4.1 Lake Water Chemistry and C Pools and Fluxes Reflect Climate, Topography, and Land Use
 The study lakes are located over a wide climatic gradient between the latitudes 60°N and 69°N. Besides climatic drivers, the spatial, seasonal, and interannual variation in lake chemistry reflects catchment land use, atmospheric deposition, and sediment water interaction, the latter being closely linked to catchment topography and lake morphology. In the boreal zone, climate is closely linked to atmospheric deposition. For example in Finland, N, P, and TOC deposition generally decreases from south to north [e.g., Kortelainen et al., 1997]. Impacts of atmospheric deposition and climate are thus difficult to separate; year-to-year variations in atmospheric deposition are closely linked to the ones in climate [Weyhenmeyer, 2008]. Despite these patterns only a few water quality variables indicated a strong south-north gradient throughout the year (Table 1). Average pH was higher in the north reflecting both lower acid deposition, lower organic acid export from northern catchments and more fertile soils in the south [Kortelainen et al., 1989]. Also, NO3-N concentrations were higher in the south reflecting higher N deposition and more intensive land use in the south, i.e., higher field percentage.
 The average TOC, TP, and Fe concentrations had slightly stronger links to climate and/or deposition compared to land use (Table 1), presumably reflecting significantly longer snow cover period in the northern catchments and longer ice cover period in the northern lakes resulting in decreased terrestrial export of TOC, TN, TP, and Fe [Kortelainen et al., 1997]. Also, CO2, NO3-N, and PO4-P had somewhat stronger links to climate and/or deposition than to land use (Table 1). The average length of the ice cover period ranges from about 5 months in southern lakes to over 7 months in northern lakes [Laasanen, 1982] although during many recent mild winters both the snow and ice cover periods have been shorter. Autochthonous production in predominantly oligotrophic boreal lakes is minor and majority of organic matter is transported from surrounding catchments [Jonsson et al., 2001; Larmola et al., 2004; Einola et al., 2011]. Majority of the annual organic matter load from boreal catchments is transported during spring and autumn high flow periods and many element fluxes, including TOC, generally follow river runoff patterns [e.g., Kortelainen et al., 1997; Laudon et al., 2004; Lepistö et al., 2008; Räike et al., 2012]. Although changing climate and hydrological conditions can alter the lateral transport of carbon through numerous mechanisms, recent studies have indicated the important role of precipitation as a key driver for C fluxes [e.g., Rantakari and Kortelainen, 2005; Raymond and Oh, 2007; Roehm et al., 2009; Ojala et al., 2011; Butman and Raymond, 2011; Sadro and Melack, 2012], whereas regional pCO2 patterns were shown to be closely linked to altitude [Lapierre and del Giorgio, 2012].
 Striegl et al.  demonstrated that Finnish lakes had generally greater pCO2 and lighter d13CDIC compared to lakes in Minnesota and Wisconsin, which indicates respiration to be the primary CO2 source in Finnish lakes. Nevertheless, the relatively stable TOC patterns over different seasons and depths compared to more variable TIC patterns (Table 4) do not support the degradation of TOC in the water column to play a major role to TIC/CO2 concentrations in Finnish lakes. The variability in the average TOC concentrations from the lake surface to the bottom was minor over different seasons, land use patterns, and a wide climatic gradient. In contrast, there was a large range in the average TOC/TIC ratio over different seasons and depth profiles, reflecting primarily the variability in TIC accumulation in the water column. In boreal dimictic lakes sediment interaction can be expected to be pronounced during winter and summer stratification when lakes are more isolated from the surrounding catchment than during spring and autumn high flow periods. This was presumably also reflected in the seasonal variation of TIC in Finnish lakes: (1) TIC and CO2 concentrations were highest in winter and linked to low O2 and high Fe, Mn, and NH4-N concentrations—presumably reflecting metabolism of organic matter in the sediment-water interface—whereas (2) during spring and autumn high flow periods TIC and CO2 concentrations were lower and more evenly distributed in the water column. Furthermore, summer stratification is stronger in the south resulting in lower O2 content and higher Fe, Mn, NH4-N, and TIC concentrations close to the sediment (Table 2). Elevated concentrations of Fe, Mn, NH4–N, TIC, and CO2 might also be due to groundwater input. However, Finnish Lake Survey in 1987, based on randomly selected lakes, demonstrated drainage lakes to be the major lake type (70 %) followed by headwater lakes (17%). Minority of the lakes were classified as seepage (10%) or closed (3%) [Kortelainen, 1993], indicating groundwater contribution in Finnish lakes to be generally small compared with many other regions in the world.
4.2 Nitrogen Is a Key Driver for Cev/Cacc
 In single lakes located in variable terrains, Cev/Cacc can show a large variability [e.g., Sobek et al., 2006; Finlay et al., 2010; Tranvik et al., 2009; Einola et al., 2011; Ferland et al. 2012], reflecting variable contributing drivers. Also, in our data the Cev/Cacc ratio varied from 4 to 86. Mean Cev/Cacc C was 30, slightly higher than the average estimate for Finnish lakes weighted by lake area (21) [Kortelainen et al., 2006a]. However, instead of focusing on most important drivers for Cev/Cacc in single lakes we use our data to study the role of lakes in landscape C balances.
 Terrestrial C balances have been intensively studied for numerous years, whereas freshwater C balance studies have traditionally been based on a few lakes neither with a focus on landscape patterns nor based on representative lake populations. Nevertheless, lakes contribute significantly to landscape C balances, evading C gases to the atmosphere and accumulating C in sediments. Finnish lakes are supersaturated both with CO2 and CH4 throughout the year, thus releasing C gases continuously to the atmosphere during the ice-free period and accumulating high concentrations of CO2 and CH4 in the water column during the winter ice cover period [Kortelainen et al., 2006a; Juutinen et al., 2009]. Although average annual C sequestration in Finnish lake sediments during the Holocene is small compared to average annual C evasion to the atmosphere, areal C stocks in sediments are larger than in upland forest soils [Kauppi et al., 1997; Kortelainen et al., 2004]. Our data demonstrate that boreal lakes play an important role in landscape C balances. The average annual CO2 emission from Finnish lakes to the atmosphere was estimated as 1.4 Tg C, approximately 20% of the average annual C accumulation in Finnish upland forest soils and tree biomass [Kortelainen et al., 2006a; Liski et al., 2006].
 In 177 randomly selected boreal lakes smaller than 100 km2, O2, NH4-N, and Mn were important drivers for CO2 evasion [Kortelainen et al., 2006a], whereas C pool in sediments was linked to lake water Fe and NO3-N concentrations [Kortelainen et al., 2004, equation (1)]. Terminal electron acceptors (i.e., O2, NO3-N, Mn, Fe oxides) thus played a key role in small boreal lakes regulating both C gas evasion to the atmosphere and average long-term C accumulation in sediments. As much as 79% (n = 2 740, p < 0.0001) of the variation in CO2 departure from the saturation in these lakes could be explained by O2 departure from saturation [Kortelainen, et al., 2006a]. Recently, Weyhenmeyer et al.  demonstrated that O2 and hydrological patterns were key drivers for CO2 concentrations both in boreal lakes and streams, whereas mean elevation was shown to play a key role in regional CO2 patterns in boreal lakes [Lapierre and del Giorgio, 2012]. Altitude and slope have been shown to be important predictors both for spatial NO3-N and DOC distributions [D'Arcy and Carignan, 1997; Kortelainen et al., 2006b; Helliwell et al., 2007; Sobek et al., 2007], although nitrate and DOC distributions often show opposite topographical patterns.
 Both lake area and maximum depth showed larger variability (0.04–1540 km2 and 1–93 m, respectively) in our data than in previous studies. Consequently, lake area and depth were important predictors both to C evasion and C accumulation. Both CO2 evasion to the atmosphere and areal C stock in the sediment were highest in small, shallow lakes, which can be considered as biogeochemical “hot spots” within the terrestrial landscape. Furthermore, the total C retention (C evasion + C accumulation) was largely explained by lake area (Figure 3). Both C evasion and C accumulation correlated also with water chemistry: positively with color, turbidity, Fe, and Mn concentrations and negatively with O2 and pH. The primary driver behind these relationships is difficult to predict, since small Finnish lakes are predominantly colored and have high TOC, Fe, and Mn concentrations, but low pH.
 When lake specific Cev/Cac was compared with lake morphometry, lake chemistry, sediment characteristics, catchment land use, and climatic drivers, only very few statistically significant correlations were found: negative correlation with sediment N pool (Figure 4) and positive with lake water NO3-N concentration (Figure 5). Furthermore, maximum depth was one of the few variables significantly correlated to Cev/Cacc. Depth of the lake is linked to topography, which was shown to play a key role in regional CO2 patterns [Lapierre and del Giorgio, 2012]. Furthermore, sediment C/N ratio was negatively linked to Cev/Cacc.
 Although N deposition and climate are closely linked in the boreal zone, none of the other drivers (including latitude and lake water temperature) showed significant correlations with Cev/Cacc. Although lake specific precipitation/runoff/deposition data was not available, atmospheric deposition in Finland decreases from south to north, precipitation is somewhat larger in the south, whereas runoff increases to the north due to higher evapotranspiration in the south. Precipitation, runoff, and deposition are thus linked to latitude.
 Molot and Dillon  studied lateral C mass balances of 20 small catchments and seven lakes in central Ontario and suggested that the ratio of evaded/accumulated C should increase with decreasing alkalinity. In our data including lakes from headwater catchments downstream to large drainage basins reflecting thus variable land use patterns and topography, Cev/Cacc correlated significantly neither with alkalinity nor with pH (Table 5). However, stepwise multiple regression models selected alkalinity and/or pH as predictors, but only after maximum depth (equation (3)) or N and catchment land use or conductivity (equations ((1))–((2))). Excluding N variables as predictors resulted in significantly lower explanation power of the model: maximum depth, pH, and alkalinity explained only 21% of the variation in Cev/Cacc (equation (3)). Our boreal data thus indicate N to be a key contributing factor contributing to the role of lakes in long term C balance in boreal zone.
 C and N accumulation rates and sediment C and N stocks were closely linked to each other (Figure 6). Furthermore, Cev/Cacc was positively linked to lake water NO3-N concentrations) indicating more effective C sequestration (i.e., lower respiration) in lakes with low NO3-N concentrations. This is in agreement with the study by Wickland et al.  showing the dominating role of inorganic nitrogen in controlling the biodegradability of DOC in the Yukon River. Furthermore, Booth et al. , conducting a synthetic analysis of 15N pool studies reported in terrestrial literature, found that soil N content exerted the strongest impact on soil N mineralization. A meta-analysis of 15N pool dilution studies of gross ammonification revealed that also the C/N ratio exerts a significant negative influence on gross ammonification indicating the higher N yield per unit of degraded soil organic matter at low C/N ratios.
 Most of the organic C in Finnish lakes originates from surrounding forests and peatlands resulting in low NO3-N concentrations and high C/N and C/P ratios in headwater streams and downstream lakes. Terrestrially produced organic matter has typically higher C/N ratio compared to organic matter produced in lakes. In our data average C/N ratio was lower in the sediment compared with lake water (12 vs. 23) (Table 3), presumably reflecting differences in the source, mineralization, and age of organic matter. Furthermore, C/N ratio reflects upstream-downstream position in the landscape; in boreal headwater streams significantly higher C/N ratio values have been recorded (average 48 [Kortelainen et al., 2006a]). Both DOC and CO2 in these headwater streams have been shown to be young [Billett et al., 2012]. Meyers and Takemura  studied Lake Biwa in Japan and concluded that during organic matter degradation organic carbon is converted to CO2 or CH4. These two gases diffuse out of the sediment, but organic N converts to NH4, which binds to clay minerals in the sediment. These contrasting fates of C and N lead to gradually smaller C/N ratios with greater time of burial.
 Phosphorus limited primary production in freshwater ecosystems has been shown in many regions and also CO2 flux from Quebec lakes was shown to be associated with total P concentrations [del Giorgio and Peters, 1994]. Recently, Lapierre and del Giorgio  demonstrated that TP:DOC ratio was important predictor for regional pCO2-DOC patterns. Nevertheless, recent studies focusing on boreal lakes have given evidence both to N limited production [Bergström and Jansson, 2006] and degradation [Kortelainen et al., 2006a] contrary to generally accepted view on P limitation. The synthesis by Dodds and Cole  showed that N regularly stimulates widely heterotrophic and autotrophic activities in freshwater and coastal ecosystems. Consistent with these findings NO3-N and NH4-N concentrations showed the highest seasonal variability among all water quality parameters: during summer inorganic N concentrations were significantly lower than during winter, whereas PO4-P concentrations showed minor seasonal variability (Table 2). Similarly, in Swedish lakes and streams NO3-N concentrations showed the highest growing season variability among all water quality variables [Khalil and Weyhenmeyer, 2009], whereas in the Yukon River and its tributaries inorganic nitrogen was shown to regulate the biodegradability of DOC [Wickland et al., 2012].
 Majority of organic matter in boreal freshwater systems is terrestrially fixed and accumulation of C and N in forests occurs through the same mechanisms, production of dead organic matter and microbial turnover, i.e., the net accumulation is regulated by the balance between production and decomposition. Globally, factors influencing decomposition rates may actually play a more important role in sequestration of organic carbon in soils than productivity [Cebrian and Duarte, 1995]. Nitrogen has been shown to be a key element regulating the production, structure, and function of terrestrial ecosystems. Meyer et al.  demonstrated strong N limitation in the northern taiga and southern tundra, and Wallenstein et al.  suggested that enzyme activity was linked to N availability in arctic soils, being low in summer presumably due to N limitation. Furthermore, Magnani et al.  showed net C sequestration to be overwhelmingly driven by N deposition in temperate and boreal forests; Gross primary production, ecosystem respiration, and Net ecosystem production only weakly correlated with temperature, and had no correlation with annual precipitation or latitude.
 Although N deposition may have increased C storage in northern forests to some extent [Pregitzer et al., 2008], ecosystem carbon storage in arctic tundra was reduced by long-term nutrient fertilization [Mack et al., 2004]. Root biomass was lower when using fertilizers, fertilization thus seemed to decrease below ground C sequestration. Minkkinen et al.  demonstrated that the annual CO2 fluxes from peatlands drained to forestry significantly increased from nutrient-poor to nutrient-rich sites. Khan et al.  analyzed results of long-term experiments in agricultural soils in the USA, and concluded that 40 to 50 year of inorganic fertilization caused a net decline in soil C content, despite increasingly massive residue C incorporation. Fertilization was of little, if any, benefit for soil C sequestration; addition of N or P was more effective for stimulating mineralization of soil organic C.
 We have shown that the role of lakes in long-term C balance in N-limited boreal landscape is linked to N. Nevertheless, changing climate and extreme weather conditions significantly contribute both to seasonal and annual C budgets thus modifying the role of lakes in landscape C balances. Einola et al.  demonstrated that C budgets in a chain of boreal lakes showed highly variable patterns during a dry and a wet year. The variability in annual precipitation regulated TOC and TIC input and output from lakes and significantly contributed also to CO2 fluxes from lakes to the atmosphere. Extreme weather conditions have been shown to modify terrestrial C budgets rapidly. Ciais et al.  demonstrated a European-wide reduction in primary productivity caused by the heat and drought in 2003; a strong anomalous net source of CO2 to the atmosphere reversed the effect of four years net ecosystem carbon sequestration. Piao et al.  demonstrated net CO2 losses from northern ecosystems due to increasing respiration during autumn warming. Also, peatland C sequestration rates have been shown to be highly sensitive even to minor climatic fluctuations, with wet periods correlating rapidly with increasing peat accumulation [Yu et al., 2003]. Climate change scenarios predict increasing temperature and precipitation for the northern latitudes [Denman et al., 2007]. The increasing frequency of heavy precipitation events and wet/dry periods might further increase the variability in the timing and magnitude of the loads. Shorter ice cover and soil frost period with earlier melting periods in winter and delayed soil frost in late autumn might result in significant feedbacks in C and N cycling, which are difficult to separate from the patterns of atmospheric deposition.