳ50 years suggest these criteria will be exceeded at more sites and by increasing margins.Résumé : Les changements climatiques sont les plus rapides dans les régions de haute latitude, et nos travaux tentent de comprendre les taux et facteurs associés à l'évolution des régimes thermiques dans les cours d'eau du centre-sud de l'Alaska et leurs répercussions sur les populations de saumons. Nous avons recueilli de manière continue des données de température de l'eau et de l'air durant des périodes d'eau libre de 2008 à 2012 dans 48 cours d'eau non glaciaires à saumons à la grandeur du bassin versant du golfe de Cook représentant un éventail de caractéristiques hydrographiques. Les plus importantes variables prédictives des températures maximums, exprimées comme étant la température moyenne en juillet, la température moyenne hebdomadaire maximum et la température maximum hebdomadaire maximum (TMHM), étaient l'altitude moyenne et la couverture de milieux humides, alors que l'altitude moyenne et la superficie étaient les variables qui expliquaient le mieux la sensibilité thermique (la pente de la relation entre la température du cours d'eau et celle de l'air). Si les températures maximums du cours d'eau variaient beaucoup (de 8,4 à 23,7°C) d'une année et d'un emplacement à l'autre, la TMHM dans la plupart des emplacements dépassait le seuil établi pour le frai et l'incubation (13°C) au-delà duquel des effets chroniques et sublétaux deviennent probables, chaque année de l'étude, donnant à penser que les saumons sont déjà en situation de stress thermique. Les projections de la TMHM pour les ϳ50 prochaines années portent à croire que ces seuils seront dépassés dans un nombre croissant d'emplacements et par une plus grande marge. [Traduit par la Rédaction]

The ecosystems supporting Pacific salmon (Oncorhynchus spp.) are changing rapidly as a result of climate change and habitat alteration. Understanding how-and how consistently-salmon populations respond to changes at regional and watershed scales has major implications for fisheries management and habitat conservation. Chinook salmon (O. tshawytscha) populations across Alaska have declined over the past decade, resulting in fisheries closures and prolonged impacts to local communities. These declines are associated with large-scale climate drivers, but uncertainty remains about the role of local conditions (e.g., precipitation, streamflow, and stream temperature) that vary among the watersheds where salmon spawn and rear. We estimated the effects of these and other environmental indicators on the productivity of 15 Chinook salmon populations in the Cook Inlet basin, southcentral Alaska, using a hierarchical Bayesian stock-recruitment model. Salmon spawning during 2003-2007 produced 57% fewer recruits than the previous long-term average, leading to declines in adult returns beginning in 2008. These declines were explained in part by density dependence, with reduced population productivity following years of high spawning abundance. Across all populations, productivity declined with increased precipitation during the fall spawning and early incubation period and increased with above-average precipitation during juvenile rearing. Above-average stream temperatures during spawning and rearing had variable effects, with negative relationships in many warmer streams and positive relationships in some colder streams. Productivity was also associated with regional indices of streamflow and ocean conditions, with high variability among populations. The cumulative effects of adverse conditions in freshwater, including high spawning abundance, heavy fall rains, and hot, dry summers may have contributed to the recent population declines across the region. Identifying both coherent and differential responses to environmental change underscores the importance of targeted, watershed-specific monitoring and conservation efforts for maintaining resilient salmon runs in a warming world.

1. Comprehensive assessments of contemporary diatom distributions across the Arctic remain scarce. Furthermore, studies tracking species compositional differences across space and time, as well as diatom responses to climate warming, are mainly limited to paleolimnological studies due to a lack of routine monitoring in lakes and streams across vast areas of the Arctic.2. The study aims to provide a spatial assessment of contemporary species distributions across the circum-Arctic, establish contemporary biodiversity patterns of diatom assemblages to use as reference conditions for future biomonitoring assessments, and determine pre-industrial baseline conditions to provide historical context for modern diatom distributions.3. Diatom assemblages were assessed using information from ongoing regulatory monitoring programmes, individual research projects, and from surface sediment layers obtained from lake cores. Pre-industrial baseline conditions as well as the nature, direction and magnitude of changes in diatom assemblages over the past c. 200 years were determined by comparing surface sediment samples (i.e. containing modern assemblages) with a sediment interval deposited prior to the onset of significant anthropogenic activities (i.e. containing pre-1850 assemblages), together with an examination of diatoms preserved in contiguous samples from dated sediment cores. 4. We identified several biotypes with distinct diatom assemblages using contemporary diatom data from both lakes and streams, including a biotype typical for High Arctic regions. Differences in diatom assemblage composition across circum-Arctic regions were gradual rather than abrupt. Species richness was lowest in High Arctic regions compared to Low Arctic and sub-Arctic regions, and higher in lakes than in streams. Dominant diatom taxa were not endemic to the Arctic. Species