Abstract
Abstract. The composition of sediment organic matter (OM) exerts a strong control on biogeochemical processes in lakes, such as those involved in the fate of carbon, nutrients and trace metals. While between-lake spatial variability of OM quality is increasingly investigated, we explored in this study how the molecular composition of sediment OM varies spatially within a single lake and related this variability to physical parameters and elemental geochemistry. Surface sediment samples (0–10 cm) from 42 locations in Härsvatten – a small boreal forest lake with a complex basin morphometry – were analyzed for OM molecular composition using pyrolysis gas chromatography mass spectrometry for the contents of 23 major and trace elements and biogenic silica. We identified 162 organic compounds belonging to different biochemical classes of OM (e.g., carbohydrates, lignin and lipids). Close relationships were found between the spatial patterns of sediment OM molecular composition and elemental geochemistry. Differences in the source types of OM (i.e., terrestrial, aquatic plant and algal) were linked to the individual basin morphometries and chemical status of the lake. The variability in OM molecular composition was further driven by the degradation status of these different source pools, which appeared to be related to sedimentary physicochemical parameters (e.g., redox conditions) and to the molecular structure of the organic compounds. Given the high spatial variation in OM molecular composition within Härsvatten and its close relationship with elemental geochemistry, the potential for large spatial variability across lakes should be considered when studying biogeochemical processes involved in the cycling of carbon, nutrients and trace elements or when assessing lake budgets.
Highlights
In lake basins, a wide range of factors are known to influence the transport and fate of sedimentary material, such as the location of inlet streams, catchment topography, land-use patterns, fetch, basin morphometry and sediment focusing
Recent studies interested in the role of lake sediments as a long-term C sink have likewise mainly treated organic matter (OM) and C as a homogeneous component (e.g., Sobek et al, 2003; Tranvik et al, 2009; Heathcote et al, 2015). Even if this approach is rational from a global perspective of calculating C budgets, treating OM as a homogeneous component is overly simplistic from the perspective of developing insights into the biogeochemical behavior of OM and its influence on C, nutrient, and trace metal cycling and does not take full advantage of the information provided by differences in the OM quality
Our specific research questions were (i) what are the spatial patterns within a single lake for various organic biochemical classes and compounds? (ii) How does the spatial pattern of the OM molecular composition relate to physical parameters and elemental, inorganic geochemistry of the sediment material? (iii) Which factors or processes appear to explain the in-lake spatial variability of the OM molecular composition?
Summary
A wide range of factors are known to influence the transport and fate of sedimentary material, such as the location of inlet streams, catchment topography, land-use patterns, fetch, basin morphometry and sediment focusing. Advanced ultra-high-resolution MS techniques, i.e., Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) or linear trap quadruple Orbitrap MS, enable the determination of a large number of organic molecular formulas in liquid samples (> 1000; e.g., Hawkes et al, 2016) These methods have been successfully used to link variability in the molecular composition of dissolved OM (DOM) with different factors and/or processes of environmental ecosystems, such as climate, hydrology and OM degradation in boreal lakes (Kellerman et al, 2014, 2015) or optical properties and DOM photochemical alterations in wetland and seawater (Stubbins and Dittmar, 2015; Wagner et al, 2015). Our specific research questions were (i) what are the spatial patterns within a single lake for various organic biochemical classes and compounds? (ii) How does the spatial pattern of the OM molecular composition relate to physical parameters (i.e., bulk density and water depth) and elemental, inorganic geochemistry of the sediment material? (iii) Which factors or processes (e.g., provenance, transport pathway and mineralization) appear to explain the in-lake spatial variability of the OM molecular composition?
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