Water renewal along the aquatic continuum offsets cumulative retention by lakes: implications for the character of organic carbon in boreal lakes
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The character of organic carbon (OC) in lake waters is strongly dependent on the time water has spent in the landscape as well as in the lake itself due to continuous biogeochemical OC transformation processes. A common view is that upstream lakes might prolong the water retention in the landscape, resulting in an altered OC character downstream. We calculated the number of lakes upstream for 24,742 Swedish lakes in seven river basins spanning from 56º to 68º N. For each of these lakes, we used a lake volume to discharge comparison on a landscape scale to account for upstream water retention by lakes (Tn tot). We found a surprisingly weak relationship between the number of lakes upstream and Tn tot. Accordingly, we found that the coloured fraction of organic carbon was not related to lake landscape position but significantly related to Tn tot when we analysed lake water chemical data from 1,559 lakes in the studied river basins. Thus, we conclude that water renewal along the aquatic continuum by lateral water inputs offsets cumulative retention by lakes. Based on our findings, we suggest integrating Tn tot in studies that address lake landscape position in the boreal zone to better understand variations in the character of organic carbon across lake districts.
KeywordsLake Landscape Time Organic carbon Colour
Many thanks go to the Swedish Meteorological and Hydrological Institute (SMHI) for providing the hydrological and geomorphological base data used in this study, as well as to the Department of Aquatic Sciences and Assessment at the Swedish University of Agricultural Sciences (SLU) for providing data from thousands of lake water samples. We thank Dolly Kothawala and Blaize Denfeld, as well as three anonymous reviewers for constructive comments. Financial support was received from the Swedish Research Council (VR), the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), and the Nordic Centre of Excellence “CRAICC—Cryosphere-atmosphere interactions in a changing arctic climate” supported by NordForsk. Martyn N. Futter was funded by the MISTRA Future Forests programme. This work developed within the research framework of the project “The Color of Water”.
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