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 À6.2 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.

Burial in sediments removes organic carbon (OC) from the short-term biosphere-atmosphere carbon (C) cycle, and therefore prevents greenhouse gas production in natural systems. Although OC burial in lakes and reservoirs is faster than in the ocean, the magnitude of inland water OC burial is not well constrained. Here we generate the first global-scale and regionally resolved estimate of modern OC burial in lakes and reservoirs, deriving from a comprehensive compilation of literature data. We coupled statistical models to inland water area inventories to estimate a yearly OC burial of 0.15 (range, 0.06–0.25) Pg C, of which ~40% is stored in reservoirs. Relatively higher OC burial rates are predicted for warm and dry regions. While we report lower burial than previously estimated, lake and reservoir OC burial corresponded to ~20% of their C emissions, making them an important C sink that is likely to increase with eutrophication and river damming.

Many boreal waters are currently becoming browner with effects on biodiversity, fish production, biogeochemical processes and drinking water quality. The question arises whether and at which speed this browning will continue under future climate change. To answer the question we predicted the absorbance (a 420 ) in 6347 lakes and streams of the boreal region under future climate change. For the prediction we modified a numerical model for a 420 spatial variation which we tested on a temporal scale by simulating a 420 inter-annual variation in 48 out of the 6347 Swedish waters. We observed that inter-annual a 420 variation is strongly driven by precipitation that controls the water flushing through the landscape. Using the predicted worst case climate scenario for Sweden until 2030, i.e., a 32 % precipitation increase, and assuming a 10 % increase in imports of colored substances into headwaters but no change in land-cover, we predict that a 420 in the 6347 lakes and streams will, in the worst case, increase by factors between 1.1 and 7.6 with a median of 1.3. This increase implies that a 420 will rise from the present 0.1-86 m −1 (median: 7.3 m ), which can cause problems for the preparation of drinking water in a variety of waters. Our model approach clearly demonstrates that a homogenous precipitation increase results in very heterogeneous a 420 changes, where lakes with a long-term mean landscape water retention time between 1 and 3 years are particularly vulnerable to climate change induced browning. Since these lake types are quite often used as drinking water resources, preparedness is needed for such waters.

16The quantity of carbon dioxide (CO 2 ) emissions from inland waters into the atmosphere 17 varies, depending on spatial and temporal variations in the partial pressure of CO 2 (pCO 2 ) in

Water renewal along the aquatic continuum offsets cumulative retention by lakes: implications for the character of organic carbon in boreal lakes. The character of organic carbon (OC) in lake waters is strongly dependent on the 8 time water has spent in the landscape as well as in the lake itself due to continuous 9 biogeochemical OC transformation processes. A common view is that upstream lakes 10 might prolong the water retention in the landscape, resulting in an altered OC character 11 downstream. We calculated the number of lakes upstream for 24,742 Swedish lakes in 12 seven river basins spanning from 56º to 68º N. For each of these lakes, we used a lake 13 volume to discharge comparison on a landscape scale to account for upstream water 14 retention by lakes (T n tot ). We found a surprisingly weak relationship between the number 15 of lakes upstream and T n tot. Accordingly, we found that the coloured fraction of organic 16 carbon was not related to lake landscape position but significantly related to T n tot when 17we analysed lake water chemical data from 1,559 lakes in the studied river basins (R 2 = 18 0.21, p < 0.0001). Thus, we conclude that water renewal along the aquatic continuum by 19 lateral water inputs offsets cumulative retention by lakes. Based on our findings, we 20 suggest integrating T n tot in studies that address lake landscape position in the boreal zone 21 to better understand variations in the character of organic carbon across lake districts. 22 23