forests. The three large Siberian rivers, Lena, Yenisei, and Ob, which also have the highest 58 proportion of forests within their watersheds, contribute about 90% of the total lignin discharge 59 to the Arctic Ocean. The composition of river DOC is also characterized by elevated levels of p-60 hydroxybenzenes, particularly during the low flow season, which indicates a larger contribution 61 from mosses and peat bogs. The lignin composition was strongly related to the average 14 C-age 62 of DOC supporting the abundance of young, boreal-vegetation-derived leachates during spring 63 flood, and older, soil-, peat-, and wetland-derived DOC during groundwater dominated low flow 64 conditions, particularly in the Ob and Yukon Rivers. We observed significant differences in 65 DOC concentration and composition between the rivers over the seasonal cycles with the 66 Mackenzie River being the most unique, the Lena River being similar to the Yenisei, and the 67 Yukon being most similar to the Ob. The observed relationship between the lignin phenol 68 composition and watershed characteristics suggests that DOC discharge from these rivers could 69 increase in a warmer climate under otherwise undisturbed conditions. 70 71 4

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Natural bacterial communities are extremely diverse and highly dynamic, but evidence is mounting that the compositions of these communities follow predictable temporal patterns. We investigated these patterns with a 3-year, circumpolar study of bacterioplankton communities in the six largest rivers of the pan-arctic watershed (Ob', Yenisey, Lena, Kolyma, Yukon, and Mackenzie), five of which are among Earth's 25 largest rivers. Communities in the six rivers shifted synchronously over time, correlating with seasonal shifts in hydrology and biogeochemistry and clustering into three groups: winter/spring, spring freshet, and summer/fall. This synchrony indicates that hemisphere-scale variation in seasonal climate sets the pace of variation in microbial diversity. Moreover, these seasonal communities reassembled each year in all six rivers, suggesting a long-term, predictable succession in the composition of big river bacterioplankton communities.16S ͉ DGGE ͉ diversity ͉ seasonality ͉ succession

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Warming in the boreal forest of interior Alaska will have fundamental impacts on stream ecosystems through changes in stream hydrology resulting from upslope loss of permafrost, alteration of availability of soil moisture, and the distribution of vegetation. We examined stream flow in three headwater streams of the Caribou-Poker Creeks Research Watershed (CPCRW) in interior Alaska over a 30-year period to determine (i) how stream flow varied among streams draining watersheds with varying extents of permafrost and (ii) evaluate if stream hydrology is changing with loss of permafrost. The three streams drained subcatchments with permafrost extents ranging from 4% to 53%. For each stream, runoff data were analyzed by separating base and storm flow contributions using a local-minimum method and with analysis of flood recession curves. Mean daily runoff during the ice-free season did not significantly vary among streams (mean = 0.57 mmÁd -1 ), although the watersheds with lower permafrost had a greater contribution of base flow. Across years, flow was variable and was related with summer temperature in the watershed with low permafrost and with precipitation in the watershed with high permafrost. With climate warming and loss of permafrost, stream flows will become less responsive to precipitation and headwater streams may become ephemeral.Résumé : Le réchauffement du climat dans la forêt boréale de l'intérieur de l'Alaska aura des impacts fondamentaux sur les écosystèmes lotiques par l'entremise de changements dans l'hydrologie des cours d'eau provoqués par la disparition du pergélisol dans les pentes, la modification de la disponibilité en eau du sol et la distribution de la végétation. Nous avons étudié le débit de trois ruisseaux situés en amont du bassin expérimental de Caribou Poker Creeks à l'intérieur de l'Alaska sur une période de 30 ans. L'étude avait pour but de (i) déterminer de quelle façon varie le débit des ruisseaux qui drainent des bassins où le pergélisol occupe différentes superficies et (ii) évaluer si l'hydrologie des ruisseaux change avec la disparition du pergélisol. Les trois ruisseaux drainaient des sous-bassins où le pergélisol occupait de 4 % à 53 % de la superficie. Pour chaque ruisseau, les données d'écoulement ont été analysées en séparant la contribution du débit de base et celle du débit d'orage à l'aide de la méthode du minimum local et de l'analyse des courbes de décrue. Le débit journalier moyen durant la saison où il n'y avait pas de glace n'a pas varié de façon significative entre les ruisseaux (moyenne = 0,57 mmÁj -1 ) mais les bassins où il y avait le moins de pergélisol contribuaient davantage au débit de base. Le débit variait d'une année à l'autre; il était relié à la température estivale dans le bassin où il y avait peu de pergélisol et aux précipitations dans le bassin où il y avait beaucoup de pergélisol. Avec le réchauffement du climat et la disparition du pergélisol, le débit des cours d'eau va devenir moins sensible aux précipitations et les ruisseaux de tête pourraient ...

Small streams and their adjacent riparian zones contribute disproportionately to the processing and transport of C compared to the catchment area and, thereby, may alter the flux of dissolved organic C (DOC) from aquatic ecosystems. We examined the factors influencing C uptake and CO 2 production in the riparian zone of a boreal stream. We tested the hypothesis that enhanced water velocity increases the supply of DOC and nutrients to microorganisms and, therefore, enhances DOC mineralization in the riparian zone. We measured in situ DOC mineralization rate in the riparian zone of a headwater stream at locations that varied with respect to the water velocity of shallow ground water. We added acetate as a tracer to measure C uptake in shallow ground water and monitored CO 2 production during tracer experiments. We assessed relationships of these measures of C dynamics with the time that the tracer remained in shallow ground water surrounding the injection site (plume residence time), pre-injection DOC concentration, nutrient availability, and aromaticity of dissolved organic matter. Acetate uptake rate varied 2 orders of magnitude from −0.002 to −0.19/min, whereas CO 2 production rate varied 5-fold from 0.01 to 0.06/min. Acetate uptake rate was negatively correlated with mean plume residence time, indicating that rapid water velocity increased C decomposition in the riparian zone. Moreover, Damköhler numbers (Da) were generally <1, indicating that substrate uptake was slower than hydrologic transport. Neither acetate nor CO 2 rates were correlated with pre-injection riparian groundwater chemistry. Spatial heterogeneity of water velocity in the subsurface has a critical influence on the processing and subsequent export of C and nutrients from the riparian zone to streams.