Temperature controls organic carbon sequestration in a subarctic lake

Abstract

Widespread ecological reorganizations and increases in organic carbon (OC) in lakes across the Northern Hemisphere have raised concerns about the impact of the ongoing climate warming on aquatic ecosystems and carbon cycling. We employed diverse biogeochemical techniques on a high-resolution sediment record from a subarctic lake in northern Finland (70°N) to examine the direction, magnitude and mechanism of change in aquatic carbon pools prior to and under the anthropogenic warming. Coupled variation in the elemental and isotopic composition of the sediment and a proxy-based summer air temperature reconstruction tracked changes in aquatic production, depicting a decline during a cool climate interval between ~1700–1900 C.E. and a subsequent increase over the 20th century. OC accumulation rates displayed similar coeval variation with temperature, mirroring both changes in aquatic production and terrestrial carbon export. Increase in sediment organic content over the 20th century together with high inferred aquatic UV exposure imply that the 20th century increase in OC accumulation is primarily connected to elevated lake production rather than terrestrial inputs. The changes in the supply of autochthonous energy sources were further reflected higher up the benthic food web, as evidenced by biotic stable isotopic fingerprints.

Spatio‐temporal cladoceran (Branchiopoda) responses to climate change and UV radiation in subarctic ecotonal lakes

“…Relative content of UV‐protective melanin pigments (UV absorbance) was measured from fossil cladoceran ( Alona affinis ) carapaces (Nevalainen & Rautio, ) as established by Rantala, Luoto, et al. (). This included sieving of the sediment through a 100‐μm mesh, extracting the A. affinis carapaces under a binocular microscope, and measuring UV absorbance of the remains using a specifically designed adapter in a UV‐VIS spectrophotometer (UV‐1800, Shimadzu Corporation, Kyoto, Japan).…”

“…The ~700‐year chronology of the core is based on an age‐depth model from 14 C of two terrestrial macrofossils (leaf fragments) and 137 Cs activity from the upper part of the core. 210 Pb dating method was used, but these measurement did not provide any reliable age estimates because of very low Pb activities in the sediment core (Rantala, Luoto, et al., ). All the analyses are based on single samples in the SEC, FIC, and down‐core data sets as replicate samples were not taken and replicate analyses from individual samples were not performed.…”

“…Variance partitioning analysis (VPA), which analyses explanatory power of different explanatory variables on a response variable, was used to examine the contribution of palaeoenvironmental factors (composite sediment biogeochemistry, climate, and solar intensity) on temporal FIC and ABS UV development. Here, previously available sediment biogeochemical data, including sediment organic matter (OM, as loss‐on‐ignition, LOI), δ 13 C OM , δ 15 N OM , carbon (as C%), nitrogen (as N%), and C/N ratio (from Rantala, Luoto, et al., ) were integrated into a single variable through PCA axis 1 scores, and used as a composite sediment biogeochemical variable. A previously available Chironomidae‐based July air temperature reconstruction from Lake Námmájávri core (Rantala, Luoto, et al., ), was used as an indicator for long‐term climate variability, and a record of sun spot numbers (Solanki, Usoskin, Kromer, Schüssler, & Beer, ) as a proxy for solar intensity.…”

“…Here, previously available sediment biogeochemical data, including sediment organic matter (OM, as loss‐on‐ignition, LOI), δ 13 C OM , δ 15 N OM , carbon (as C%), nitrogen (as N%), and C/N ratio (from Rantala, Luoto, et al., ) were integrated into a single variable through PCA axis 1 scores, and used as a composite sediment biogeochemical variable. A previously available Chironomidae‐based July air temperature reconstruction from Lake Námmájávri core (Rantala, Luoto, et al., ), was used as an indicator for long‐term climate variability, and a record of sun spot numbers (Solanki, Usoskin, Kromer, Schüssler, & Beer, ) as a proxy for solar intensity. RDAs and VPAs were performed with the Canoco5 software (Šmilauer & Lepš, ).…”

“…Due to their remoteness and lack of appropriate long‐term monitoring data, palaeolimnological approaches have been successfully utilized in examining biodiversity, species distribution, and aquatic responses to environmental changes during the Anthropocene. Previous studies have recorded significant aquatic responses in northern lakes, including, for example, increases in organic carbon burial (Heathcote, Anderson, Prairie, Engstrom, & del Giorgio, ; Rantala, Luoto, & Nevalainen, ), changes in organisms’ UV responses (Nevalainen, Rantala, Luoto, Ojala, & Rautio, ), and community reorganizations in different trophic levels (Luoto, Oksman, & Ojala, ; Smol et al., ).…”