Long-term patterns in dissolved organic carbon, major elements and trace metals in boreal headwater catchments: trends, mechanisms and heterogeneity

Abstract

The boreal landscape is a complex, spatio-temporally varying mosaic of forest and mire landscape elements that control surface water hydrology and chemistry. Here, we assess long-term water quality time series from three nested headwater streams draining upland forest (C2), peat/mire (C4) and mixed (C7) (forest and mire) catchments. Acid deposition in this region is low and is further declining. Temporal trends in weather and runoff (1981–2008), dissolved organic carbon concentration [DOC] (1993–2010) and other water quality parameters (1987–2011) were assessed. There was no significant annual trend in precipitation or runoff. However, runoff increased in March and declined in May. This suggested an earlier snowmelt regime in recent years. Significant monotonic increasing trends in air temperature and length of growing season suggested a decrease in snowfall and less spring runoff. Stream [DOC] was positively correlated with some trace metals (copper, iron and zinc) and negatively with several other chemical parameters (e.g. sulfate, conductivity, calcium). Both sulfate and conductivity showed declining trends, while a significant increase was observed in pH during winter and spring. Calcium and magnesium showed monotonic decreasing trends. The declining trajectories of stream base cation and sulfate concentrations during other times of the year were not accompanied by changes in pH and alkalinity. These results indicate subtle effects of recovery from acidification. Water temperature increased significantly both annually and in most months. A simultaneous monotonic increase in iron (Fe) and [DOC] in autumn suggests co-transport of Fe-DOC in the form of organometallic complexes. A monotonic increase in UV absorbance in most months without co-occurring changes in DOC trend suggests a shift in DOC quality to a more humic-rich type. The observed increase in soil solution [DOC] and subtle trends in stream [DOC] suggest that climate rather than recovery from acidification is the dominant driver of DOC trends in the Svartberget catchment

Cross-scale ensemble projections of dissolved organic carbon dynamics in boreal forest streams

“…The long-term mean air temperature in the Svartberget catchment is 1.7°C with minimum and maximum temperatures being -9.5 ± 1.4°C in January and 14.5 ± 1.7°C in July respectively, which provides a longterm mean growing season of *148 days (Oni et al 2013). Long-term mean annual precipitation is 610 ± 109 mm with snow cover lasting for *170 days (Köhler et al 2008).…”

“…Samples were collected in acid-washed polyethylene bottles and stored frozen before being transported to the laboratory for analysis. Water samples were analyzed in the lab using a Shimadzu 5000 TOC analyzer following common lab protocols as described elsewhere (Köhler et al 2008;Laudon et al 2011Winterdahl et al 2011a;Oni et al 2013 (Ehret et al 2012;Teutschbein and Seibert 2013), and do not agree well with observed time series (Fig. 3a, b), these simulations were not used directly as input to the hydrological model in the present study (Fig.…”

“…DOC can also act as a medium of carbon exchange in the form of CO 2 emissions from aquatic conduits in boreal systems (Wallin et al 2013). It is a matter of concern that DOC concentrations ([DOC]) in many parts of the world have been increasing in surface waters Eimers et al 2008;Oni et al 2013). Several factors have been identified as drivers of these increases, including declines in acid deposition (Monteith et al 2007) and landscape disturbances (Schelker et al 2012) but no scientific consensus has, as yet, been reached because the dominant drivers seem to differ depending on whether local or regional scales of observation are used.…”

“…Understanding how climatic factors affect future trajectories of DOC requires an analysis of long-term datasets (Oni et al 2013), combined with new experimental data (Cleveland et al 2004) and the development of simulation models (Futter et al 2007;Wu et al 2013). While numerous biogeochemical models have been developed to aid our understanding of how climate affects DOC, all contain inherent uncertainties because of our limited understanding of the dominant mechanisms that drive processes at local or regional scales, and the ways in which such processes are represented within models.…”

Seasonal contribution of terrestrial organic matter and biological oxygen demand to the Baltic Sea from three contrasting river catchments

“…Several factors have been suggested as drivers of this increasing organic matter trend, from decreasing acidification (Evans et al, 2012;Monteith et al, 2007) changing hydrology and warming climates (Evans, 2005) to increasing ionic strength in soils (Hruška et al, 2009) and changes in land use (Garnett et al, 2000). The specific processes behind these trends are likely to at least partially vary regionally (Oni et al, 2013).…”