Boreal regions store most of the global terrestrial carbon, which can be transferred as dissolved organic carbon (DOC) to inland waters with implications for both aquatic ecology and carbon budgets. Headwater riparian zones (RZ) are important sources of DOC, and often just a narrow 'dominant source layer' (DSL) within the riparian profile is responsible for most of the DOC export. Two important questions arise: how long boreal RZ could sustain lateral DOC fluxes as the sole source of exported carbon and how its hydromorphological variability influences this role. We estimate theoretical turnover times by comparing carbon pools and lateral exports in the DSL of 13 riparian profiles distributed over a 69 km 2 catchment in northern Sweden. The thickness of the DSL was 36 AE 18 (average AE SD) cm.Thus, only about one-third of the 1-m-deep riparian profile contributed 90% of the lateral DOC flux. The 13 RZ exported 8.7 AE 6.5 g C m À2 year À1, covering the whole range of boreal stream DOC exports. The variation could be explained by local hydromorphological characteristics including RZ width (R 2 = 0.90). The estimated theoretical turnover times were hundreds to a few thousands of years, that is there is a potential long-lasting supply of DOC. Estimates of net ecosystem production in the RZ suggest that lateral fluxes, including both organic and inorganic C, could be maintained without drawing down the riparian pools. This was supported by measurements of stream DO 14 C that indicated modern carbon as the predominant fraction exported, including streams disturbed by ditching. The transfer of DOC into boreal inland waters from new and old carbon sources has a major influence on surface water quality and global carbon balances. This study highlights the importance of local variations in RZ hydromorphology and DSL extent for future DOC fluxes under a changing climate.

The boreal ecoregion supports about one-third of the world's forest. Over 90% of boreal forest streams are found in headwaters, where terrestrial-aquatic interfaces are dominated by organic matter (OM)-rich riparian zones (RZs). Because these transition zones are key features controlling catchment biogeochemistry, appropriate RZ conceptualizations are needed to sustainably manage surface water quality in the face of a changing climate and increased demands for forest biomass. Here we present a simple, yet comprehensive, conceptualization of RZ function based on hydrological connectivity, biogeochemical processes, and spatial heterogeneity. We consider four dimensions of hydrological connectivity: (1) laterally along hillslopes, (2) longitudinally along the stream, (3) vertically down the riparian profile, and (4) temporally through eventbased and seasonal changes in hydrology. Of particular importance is the vertical dimension, characterized by a 'Dominant Source Layer' that has the highest contribution to solute and water fluxes to streams. In addition to serving as the primary source of OM to boreal streams, RZs shape water chemistry through two sets of OM-dependent biogeochemical processes: (1) transport and retention of OM-associated material and (2) redox-mediated transformations controlled by RZ water residence time and availability of labile OM. These processes can lead to both retention and release of pollutants. Variations in width, hydrological connectivity, and OM storage drive spatial heterogeneity in RZ biogeochemical function. This conceptualization provides a useful theoretical framework for environmental scientists and ecologically sustainable and economically effective forest management in the boreal region and elsewhere, where forest headwaters are dominated by low-gradient, OM-rich RZs.

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Riparian zones (RZ) are a major factor controlling water chemistry in forest streams. Base cations' (BC) concentrations, fluxes, and cycling in the RZ merit attention because a changing climate and increased forest harvesting could have negative consequences, including re-acidification, for boreal surface waters. We present a two-year study of BC and silica (Si) flow-weighted concentrations from 13 RZ and 14 streams in different landscape elements of a boreal catchment in northern Sweden. The spatial variation in BC and Si dynamics in both RZ and streams was explained by differences in landscape element type, with highest concentrations in silty sediments and lowest concentrations in peat-dominated wetland areas. Temporal stability in BC and Si concentrations in riparian soil water, remarkably stable Mg/Ca ratios, and homogeneous mineralogy suggest that patterns found in the RZ are a result of a distinct mineralogical upslope signal in groundwater. Stream water Mg/Ca ratios indicate that the signal is subsequently maintained in the streams. Flow-weighted concentrations of Ca, Mg, and Na in headwater streams were represented by the corresponding concentrations in the RZ, which were estimated using the Riparian Flow-Concentration Integration Model (RIM) approach. Stream and RZ flow-weighted concentrations differed for K and Si, suggesting a stronger biogeochemical influence on these elements, including K recirculation by vegetation and retention of Si within the RZ. Potential increases in groundwater levels linked to forest harvesting or changes in precipitation regimes would tend to reduce BC concentrations from RZ to streams, potentially leading to episodic acidification

Globally, artificial river impoundment, nutrient enrichment and biodiversity loss impair freshwater ecosystem integrity. Concurrently, beavers, ecosystem engineers recognized for their ability to construct dams and create ponds, are colonizing sites across the Holarctic after widespread extirpation in the 19th century, including areas outside their historical range. This has the potential to profoundly alter hydrology, hydrochemistry and aquatic ecology in both newly colonized and recolonized areas. To further our knowledge of the effects of beaver dams on aquatic environments, we extracted 1366 effect sizes from 89 studies on the impoundment of streams and lakes. Effects were assessed for 16 factors related to hydrogeomorphology, biogeochemistry, ecosystem functioning and biodiversity. Beaver dams affected concentrations of organic carbon in water, mercury in water and biota, sediment conditions and hydrological properties. There were no overall adverse effects caused by beaver dams or ponds on salmonid fish. Age was an important determinant of effect magnitude. While young ponds were a source of phosphorus, there was a tendency for phosphorus retention in older systems. Young ponds were a source methylmercury in water, but old ponds were not. To provide additional context, we also evaluated similarities and differences between environmental effects of beaver-constructed and artificial dams (767 effect sizes from 75 studies). Both are comparable in terms of effects on, for example, biodiversity, but have contrasting effects on nutrient retention and mercury. These results are important for assessing the role of beavers in enhancing and/or degrading ecological integrity in changing Holarctic freshwater systems.