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Abstract: This review summarizes biogeochemical processes that operate on diel, or 24-h, time scales in streams and the changes in aqueous chemistry that are associated with these processes. Some biogeochemical processes, such as those producing diel cycles of dissolved O2 and pH, were the first to be studied, whereas processes producing diel concentration cycles of a broader spectrum of chemical species including dissolved gases, dissolved inorganic and organic carbon, trace elements, nutrients, stable isotopes, and suspended particles have received attention only more recently. Diel biogeochemical cycles are interrelated because the cyclical variations produced by one biogeochemical process commonly affect another. Thus, understanding biogeochemical cycling is essential not only for guiding collection and interpretation of water-quality data but also for geochemical and ecological studies of streams. Expanded knowledge of diel biogeochemical cycling will improve understanding of how natural aquatic environments function and thus lead to better predictions of how stream ecosystems might react to changing conditions of contaminant loading, eutrophication, climate change, drought, industrialization, development, and other factors. [Copyright &y& Elsevier]

Abstract: It has previously been concluded that the stable Si isotopes are fractionated during geochemical and biogeochemical elemental transfers, such as weathering and precipitation of clays and biogenic Si, which has opened up the possibility of using Si as a tracer in natural terrestrial ecosystems. Furthermore, quantification of the biogenic impact on the biogeochemical Si cycle has attracted significant scientific interest since biological control has been suggested. Previous observations of seasonal variations in the dissolved Si isotopic pattern further imply that high-frequency riverine sampling during main hydrological events might provide important information about natural processes governing the fluvial biogeochemical Si cycle. Therefore, temporal variations in the isotopic composition of riverine dissolved Si were investigated for the Kalix River, Northern Sweden, the largest pristine river system in Europe, based on high-frequency sampling during a period of 25weeks from early April to early October 2006. Temporal variations spanning 0.4‰ for δ 29Si and 0.8‰ for δ 30Si of dissolved Si in the Kalix River were observed during the period, suggesting that the riverine dissolved Si input to the oceans cannot be considered to have a constant Si isotopic composition on a short time scale. The results implicate biogeochemical Si-cycling via formation and dissolution of biogenic silica as processes significantly affecting the dissolved Si transport in boreal systems during April to early October. The Si budget in the river system appeared to be controlled by relative Si enrichment during high discharge events and relative Si depletions in the subarctic mountainous and lake dominated areas. The Si enrichments and depletions were accompanied by decreasing and increasing riverine δ 29Si and δ 30Si, respectively. These isotope variations can be explained by release of plant derived silica, depleted in heavier Si isotopes, during the spring snowmelt. Further, increased volumetric contribution from the headwater and losses of dissolved Si due to biogenic silica formation by diatoms in the subarctic lakes at a later period are expected to be responsible for the preferential losses of lighter isotopes, as further verified by land cover analysis. [Copyright &y& Elsevier]

Assessing multi-annual riverine export fluxes of carbon, nutrients and metals from the continent to the Arctic Ocean is crucial for constraining the current status and foreseeing future changes in riverine export induced by climate change. Over 3 consecutive years (2012–2014), we measured daily to weekly concentrations of C, N, P, all major and 45 trace elements (TE) in filtered (<0.45 μm) river water at the terminal station of Severnaya Dvina, the largest European Arctic river, gauged since 1882. There were 3 main groups of elements according to their seasonal behavior, demonstrating distinct links between concentration and the discharge. Dissolved Inorganic Carbon (DIC), major anions (Cl, SO 4), alkali and alkaline-earth elements (Li, Na, Mg, K, Ca, Rb, Sr, Ba), and oxyanions (B, Mo, Ge, As) and U exhibited minimum concentrations during spring flood and autumn high flow events, being significantly (p < 0.01) negatively correlated with discharge and dissolved organic carbon (DOC) concentration. These elements reflected the dominant lithology of Severnaya Dvina watershed (carbonates, gypsum) and were controlled by dilution of underground (baseflow) influx by floods. The concentration of DOC, low-soluble trace elements (Al, Ti, Ga, Y, rare earth elements, Zr, Nb, Hf, Th) and organically-bound metals (V, Cr, Co, Ni, Cu, Cd, Pb) had the maximum at the spring flood peak. The input of these elements to the river occurred via surface flow and they were significantly (p < 0.01) increasing with water discharge. Finally, N, P, Si, Mn and Fe originated from both surface and underground sources and were impacted by autochthonous biotic processes in the river channel; they exhibited a strong winter maximum and a summer minimum. The spring flood (May and June) provided 50% of overall water and DOC, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sb, and Pb flux and 60% of low-soluble TE (Al, Y, Nb, REEs, Ti, Zr, Hf and Th). The nutrients (K, Rb, Si, P, N) exhibited around 40% share of annual export during spring flood, whereas the export of soluble mobile elements (DIC, Cl−, SO 4 2−, Li, B, Na, Mg, Ca, Sr, Mo and U) mainly occurred during winter (35–40%) with only 20–25% of annual flow provided by spring flood. The 3-year mean export fluxes of dissolved components were 30 to 50% lower than (Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cu, Cd, and Pb) or comparable to (±30% for all other elements including DOC and for DIC) previous estimates of this river from 2007 to 2008 and the historic data of major components from 1930 to 1950. In particular, there was no trend in mean monthly seasonal and annual concentrations and export fluxes of DOC, DIC, SO 4 , Cl since the beginning of chemical analysis between 1929 and 1930 and in Ca, Mg, Si, and Fe since the 1950's through the duration of this study (2012–2014). The lack of seasonal and annual increases in discharge, carbon, and element concentrations in the permafrost-free Severnaya Dvina River for >120 years of discharge and over 60 years of chemical composition measurements is at odds with the reported short-term trend (based on measurements that began in 1970s) showing that element export fluxes increased in other Arctic rivers due to on-going climate warming and permafrost thaw. Historical and contemporary measurements of element fluxes in the largest European Arctic river did not reveal sizeable changes in discharge and DOC, DIC, Ca, Mg, Fe, Si, SO 4 , Cl concentrations and fluxes over past >60 years. Unlabelled Image • High resolution export fluxes of DOC, major and trace element (TE) in the largest European Arctic river • Soluble highly mobile elements exhibited minimal concentrations in spring and maximal concentrations in winter • Trivalent and tetravalent TEs and organically bound metals showed a maximum at the spring flood peak • There is no statistically significant trend in element export fluxes since the beginning of observations (1930s) • Chemical runoff in the European Arctic exhibits high inter-annual variability, exceeding any possible long-term trend [ABSTRACT FROM AUTHOR]