Long chain n-alkanes in lake sediment track differences in adjacent land vegetation.

Abstract

We conducted an analysis of n-alkanes, lignin-derived phenols, and other sediment markers from 19 lakes across four ecoregions in Saskatchewan, Canada, spanning from Prairie Grassland to Boreal Upland. Our goal was to establish whether these biomarkers relate to different ecoregions and land cover types (herbaceous plants vs trees) in the catchments of these lakes. Our findings revealed a significant inverse correlation between the proportion of herbaceous plants to trees in a lake’s catchment and the proportion of aquatic n-alkanes P_aq (C₂₃ + C₂₅)/(C₂₃ + C₂₅ + C₂₉ + C₃₁) indicating that aquatic plants contributed proportionally more to sedimentary n-alkanes when the catchments were mostly in Boreal forest. We also observed significant positive correlations between the proportion of herbaceous plants to trees in a lake’s catchment and the n-alkane composition ratios C₃₁/(C₂₇ + C₃₁) and C₃₁/(C₂₇ + C₂₉ + C₃₁), reflecting higher relative inputs of C₃₁ from herbaceous vegetation. These findings suggest that these ratios could potentially be utilized to infer historical land cover composition based on dated sediment records. Moreover, variations in the C₃₁/(C₂₇ + C₃₁) alkane ratio were observed among ecoregions, particularly between Prairie Grassland and the forest-dominated areas. We found no correlations between chlorophyll-a concentrations in lake water and the above-mentioned n-alkane ratios in sediment, suggesting that these ratios primarily reflect land cover composition rather than autochthonous production in the lakes. Additionally, the C/N ratio and δ¹³C were not effective in distinguishing ecoregions or land cover composition, likely due to influences from algal production and perhaps agricultural activities in surrounding farmland. In contrast, lignin-derived phenols in sediments showed relatively little association with their respective ecoregions and appeared to be influenced by decomposition as evidenced by high ratios of carboxylic acids relative to aldehydes (Ad/Al). Overall, our research highlights the potential of n-alkanes as biomarkers for tracking distinct land cover types due to their strong associations with the proportion of grasses and trees.