Almost 50 years of monitoring shows that climate, not forestry, controls long‐term organic carbon fluxes in a large boreal watershed

Abstract

Here, we use a unique long-term data set on total organic carbon (TOC) fluxes, its climatic drivers and effects of land management from a large boreal watershed in northern Finland. TOC and runoff have been monitored at several sites in the Simojoki watershed (3160 km(2) ) since the early 1960s. Annual TOC fluxes have increased significantly together with increased inter-annual variability. Acid deposition in the area has been low and has not significantly influenced losses of TOC. Forest management, including ditching and clear felling, had a minor influence on TOC fluxes - seasonal and long-term patterns in TOC were controlled primarily by changes in soil frost, seasonal precipitation, drought, and runoff. Deeper soil frost led to lower spring TOC concentrations in the river. Summer TOC concentrations were positively correlated with precipitation and soil moisture not temperature. There is some indication that drought conditions led to elevated TOC concentrations and fluxes in subsequent years (1998-2000). A sensitivity analysis of the INCA-C model results showed the importance of landscape position, land-use type, and soil temperature as controls of modeled TOC concentrations. Model predictions were not sensitive to forest management. Our results are contradictory to some earlier plot-scale and small catchment studies that have shown more profound forest management impacts on TOC fluxes. This shows the importance of scale when assessing the mechanisms controlling TOC fluxes and concentrations. The results highlight the value of long-term multiple data sets to better understand ecosystem response to land management, climate change and extremes in northern ecosystems.

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

“…A growing number of studies show that climate change will greatly affect hydroclimatic regimes and DOC exports in environments at high latitudes if future conditions are warmer and wetter (Jungqvist et al 2014;Carey et al 2010;Tetzlaff et al 2013;Mellander et al 2007;Lepistö et al 2014). This study supports these earlier findings, with the RCM ensemble projecting a wide range of possible future hydroclimatic conditions (Table 3).…”

“…2). It has been used at a range of spatial scales including headwater catchments (Futter et al 2007(Futter et al , 2009) and larger rivers (Oni et al 2011;Ledesma et al 2012) and has been used for simulations based on nearly 50 years of measured data (Lepistö et al 2014). INCA-C has been widely used for projecting future [DOC] in boreal and temperate surface waters (Aherne et al 2008;Futter et al 2009;Oni et al 2012;Holmberg et al 2014 inter alia).…”

“…For example, recovery from acidification was the main driver of the increase in DOC observed in southern Fennoscandia, the UK and parts of North America Monteith et al 2007). However, in boreal and other high latitude regions which have not been significantly affected by acidification, there is increasing recognition of the role of climate variability and change as a long-term control of [DOC] (Couture et al 2012;Laudon et al 2012Laudon et al , 2013aLepistö et al 2014;Tetzlaff et al 2013;Pastor et al 2003). Such trends of increasing [DOC] might continue as the future climate warms and precipitation patterns shift in an unpredictable manner (IPCC 2007).…”

Aquatic DOC export from subarctic Atlantic blanket bog in Norway is controlled by seasalt deposition, temperature and precipitation

“…The model is process-based and simulates soil organic matter cycling controlled by temperature, soil moisture and chemistry in addition to vertical and lateral hydrological transport of DOC. INCA-C has been used for simulating DOC under present (Futter et al 2007;Futter and de Wit 2008;Lepisto et al 2014) and future (Futter et al 2009;Oni et al 2014) conditions at a range of headwater and larger catchments in Fennoscandia and Canada.…”

Quality transformation of dissolved organic carbon during water transit through lakes: contrasting controls by photochemical and biological processes

“…Recent concerns have been raised regarding widespread increases in color and DOC concentrations in the Northern Hemisphere caused by a combination of factors involving a warmer climate (Lepistö et al, 2014;Pagano et al, 2014), an intensified hydrological cycle (Weyhenmeyer et al, 2012;Fasching et al, 2016), and the release of DOC previously immobilized in soils due to acidification (Monteith et al, 2007). This rise in colored DOC, reviewed by Solomon et al (2015), is predicted to reduce aquatic productivity (Karlsson et al, 2009), change food webs and population structures , alter the stoichiometry and magnitude of bioavailable nutrients pools (Berggren et al, 2015b), and cause increased freshwater CO 2 outgassing (Lapierre et al, 2013).…”