Ecosystem responses to increased precipitation and permafrost decay in subarctic Sweden inferred from peat and lake sediments

Abstract

Recent accelerated decay of discontinuous permafrost at the Stordalen Mire in northern Sweden has been attributed to increased temperature and snow depth, and has caused expansion of wet minerotrophic areas leading to significant changes in carbon cycling in the mire. In order to track these changes through time and evaluate potential forcing mechanisms, this paper analyses a peat succession and a lake sediment sequence from within the mire, providing a record for the last 100 years, and compares these with monitored climate and active layer thickness data. The peat core was analysed for testate amoebae to reconstruct changes in peatland surface moisture conditions and water table fluctuations. The lake sediment core was analysed by near infrared spectroscopy to infer changes in the total organic carbon (TOC) concentration of the lake-water, and changes in d 13 C and C, N and d 15 N to track changes in the dissolved inorganic carbon (DIC) pool and the influence of diagenetic effects on sediment organic matter, respectively. Results showed that major shifts towards increased peat surface moisture and TOC concentration of the lake-water occurred around 1980, one to two decades earlier than a temperature driven increase in active layer thickness. Comparison with monitored temperature and precipitation from a nearby climate station indicates that this change in peat surface moisture is related to June-September (JJAS) precipitation and that the increase in lakewater TOC concentration reflects an increase in total annual precipitation. A significant depletion in 13 C of sediment organic matter in the early 1980s probably reflects the effect of a single or a few consecutive years with anomalously high summer precipitation, resulting in elevated DIC content of the lake water, predominantly originating from increased export and subsequent respiration of organic carbon from the mire. Based on these results, it was not possible to link proxy data obtained on peat and lake-sediment records directly to permafrost decay. Instead our data indicate that increased precipitation and anomalously high rainfall during summers had a significant impact on the mire and the adjacent lake ecosystem. We therefore propose that effects of increased precipitation should be considered when evaluating potential forcing mechanisms of recent changes in carbon cycling in the subarctic.

Contrasting trends in floods for two sub-arctic catchments in northern Sweden – does glacier presence matter?

“…This progressive increase in active layer thickness is likely to cause increased water flow via subsurface flow pathways as indicated by the negative trend in mean summer discharge in the Abisko catchment. Similar indicators of a progressive increase in catchment storage have been documented by means of a greater annual discharge (Lindström and Bergström, 2004), increased total organic carbon (TOC) export rates to lakes (Kokfelt et al, 2009), and increased dissolved inorganic carbon (DIC) exports to streams (Lyon et al, 2010).…”

A new climate era in the sub‐Arctic: Accelerating climate changes and multiple impacts

“…Palsas degraded and a mire became wetter Palsa mire vegetation changed [Malmer et al, 2005]. Methane emissions increased from a mire with thawing permafrost [Christensen et al, 2004] Tree radial growth Increased growth of pine [Grudd et al, 2002] The abundance of shrubs has increased particularly in the absence of herbivores [Olofsson et al, 2009] Freshwater chemistry and biology Lake water TOC concentration increased post 1980 in one lake [Kokfelt et al, 2009] and post 1989 in another lake [Rosén et al, 2009], both lakes close to thawing permafrost Changes in diatom assemblages occurred post 1989 in a lake close to thawing permafrost [Rosén et al, 2009] a Data interpolated from B-spline and Legendre polynomial smoothing of annual and seasonal data. Statements without references are supported in the text of the paper.…”

A First Regional-Scale Estimate of Climate-Driven Terrestrial Carbon Export in Boreal Catchments

“…The evidence that high TOC exports take place particularly in autumn indicates that the process of TOC export is most sensitive to changes in autumn precipitation [31]. In sub-Artic Sweden, Kokfelt et al [36] showed that major shifts towards increased TOC concentration in lake water occurred around 1980, which reflects an increase in annual total precipitation. The mean annual TOC concentration also increased significantly in seven of the eight catchments analyzed by Sarkkola et al [19] in Finland.…”