No abstract

We examined the hypothesis that the extent of vegetation cover governs the fluxes of nutrients from boreal and subarctic river catchments to the sea. Fluxes of total organic carbon (TOC) and dissolved inorganic nitrogen, phosphorus, and dissolved silicate (DIN, DIP, and DSi, respectively) are described from 19 river catchments and subcatchments (ranging in size from 34 to 40,000 km2) in northern Sweden with a detailed analysis of the rivers Luleälven and Kalix%lven. Fluxes of TOC, DIP, and DSi increase by an order of magnitude with increasing proportion of forest and wetland area, whereas DIN did not follow this pattern but remained constantly low. Principal component analysis on landscape variables showed the importance of almost all land cover and soil type variables associated with vegetation, periglacial environment, soil and bedrock with slow weathering rates, boundary of upper tree line, and percentage of lake area. A cluster analysis of the principal components showed that the river systems could be separated into mountainous headwaters and forest and wetland catchments. This clustering was also valid in relation to river chemistry (TOC, DIP, and DSi) and was confirmed with a redundancy analysis, including river chemistry and principal components as environmental variables. The first axis explains 89% of the variance in river chemistry and almost 100% of the variance in the relation between river chemistry and landscape variables. These results suggest that vegetation change during interglacial periods is likely to have had a major effect on inputs of TOC, DIP, and DSi into the past ocean.

No abstract

This case study tests the hypothesis that damming leads to a depletion of major elements in river systems. It determines the effect of river dams on the weathering regime, and thus on dissolved silicate (DSi) fluxes from land to the Sea by comparing two headwater areas in northern Sweden. In the pristine River Kalixälven, major dissolved elements are enriched within a few kilometers downstream from a high mountainous provenance, coming from an area low in vegetation and a thin active soil layer to a forested landscape. Also, alkalinity increased from 30 μeq L−1 to 110 μeq L−1, compared to 240 μeq L−1 measured at the river mouth. In the headwater of the River Luleälven, regulations led to inundation of the river valley and associated loss in vegetated soils. In reaches between the reservoirs, underground channeling of water and a reduction of water level fluctuations result in further decrease in soil‐water contact, and consequently diminishing weathering rates. The ratio of forest area to lentic area in the headwater was reduced dramatically with damming, from 2.65 to 0.84. As a consequence, geochemical variables in the river water show uniformity in space and in time. Alkalinity values at the River Luleälven mouth (155 μeq L−1) remained unchanged from the two main mountainous storage reservoirs (161 μeq L−1 and 166 μeq L−1). These results indicate that loss of vegetated soils through damming in river headwater critically reduces weathering fluxes and also suggest that changes in vegetation coverage in the Quaternary have altered DSi inputs significantly to the global Ocean.