Abstract
Inland waters transport large amounts of dissolved organic matter (DOM) from terrestrial environments to the oceans, but DOM also reacts en route, with substantial water column losses by mineralization and sedimentation. For DOM transformations along the aquatic continuum, lakes play an important role as they retain waters in the landscape allowing for more time to alter DOM. We know DOM losses are significant at the global scale, yet little is known about how the reactivity of DOM varies across landscapes and climates. DOM reactivity is inherently linked to its chemical composition. We used fluorescence spectroscopy to explore DOM quality from 560 lakes distributed across Sweden and encompassed a wide climatic gradient typical of the boreal ecozone. Six fluorescence components were identified using parallel factor analysis (PARAFAC). The intensity and relative abundance of these components were analyzed in relation to lake chemistry, catchment, and climate characteristics. Land cover, particularly the percentage of water in the catchment, was a primary factor explaining variability in PARAFAC components. Likewise, lake water retention time influenced DOM quality. These results suggest that processes occurring in upstream water bodies, in addition to the lake itself, have a dominant influence on DOM quality. PARAFAC components with longer emission wavelengths, or red-shifted components, were most reactive. In contrast, protein-like components were most persistent within lakes. Generalized characteristics of PARAFAC components based on emission wavelength could ease future interpretation of fluorescence spectra. An important secondary influence on DOM quality was mean annual temperature, which ranged between -6.2 and +7.5°C. These results suggest that DOM reactivity depends more heavily on the duration of time taken to pass through the landscape, rather than temperature. Projected increases in runoff in the boreal region may force lake DOM toward a higher overall amount and proportion of humic-like substances.
Highlights
Dissolved organic matter (DOM) has a central role in biogeochemical processes within lakes and has an active role in the global carbon cycle (Cole et al, 2007; Battin et al, 2009; Tranvik et al, 2009)
We used fluorescence spectroscopy to explore dissolved organic matter (DOM) quality from 560 lakes distributed across Sweden and encompassed a wide climatic gradient typical of the boreal ecozone
The relative positioning of Y and X variables differed substantially when Y variables were expressed as parallel factor analysis (PARAFAC) component intensities (C1 to C6; referred to as the C-intensity model) (Fig. 2a), or as a percentage (%C1 to %C6; referred to as the C-percentage model) (Fig. 2b)
Summary
Dissolved organic matter (DOM) has a central role in biogeochemical processes within lakes and has an active role in the global carbon cycle (Cole et al, 2007; Battin et al, 2009; Tranvik et al, 2009). There is widespread awareness that concentrations of DOM exported to aquatic ecosystems have increased in parts of Northern Europe and North America (De Wit et al, 2007; Monteith et al, 2007; Couture et al, 2012). Previous links to declining sulfate deposition have relied on correlative analysis (Monteith et al, 2007), yet, recent experimental work has provided support that increasing dissolved organic carbon (DOC) export from soils is associated with decreasing soil acidity (Evans et al, 2012). It is expected that catchment soils will continue to recover from acidification in the upcoming decade; regions, such as the boreal ecozone anticipating increasing precipitation levels, may observe continued inputs of additional DOM fluxes due to greater runoff (Hongve et al, 2004; Eimers et al, 2008; Lo€fgren & Zetterberg, 2011)
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