Long‐term development and trajectories of inferred lake‐water organic carbon and pH in naturally acidic boreal lakes

Abstract

Monitoring of surface waters in the boreal region over the last decades shows that waters are becoming browner. This timeframe may not, however, be sufficient to capture underlying trajectories and driving mechanisms of lake-water quality, important for prediction of future trajectories. Here we synthesize data from seven lakes in the Swedish boreal landscape, with contemporary lake-water total organic carbon (TOC) concentrations of 1.4-14.4 mg L −1 , to conceptualize how natural and particularly human-driven processes at the landscape scale have regulated lake-water TOC levels over the Holocene. Sediment-inferred trends in TOC are supported by several proxies, including diatom-inferred pH. Before $ 700 CE, all lakes were naturally acidic (pH 4.7-5.4) and the concentrations of inferred lake-water TOC were high (10-23 mg L −1 ). The introduction of traditional human land use from $ 700 CE led to a decrease in lake-water TOC in all lakes (to 5-14 mg L −1 ), and in four poorly buffered lakes, also to an increase in pH by > 1 unit. During the 20 th century, industrial acid deposition was superimposed on centuries of land use, which resulted in unprecedentedly low lake-water TOC in all lakes (3-11 mg L −1 ) and severely reduced pH in the four poorly buffered lakes. The other lakes resisted pH changes, likely due to close connections to peatlands. Our results indicate that an important part of the recent browning of boreal lakes is a recovery from human impacts. Furthermore, on a conceptual level we stress that contemporary environmental changes occur within the context of past, long-term disturbances.

Application of mid-infrared spectroscopy for the quantitative and qualitative analysis of organic matter in Holocene sediment records

“…The earliest of the three phases identified by Ninnes et al (2017) is the post-glacial phase (>7900 BCE), which represents the time of lake formation. Mineral matter initially dominates the sediment but as the catchments become vegetated and soils stabilize (Myrstener et al, 2021), the relative abundances of most pyrolytic organic compound groups increase rapidly (Figure 6). This phase is equally apparent in the spectral data, with initially low scores for all organic components, corroborating the relatively low, but increasing OM content.…”

“…The first two are characterized by large increases in labile pyrolytic compound groups such as proteins and amino acids, anhydrosugars and 4-isopropenylphenol, but also in the aliphatic n -alkan-2-ones. These changes are probably linked to altered catchment inputs due to expansion of adjacent peatlands and/or the development of the floating, shoreline peat mats (Myrstener et al, 2021; Ninnes et al, 2017). Increased supply of OM from peat or Sphagnum , in combination with more rapid burial, would increase the proportion of polysaccharides in the sediment and also promote preservation of labile compounds (Ninnes et al, 2017).…”

“…Dragsjön was sampled in 2014 and Lång-Älgsjön in 1998. Dating and initial age-depth modeling were done by Ninnes et al (2017) and Meyer-Jacob et al (2015) for Dragsjön and Lång-Älgsjön, respectively, and both age-depth models updated by Myrstener et al (2021).…”

Spatial Predictors and Temporal Forecast of Total Organic Carbon Levels in Boreal Lakes

“…The intensification of forest management in the 1950ies and 60ies increased the forest volume in Nordic countries. Moreover, abandonment of out-field resources as grazing pastures for husbandry, has resulted in increased standing forest biomass during the last centuries (Myrstener et al, 2021). In the coming decades, afforestation is expected to expand further, as the governments of Norway, Sweden, and Finland (collectively referred to here as Fennoscandia) consider forest as a necessary trade-off leverage for offsetting their carbon emissions and reaching their zero-emission goals (Nordic Council of Ministers, 2021;Vogt et al, 2022).…”