Release and Chemical Composition of Riverine DOC 1631 Seasonality of Riverine DOC Export

Based on seasonal patterns of discharge and the chemical characteristics of DOC in subarctic rivers, there is a common division of annual hydrographs into spring flood, summer through autumn, and winter flow periods (Fig. 16.6). Although the start and duration of these periods may vary greatly among basins and annually, such separation is motivated by distinct changes of sources and flowpaths of water and DOC in riverine systems.

Fig. 16.6 Dynamics of S18O in water, concentration of DIC, DOC and specific ultraviolet absorb-ance (SUVA, 280 nm) in Kochechumo river (Central Siberia) in 2006. 1 winter; 2 spring flood; 3 summer and fall flow periods. DOY = day of year

A general concept observed across the subarctic area is that there are two major controls on runoff and DOC export: (1) permafrost distribution defines basin-contributing areas, as lateral flow is confined to permafrost-underlain terrains due to their ability to restrict deep percolation, and (2) surface organic soils play a key role in rapidly conveying water to the stream (Quinton et al. 2000). During the melt period, meltwater percolating from the snowpack in terrains with shallow permafrost soils infiltrates through organic soil, since deeper infiltration is restricted by the impermeable permafrost table. In areas with deeper frost (e.g. south-facing slopes) or in the absence of frost (discontinuous or sporadic permafrost regions), percolation is uninhibited unless there are ice-rich layers at depth. The isotopic signature of river water at this time becomes strongly depleted with respect to 518O, suggesting large meltwater recharge (Fig. 16.6).

Chemically, DOC removed from organic soils in the meltwater solution and flushed during this runoff pulse demonstrates an enrichment in aromatic structures, originating from lignocellulose decomposition products (Kawahigashi et al. 2004; Prokushkin et al. 2007) and demonstrating contemporary ages (Neff et al. 2006). These are all attributed to relatively fresh organic matter entering the riverine systems. Such findings prove that organic solutes do not infiltrate to mineral soil, and bypass the interaction with mineral soil that remains frozen in spring. As a result, more DOC reaches rivers; therefore, subarctic river waters contain generally higher concentrations of DOC than rivers in permafrost-free areas. Furthermore, a peak in DOC concentrations is measured during spring breakup, when 40-80% of arctic river discharge occurs (Gordeev et al. 1996). Both streams and rivers of high latitudes release more than half of the annual DOC export during the 2- to 4-week-long snow melt period.

As the active layer deepens in the course of the frost-free period, deeper infiltration of organic solutes and higher retention time in soil cause a decrease of DOC concentration in subarctic rivers (Fig. 16.6) and streams (Fig. 16.7a), and an alteration of its chemical composition (Neff et al. 2006; Prokushkin et al. 2007). In particular, chemical and isotopic fingerprints of summer-autumn DOC suggest a higher input of microbially transformed and/or derived material. Therefore, the release of terrestrial DOC from permafrost-affected watersheds is controlled by the seasonal cycle of the active layer over permafrost, as shown by increasing 518O values (Carey and Quinton 2004) and DO13C in river waters and on the other hand, decreasing aromaticity and older 14C signature of dissolved organic matter (Neff et al. 2006).

Reduced DOC export during summer through autumn in subarctic rivers contradicts suggestions that rising temperature in northern latitudes will result in a significant increase of DOC flux to the marine system. Comparative analysis of watersheds with different extent of permafrost distribution in Alaska supports the reduction scenario of DOC export in a warmer climate (MacLean et al. 1999). Recent data of Kawahigashi et al. (2004) provide further evidence of decreased riverine DOC export in Siberia, due to a significant drop of DOC concentrations in small streams along a gradient from continuous to discontinuous permafrost in the lower Yenisey River basin. Simultaneous major alteration of biochemical composition (i.e.,

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Fig. 16.7 Changes in mean concentrations of DOC in the Kulingdakan stream from May to September in 2001-2005. Relationships between stream DOC concentration (a) and discharge (b), and monthly mean DOC concentration and precipitation amount (c) for July 1998-2005

c decrease of lignocellulose complex, increase of hydrophilic fraction) confirms the significant influence of a thickness of the active layer and distribution of permafrost on flux, composition and biodegradability of DOC in Siberian soils.

The connection between river DOC and old (aged) OC stored in permafrost remains unclear. While there is evidence that permafrost in Arctic regions is undergoing rapid change (Serreze et al. 2000), the recent (younger) DOC observed for arctic rivers shows that the release of old DOC from permafrost into the hydrologi-cal cycle is not substantial (Benner 2004; Guo et al. 2006). Extended sampling during the growing season clearly demonstrated increasing age of DOC in upland streams and the Kolyma River in Eastern Siberia (Neff et al. 2006). These findings, however, are indicative also for an increased input of deep groundwater from "taliks" (liquid water reservoirs within frozen ground) located beneath river beds.

Winter base flow in permafrost-dominated basins is largely deep beneath permafrost groundwater, having low DOC concentrations and DOC chemistry consistent with high water residence and DOC withdrawal (Striegl et al. 2005). A number of recent studies have pointed to recent trends toward increased winter discharge from the major Siberian rivers (Peterson et al. 2002). Changes in active layer depth over permafrost directly affect potential groundwater storage and river discharge throughout the winter season. The thicker active layer has more groundwater storage capacity, due to the melting of ground ice and an increased precipitation input. This increased groundwater storage in turn results in a greater contribution of subsurface water to the river systems and, hence, increases the winter season stream flow. Thus, permafrost degradation forces an elongation of the period of hydrologi-cally active soil and an increase of the soil-water storage capacity, which in turn contribute to higher concentrations of DOC to rivers. Therefore, changes associated with the deepening of the active layer induce a reduction of DOC export from watershed in frost-free periods, and in contrast may enhance winter DOC flux.

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