5.1. DOM Temporal Variability and Patterns
 Our results indicate that during spring snowmelt, lakes buffered outflows from DOM flushing. We observed a decline in the DOC temporal variability, the DOC response to snowmelt flushing (both the timing and the rate of increase), and the magnitude of DOC concentrations in the outflow relative to the inflow (Figures 2 and 3 and Table 2), signifying in-lake DOC retention during snowmelt. LRF inflow was the only lake inflow that did not exhibit a rapid increase in DOC concentration on the ascending limb of the snowmelt hydrograph (Figure 3) or a DOC lag (Table 2). This result is likely due to a large (6 km2) lake only 2.6 km upstream (Figure 1) causing LRF inflow to behave more like lake outflows. In contrast, lakes do not buffer the timing or magnitude of peak stream water discharge (Figure 2), consistent with data from Bull Trout presented by Arp et al. . The inability of lakes to buffer spring snowmelt flows is likely a result of low in-lake storage capacity as lake water levels rise [Arp et al., 2006]. These results indicate that while hydrologic factors have a large control on stream snowmelt DOC dynamics, in-lake biogeochemical factors also influence lake outflow DOC dynamics.
 The inflow DOC flushing responses observed in the Sawtooth Mountain lake district were similar to other snowmelt-dominated systems, where snowmelt represents a flushing of a finite terrestrial DOC source in soil pore water [Boyer et al., 1997; Brooks et al., 1999; Hood et al., 2003; Hornberger et al., 1994]. However, our results indicate that lakes can dampen this DOC flushing response in lake outflows. The ability of lakes to dampen this DOC pulse was variable across watersheds and is likely related to an interaction of many factors including controls on DOC pulse dynamics, such as hillslope connectivity [McGlynn and McDonnell, 2003], topography, elevation and patterns of snow accumulation and melting [Boyer et al., 1997; Hornberger et al., 1994], as well as controls on lake DOC dynamics, such as lake residence time [Arp et al., 2006] and in-lake microbial processing [Crump et al., 2003].
 Pace and Cole  hypothesized that DOC may build up in lakes during times of ice cover due to a lack of light, and hence a lack of photodegradation. If this occurred in our study, we would have expected lake outflows to exhibit high DOC concentrations during spring snowmelt flushing. However, pre-ice-out DOC concentrations in outflows of our study lakes were not above base flow concentrations (Figure 2), and their peak snowmelt DOC concentrations were lower than inflows. The fact that we did not observe a buildup of DOC in lakes during ice cover may be a result of their oligotrophic nature, and hence low productivity [Budy et al., 1995].
 As expected, we observed significantly lower CVs for DOM concentrations and characteristics in lake outflows than inflows (Table 3). These results illustrate the ability of lakes to buffer stream DOM temporal variability. In contrast, Cattaneo and Prairie  did not observe a dampening effect of lakes on lake outlet stream chemistry (DOC concentration was not included) in a low-gradient Canadian stream, which they attributed to the relatively small lake size (0.03–5.29 km2) and rapid flushing (not reported) of their study lakes. However, their sampling began following spring runoff, and therefore may not have captured the period when stream chemical concentrations and characteristics are changing drastically in snowmelt-dominated systems [Boyer et al., 1997; Lewis and Grant, 1979]. Wurtsbaugh et al.  also argued that lakes would dampen nutrient pulses through watersheds and used a mixed-reactor model to demonstrate the possible magnitude of this effect.
 We observed an inverse relationship between lake outflow DOC temporal variability and the size of the lake (Table 4), suggesting that the extent to which lakes can stabilize temporal variability in DOC concentration in mountain landscapes is related to their storage capacity. Furthermore, we observed an inverse relationship with inflow DOC variability and the volume of the closest upstream lake, indicating that upstream water residence time can regulate downstream DOC temporal variability.
 Water residence time and flushing rates are related to both watershed and lake size. Lake: watershed area ratios (i.e., drainage ratio) provide insight into the amount of water being routed from the surrounding landscape to a lake of a known size. Thus, the drainage ratio can provide considerable information on lake storage capacity and flushing rates [Canham et al., 2004] and boreal DOC concentration [Engstrom, 1987; Kortelainen, 1993; Rasmussen et al., 1989]. We observed negative relationships between drainage ratios and DOM variability (Table 4). We also observed a negative correlation, albeit not as strong for DOM characteristics (SUVA254), between DOM variability and lake volume. These results illustrate that both lake volume and watershed area influence DOM temporal variability.
 Upstream watershed characteristics also appear to influence DOM temporal variability. Increasing percentages of upstream lake area dampened temporal variability in DOC concentrations (Table 4). Additionally, we observed strong relationships between the temporal variability of SUVA254 and watershed land cover characteristics, where the greater the % barren area and lower the % vegetated area, the greater the variability. Similar to our results, Little et al.  found the proportion of barren land to be a significant predictor of N (primarily DON) export during base flow conditions in Chilean watersheds, and attributed this result to a low capacity for water and nutrient retention.
 DOM temporal patterns were different between most lake inflows and outflows (Table 5). However, two (HR and LRF) of our seven study watersheds did not exhibit a significant difference in DOC temporal patterns between inflow and outflow streams. These two watersheds have extremely low residence times during base flow (0.29 and 0.02 years, respectively) conditions (Table 1), and residence times would be even lower during snowmelt conditions [Arp et al., 2006]. Therefore, high flushing rates likely resulted in the decreased buffering capacity of these lakes [Arnott et al., 2003]. Additionally, both of these lakes are influenced strongly by relatively close upstream lakes (Figure 1).
 Similar to temporal patterns of DOC concentration, DOM characteristics in lake inflow streams were more temporally dynamic than lake outflow streams (Figure 4 and Table 3). Surprisingly, in late July and early August (approximate days of the year 200–214), SUVA254 values increased in lake inflows. This shift in DOM characteristics was more dramatic in lake inflows than outflows, and may be a result of dramatic decreases in lake inflow DOC concentrations (Figure 2). However, lakes buffered outflow streams from this increase in aromatic DOM observed in the inflows during mid–late summer (Figure 4), which may be a result of microbial DOM production and processing within the lakes [Goodman, 2010].
 Our DOC:DON results illustrate the high temporal variability of DOC:DON ratios (Table 3), which may be a result of high variability of DON analytical measurements and/or differences in flushing rates [Kaiser and Zech, 2000; Kaushal and Lewis, 2003] and cycling [Caraco and Cole, 2003; Kaushal and Lewis, 2005; Wiegner and Seitzinger, 2001]. Additionally, rapid uptake of DON can occur in N-limited systems [Kaushal and Lewis, 2005; Stepanauskas et al., 2000], such as the Sawtooth Mountains, which further obscures the use of DOC:DON ratios as an indicator of source DOM quality and bioavailability. Kaushal and Lewis  found DOC bioavailability to be related to nonhumic (i.e., carbohydrates) fraction C:N ratio, yet a substantial portion of DOC is humic substances [Hood et al., 2003; Kaushal and Lewis, 2005]. Similarly, DOC:DON ratios of DOM isolates, such as fulvic and transphilic acids, may be a more reliable indicator of DOM temporal variability in chemical characteristics [Hood et al., 2005]. Therefore, we suggest care should be taken when evaluating DOC:DON ratios as an indicator of DOM bioavailability, and further suggest the use of DOM chemical isolates or a large sample size when attempting to evaluate DOM characteristics using DOC:DON ratio.
5.2. Controls on DOC Dynamics
 Annual export from our study watersheds was similar to those reported from Colorado Front Range systems (∼11–16 kg C ha−1; Table 6) [Hood et al., 2003], albeit in the low range of DOC yields across the United States, 7–74 kg C ha−1 [Aitkenhead and McDowell, 2000; Tate and Meyer, 1983]. Similar to other studies, DOC export in our systems was driven by spring snowmelt runoff [Baker et al., 2000; Boyer et al., 1997; Kortelainen et al., 1997; Schindler et al., 1997].
 Recent studies have shown that lakes can be a sink for inorganic nutrients [Brown et al., 2008; Kling et al., 2000; Robinson and Matthaei, 2007], and may be a source of particulate and dissolved organic nutrients [Brown et al., 2008; Robinson and Matthaei, 2007]. Furthermore, Brown et al.  and Wurtsbaugh et al.  suggested that lakes may switch from sinks to sources of nitrogen from spring snowmelt to summer base flow. Our results support this hypothesis. We observed a shift in the role of lakes from a DOC sink to a source (that is, in-lake DOC production increased) throughout the summer, as evaluated by differences in DOC concentration between lake inflows and outflows relative to the inflow concentration (Figure 6).
 Following peak snowmelt discharge in early June, DOC production in lakes increased for approximately 2 months, which may be a result of increased lake primary production [Crump et al., 2003]. DOC and DON production in lakes has been observed in small oligotrophic [Kling et al., 2000; Lockwood, 2009] and eutrophic [Fairchild and Velinsky, 2006] lakes and ponds. Our prediction that larger lakes would have greater DOC production due to longer residence time was not supported and may be a result of the inherently low pelagic primary production of these systems [Budy et al., 1995].
 The fact that the percentage of DOC production declined dramatically in all lakes by mid August, in accordance with shifts in SUVA254 (i.e., aromaticity increased, lower quality carbon; Figure 4) suggests a shift in DOM source and/or microbial processing [Goodman, 2010]. Previous work in these systems indicates that in July and August, nutrients (i.e., N and P) become limiting to autotrophic [Marcarelli and Wurtsbaugh, 2007; Spaulding, 1993] and heterotrophic [Goodman, 2010] production and processing. Thus, altered microbial processes due to nutrient limitation may have altered inflow DOM characteristics. Conversely, in Bull Trout and Stanley Lake we observed higher SUVA254 values in the lake outflow than inflow (Figure 4 and Table 3). This result is likely due to the fact Bull Trout and Stanley Lakes contains dense submerged macrophytes (W. Wurtsbaugh, unpublished data, 2008), which likely contribute large amounts of DOM to the lake and lake outflow [Goodman, 2010; Rich and Wetzel, 1978].
 Previous research has shown that lakes in oligotrophic boreal systems can decrease annual DOC export [Mattsson et al., 2005], while autotrophic production in eutrophic lakes can increase annual DOC production [Fairchild and Velinsky, 2006]. We observed a shift in the role of oligotrophic subalpine lakes over time, where lakes decreased DOC export during the spring, and increased DOC export during the summer (Table 6). This higher DOC export during base flow conditions results in an increase in annual DOC export and may provide an important DOC source to downstream locations at a time when the terrestrial DOC supply has been exhausted [Boyer et al., 1997; Brooks et al., 1999; Hood et al., 2003; Hornberger et al., 1994].
 Six of the seven lakes we studied exported more DOC than was imported. Two lakes (Bull Trout and Stanley) that have extensive macrophyte beds also had the highest net production of DOC (Figure 6), suggesting that in-lake primary production may be quite important. The lakes also receive particulate organic carbon, particularly during spring runoff. Based on previous work on nitrogen import to Bull Trout Lake [Hall et al., 2009], we calculate that the inflow stream delivered an average of 0.23 mg POC L−1 during the descending limb of the hydrograph, and concentrations as high as 2.9 mg C L−1 occurred during a spate (B. Hall, unpublished data, 2002). Much of this POC can be processed in lakes and likely contributes to the higher DOC concentrations in the outflows than in the inflows [Cole et al., 2002]. Although net export of organic matter may allude to a positive or autotrophic metabolic balance [Lovett et al., 2006], without a complete carbon budget or measurements of CO2 saturation it is not possible to assess whether the lakes are heterotrophic or autotrophic [Duarte and Prairie, 2005].
 One limitation for the interpretation of DOM fluxes in our study is that we could not account for all of water flowing into the lakes. Gross and Wurtsbaugh  calculated that, on average, 6% of the total inflow of Sawtooth Lakes is from nonchannelized hillslope runoff. Hence, we were forced to assume that inflow volumes were equal to outflow volumes. We also assumed that the unaccounted for water had DOM concentrations similar to those measured in the inflow streams. We believe that a substantial portion of unaccounted water is from large numbers of rivulets that entered around the lake during snowmelt, but it is reasonable to assume that their DOM concentrations were approximately like those in the mainstreams. Lake evaporation was also not measured, and could have contributed somewhat to the increased concentrations of DOC in the lake outflow, however lake evaporation is small in this mountainous region [Molnau et al., 1992].
 Our study characterized the ability of lakes to buffer DOM temporal patterns in mountain streams by evaluating how the function of subalpine lakes can switch throughout a sampling season. Our results illustrate that lakes within high-elevation ecosystems can buffer DOM temporal patterns in streams, by acting as a DOM sink during springtime snowmelt flushing and a DOM source during summer base flow. Therefore, lakes dampen snowmelt-flushing responses in lake outflows and increase DOM export during base flow, ultimately buffering solute fluxes by lakes in subalpine regions. The fact that lake inflow variability was strongly related to % upstream lake area further illustrates the ability of lakes to reduce the control of hydrologic transport on stream DOM dynamics. Our results demonstrate that oligotrophic lakes within mountain fluvial networks can play a role as a landscape regulator of DOC concentration and export, and may provide an important energy source to downstream locations when terrestrial supplies have been exhausted. As such we suggest that mountain lakes provide stability to surface stream networks.