The DOC concentration record and hydrologic characteristic at the bog site has been described previously in Broder and Biester (2015). In short, bog discharge exhibited a flashy regime with an instantaneous response to rain events. The rain event with the highest recorded discharge peak occurred in spring, while in summer a longer drought period resulted in very low discharge and little response to rainfall due to recovery of water storage within the bog. More frequent rain events in fall at wetter antecedent moisture conditions caused again more flashy discharge and concentration responses.

The variability of DOC concentrations over the year ranged between 5.0 and 45.8 mg L-1 (SD of 7.0 mg L-1 at the forested site and 8.1 mg L-1 at the bog site) and was larger than during single rain events at both sites, where standard deviations ranged between 1.3 and 1.9 mg L-1 at the bog site and from 0.6 to 4.2 mg L-1 at the forested site (Figs. 2 and 3 and Table 1). However, the rain event in fall was responsible for the highest recorded DOC concentrations of 37.3 and 45.8 mg L-1 during the entire study period at both sites. The concentration trend generally followed the vegetation period with highest concentrations in late summer and fall. Spearman's correlation of DOC concentrations with mean daily air temperature was significant and positive at the bog site (coefficient of 0.591; p < 0.01, two-tailed), but not at the forested site. The lowest concentrations, of 5.0 and 10.2 mg L-1, were measured during snowmelt at both sites. Concentrations of DOC were significantly higher at the forested site (median of 32.1 mg L-1) than at the bog outlet (median of 25.7 mg L-1) over the whole sampling period (see Table 1).

The abs254nm as index for total aromaticity of the DOM exhibited a similar trend to the DOC concentrations at both sites over the year (Fig. 2). Nonetheless, SUVA254 values as index for proportional aromaticity of DOM varied between 3.5 and 5.2 at the bog site and between 3.4 and 5.9 at the forested site, but with no seasonal trend as observed for DOC concentrations. According to Weishaar et al. (2003), calculated SUVA254 values corresponded to a DOM aromaticity of 27–38 % for the bog site and 29–42 % for the forested site. Variations were mainly induced by hydrologic conditions with high values during rain events in spring (up to 5.5). Mean SUVA254 values were higher at the forested site (SUVA254 of 4.6, SD of 0.5), but showed a larger variability than at the bog site (mean SUVA254 of 4.4, SD of 0.4, see also Table 1). During the summer drought period SUVA254 values decreased at both sites, but this decrease was stronger at the forested site (Fig. 2).

As expected, the SR as reciprocally proportional index for molecular weight of DOM exhibited an opposite trend to SUVA254 (Fig. 2), expressed in a negative correlation on a 0.01 level of significance (Spearman correlation coefficient of -0.616 at the bog site and -0.598 at the forest site). The annual dynamic was similar at both sites, but with higher SR during snowmelt (SR up to 2.2) and sampled rain events (SR 1.7–2.0) at the forested site (Fig. 3), indicating a lower molecular weight than at the bog site. During the summer drought SR steadily increased from 1.7 to 2.0, indicating decreasing molecular weight. With the onset of fall rain events, molecular weight increased again, indicated by lower SR values.

The HIX, as well as the FI, also exhibited no annual trend (Fig. 4). At the bog and the forested site HIX only varied during snowmelt and spring events with lower values down to 0.76 and 0.81, respectively, compared to HIX in summer and fall, where values remained between 0.90 and 0.94 and between 0.91 and 0.95, respectively. FI exhibited values between 1.5 and 1.75, with significantly higher values at the forested site. At the forested site FI increased during summer drought (from 1.5 to 1.7).

To facilitate a description of the PARAFAC results, the identified components are briefly described here and compared to hypothetical fluorophores typically observed in other studies. PARAFAC modeling resulted in a five-component model with four humic-like and one protein-like hypothetic fluorophores. Excitation–emission regions of each component can be found in Fig. 5. The modeled PARAFAC component C1 can be compared to a terrestrial, humic-like fluorophore originating from forest and wetland soils, as described by Perdrial et al. (2014), and C2 can again be described as humic-like (see, e.g., C3 of Singh et al., 2014), but the excitation–emission region is shifted to higher excitation and emission wavelengths compared to C1, indicating more conjugated and more aromatic fluorescent molecules. A component similar to C3 has previously been described as humic, but also of terrestrial origin, small molecular size, recalcitrant and reduced (Cory and McKnight, 2005; Singh et al., 2014; Fellman et al., 2008; Perdrial et al., 2014). C4 is only slightly shifted compared to the excitation–emission region of C3 and compares to C2 from Fellman et al. (2008) and Ohno and Bro (2006), another humic-like fluorophore. C5 could be described as tryptophan-like, of microbial origin, labile and of recent biological production (described in e.g., Fellman et al., 2008 as C8). It can be used as proxy for BDOM (Fellman et al., 2008). In C1 and C3 fulvic-like fluorophores might also be included, which are more hydrophilic and therefore more mobile than the humic-like DOM (in Fellman et al., 2008: C3, C4), but could not be clearly separated into individual components.

The fDOM, as the sum of all fluorescent components modeled by PARAFAC, showed changes with discharge events with both minimum and maximum intensities during sampled discharge events (Fig. 4). Fluorescence was elevated during summer and lower values occurred in spring and fall at both sites. Normalizing fDOM to DOC concentrations, a decrease in the fluorophore fraction in DOM from spring to fall could be observed (Fig. 4). This fluorophore fraction also decreased during individual sampled rain events.

The fDOM at the bog site showed few seasonal changes in the contribution of the four components over the year. The greatest changes were perceived during snowmelt with the highest protein-like C5 % (∼ 10–17 %) and variable humic-like C1 %, C2 % and C4 % contributions at the bog site (Fig. 7). The protein-like C5 % contributed least to fDOM with about 5 % during most of the record. The components C1 % and C4 % increased during summer drought at the bog site (Fig. 6). The protein-like C5 % contributed largely to fDOM during snowmelt and a wet spring event (Fig. 6). The humic like C1 % increased during summer drought, while C2 % decreased at the same time. The components C2 % and C3 % were significantly elevated at the forested site, while significantly higher contributions of C4 % were attributed to the bog site (Figs. 6 and 7).

Snowmelt samples exhibited a distinct pattern compared to discharge events in other seasons. DOC concentrations (5.0–11.7 mg L-1), abs254nm (45–128 m-1), HIX (0.76–0.93) and fDOM (0.6–1.1) were lowest and normalized fDOM / DOC (0.06–0.14) was highest during snowmelt at both sites (Figs. 3 and 4). Differences to other events were significant, excluding the wet spring event, though. Values of SR during snowmelt were high at the forested site with about 1.96–2.17, while SR at the bog site exhibited mean values of 1.78–1.88 (Fig. 3), which was still higher than following spring samples, though. SUVA254 was significantly lower (4.2–4.5) than in following spring events at the forested site, while at the bog site SUVA254 of 3.9–4.4 were similar to values of the rest of the record (Table 1, Fig. 2). The protein-like PARAFAC component C5 % was elevated during snowmelt (7–16 %) and was significantly higher than at the fall event at both sites (Fig. 7). Values of the humic-like C2 % (median of 23 %) and C4 % (median of 7 %) were low at the bog site compared to median annual values of 26 and 10 %, respectively. At the forested site the bog-derived C4 % was almost absent and the humic-like forest-derived C3 % was strongly elevated (contributing about 20 % to total fluorescence, Fig. 7). It should be noted, though, that the snowmelt event was not entirely covered by our sampling period as the upstream water was still covered with snow.

The sampled spring rain events could be differentiated by hydrologic preconditions as the first event occurred under dry preconditions, while the second described spring event followed after preceding rain events under wet hydrologic preconditions. DOC concentrations and abs254nm were significantly higher after dry preconditions (21.9–31.7 mg L-1 and 225–341 m-1) than after wet preconditions (9.4–23.1 mg L-1 and 102–281 m-1) at both sites (Fig. 3). SUVA254 values at the forested site were persistently high during the whole spring time (4.5–5.5), but decreased during each rain event. At the bog site, SUVA254 values were significantly lower than at the forested site during spring rain events, but were still significantly elevated compared to the fall event at the bog site. High-resolution sampling before the wet-precondition spring event during low-flow conditions showed a steady decrease in SR until the onset of the rain event where values momentarily peaked at the forested site. However, at the first spring event with dry preconditions, the SR values peaked later at the declining limb of the hydrograph. At the bog site SR values during spring events were low, but again with increasing values at the declining limb of the hydrograph, which indicates a decrease in apparent molecular size.

Contribution of protein-like component C5 % exhibited elevated values during the second spring event with wet preconditions at the bog site (Figs. 6 and 7). The bog-derived humic-like C4 % decreased during spring events at both sites (Fig. 6). This trend was more distinct at the forested site, with a quick drop to zero at the second event. The humic-like C1 % dropped from the first sampled spring event to the second one at the bog site, while the more aromatic C2 % increased at the forested site from the first to second event. The predominantly forest-derived humic-like C3 % also increased at all storm events and was significantly higher at the forested site.

The year 2013 was characterized by a strong summer drought, which caused low discharge over the summer months. During the prevailing drought period DOC concentrations, abs254nm and SR increased, while SUVA254 decreased, especially at the forested site (from 5.4 to 3.7, Fig. 2). The humic-like C1 % increased and the humic-like, but more aromatic C2 % decreased at the forested site, while at the bog outlet an increase in the humic-like bog-derived C4 % was perceived (Fig. 6).

The fall event following the summer drought generated the highest DOC concentrations of the annual record with 45.8 mg L-1 DOC at the forested site and 37.3 mg L-1 DOC at the bog site. Even though abs254nm was high and even increasing during the event at the forested site, SUVA254 values were significantly lower than during spring events, indicating a lower aromaticity of DOM in fall (Fig. 3). Congruently, SR values were higher at both sites, indicating smaller DOM during the fall event. The humic-like PARAFAC components C1 %, C4 % and the protein-like C5 % decreased during the fall rain event at the forested site, while the forest-derived humic-like C2 % and C3 % increased (Fig. 6). The bog-derived C4 % contributions were significantly higher than during snowmelt and in spring at the bog site (Fig. 7), but exhibited the same decreasing trend with the ongoing rain event at both sites. The protein-like C5 % exhibited lower values during the fall event than at the spring rain.

Organic-rich riparian zones are known to generate high DOC concentrations (Grabs et al., 2012). As a large part of the organic-rich upper soil layer is hydrologically connected and contributes to discharge only during events, high DOC concentrations at the forested site are due to the repeated flushing of peaty soils in the riparian zone. Additionally, upper organic layers of shallow hillslope soils may get connected via surface or near-surface flow networks during such events. Therefore, dry preconditions within the catchment facilitated high DOC concentrations during events of a certain magnitude when the upper soil layer gets hydrologically connected. The DOC concentrations at the bog site were less sensitive to rain events and not as elevated during those events than at the forested site. Here, partly decreasing concentrations were observed. Due to the usually high water level, rain events here do not connect additional DOM pools, but lead to dilution by surface flow or an exhaustion effect (Broder and Biester, 2015). We conclude that DOC concentration peaks during rain events were mainly induced by peaty forest soils and not by bogs. Although the latter are strong C sources to the aquatic, they are less susceptible to rain events. DOC concentration trends of the bog site were further disentangled in Broder and Biester (2015).

During snowmelt, DOC concentration at both sites were lowest. This has been reported elsewhere (e.g., Laudon et al., 2004; Clark et al., 2008) and can be attributed not only to dilution by snow packs, but also to low microbial and plant activity. The absolute values of chromophore and fluorophore DOM were also lowest. Overall, spectrofluorometric indices point to rather small molecular size, which were less aromatic, especially at the forested site. Additionally, PARAFAC components indicate a strong flush of labile, protein-like DOM. This characteristic was also more pronounced at the forested site than at the bog site. The elevated export of a protein-like fraction, i.e., easily biodegradable DOM (Fellman et al., 2009), can be explained by less biotic demand and a domination of shallow flow paths during snowmelt bypassing large, strongly modified and aromatic DOM pools in the subsurface (Fellman et al., 2009). Also, near-surface freeze–thaw cycles during winter provide fresh DOM from microbial cell lysis and root mortality, which is not utilized due to the low productivity (Haei et al., 2012; Fellman et al., 2009). Differences between the two sites were more evident in other PARAFAC components. At the forested site a large fraction of humic-like fluorescence was attributed to C3 %, while C4 % was absent. C3 % only increased strongly at the forested site during rain events, while C3 % remained constant at the bog site. This suggests that component C3 % is predominantly sourced in the upper, organic layer of the forest soil and is only mobilized during rain events, when these near-surface layers get hydrologically connected, even during snowmelt. As this component increases the most out of all identified components, higher DOC concentrations during rain events should be mainly caused by a connection of near-surface organic-rich layers to the streams, irrespective of the season. In our study an increase of C3 % coincides with lower SUVA254 and higher SR and FI values. This points to a microbially modified, recalcitrant, but less aromatic DOM fraction of smaller apparent molecular size, confirming previous descriptions of a largely similar fluorophore (e.g., Fellman et al., 2008; Singh et al., 2014). Overall, the snowmelt DOM can be described as smaller, less chromophoric, less aromatic and more biodegradable due to a higher protein-like fraction, which is more pronounced at the forested site and has its source in the upper soil layer.

A drought period during summer 2013 caused a strong decrease in the bog water level down to 35 cm depth, and exceptionally low discharge representing pronounced baseflow conditions. This dry period induced concomitant changes in DOM quality in stream discharge. While DOC concentrations at both sites continuously increased, aromaticity and apparent molecular size decreased. While FI at the bog site remained constant (around 1.55), FI at the forested site increased during drought up to 1.75, indicating a greater fraction of microbially derived, strongly modified DOM. Under these drought conditions, the indices congruently illustrate a less aromatic, smaller, and more microbial DOM at the peaty forest soils compared to the bog site and to the rest of the record. This can be explained by a higher contribution of shallow groundwater and decreasing discharge through the peaty surface layers of the riparian zone. Due to the adsorption of larger, more aromatic compounds to mineral phases, groundwater DOM is typically of smaller molecular size and less aromatic (Meier et al., 2004; Inamdar et al., 2012). For the bog site, increasing C1 % and bog-derived humic-like C4 %, as well as decreasing molecular size, also hints at a DOM source change toward deeper peat layers. Summarizing the summer drought fingerprint, DOM during this period approaches characteristics observed for shallow groundwater at the forested site and resembles DOM from deeper peat layers at the bog site. This caused a change in DOM quality to smaller, more microbial and less aromatic components.

The three sampled rain events distributed over spring and fall clearly differed with respect to observed changes in DOM quality. The spring events can be differentiated between dry (first event) and wet (second event) preconditions, while the fall event occurred again under dry preconditions. The main difference between these two preconditions was a significantly higher DOC concentration under dry preconditions at both sites. While this difference in DOM concentration could be attributed to a dilution effect under wet preconditions at the bog site, notable differences in DOM quality also occurred: the non-fluorescent DOM fraction was clearly elevated following dry preconditions. This indicates that under dry preconditions a specific DOM fraction, which cannot be separated by fluorometric indices, is exported compared to wet preconditions. As non-fluorescent DOM is probably more easily degradable DOM like organic acids or products of biotic activity; this might be a flushing effect, when the upper soil layer gets hydrologically connected after prolonged aeration, decomposition and concomitant enrichment of potentially mobile DOM, while under wet preconditions this DOM fraction gets exhausted. As this flushing effect also occurred at the dry fall event, the export of this DOM fraction may be mainly attributed to the hydrologic preconditions and not to the low demand of labile DOM in early spring, as has been suggested elsewhere (Fellman et al., 2009). This would further imply that focusing on the protein-like C5 % as proxy for BDOM (Fellman et al., 2008) neglects a further labile DOM fraction, which might serve as a nutrient source downstream and does not correlate with protein-like fluorescence.

A major seasonal difference between the spring and fall events is that there were no elevated protein-like DOM exports in fall. However, elevated contributions of protein-like fluorescence as described before (Singh et al., 2014) were ascribed to leaf fall at that time of the year. As in our catchment only coniferous trees occurred, this may not be observed in this study. However, the contribution of the non-fluorescent fraction in fall was even higher than in spring. This might be indicative of higher biotic activity, generating small, non-fluorescent molecules, and demonstrates inherent limitations of the spectrofluorometric approach of DOM characterization.

At the forested site, trends of SUVA254, HIX and SR indicated a decreasing aromaticity and apparent molecular size during all rain events. This is confirmed by an increasing FI up to 1.74, meaning a shift to more microbial DOM. The DOM was of even smaller apparent molecular size and less aromatic during the fall event even though trends of the indices were partly reversed. This reversal was due to the extreme dry preconditions and prevailing baseflow with a high microbially derived DOM fraction, primarily originating from groundwater and deep peat. With the onset of a rain event the organic soil layers get hydrologically connected, leading to a decrease of microbial- and shallow-groundwater-derived compounds and an increase in more aromatic DOM originating from the strongly humified organic matter of the peaty layers. Taking together the trend of the humic-like C1 % at the forested site during events and drought periods reveals that this component probably represents a rather microbially processed DOM fraction from groundwater, while humic-like C2 % and C3 % largely contribute to DOM under high discharge conditions and therefore represent a DOM sourced in the upper soil layers. In contrast to other studies (Hood et al., 2006; Inamdar et al., 2011), a general increase of aromatic DOM during rain events at the forested site was not recorded. However, there is an increase in humic and microbial DOM which indicates that the humic-like C3 % is also microbially derived DOM, but from another source. However, under wet preconditions the decrease in SUVA254 was less distinct and C2 %, indicative of a shift to rather aromatic structures, contributed more to fDOM than under dry preconditions. This indicates a stronger aromatic contribution to DOM export from upper soil layer under wet preconditions at the forested site, while under dry preconditions longer aeration may yield more modified, less aromatic DOM that has pooled up during drought.

Under wet preconditions, DOM quantity and quality differed significantly between sites, as expressed by significant differences of all indices except of the PARAFAC component C1 %. At the bog site much less DOC was exported than at the forested site, indicating an exhaustion, as well as a dilution effect of the surficial DOM pool here (Broder and Biester, 2015). Hydraulic conductivities at greater depths, where high DOM concentrations prevail, are presumably too low for rapid mobilization. Compared to changes at the forested site, aromaticity only moderately shifted during rain events and over seasons. However, even though aromaticity was not elevated at the bog site, SR indicated DOM of a rather large molecular size being mobilized under wet preconditions. Also the protein-like component was highest at the wet spring event, which is explained by flushing of fresh biotic material in the surface layer. Moreover, during this event overland flow is very likely (Broder and Biester, 2015), which might further leach larger polymers of proteins, cellulose or polysaccharides from the living biomass. Also, Fellman et al. (2009) reported an increase in protein-like DOM export and related this to lower residence times and low biotic demand in a wetland catchment. Unfortunately, SR was not monitored in that study. The bog-specific humic-like C4 % component only moderately decreased during rain events and increased over summer drought at the bog site. This dynamic in C4 % presumably describes a component from a deeper peat layer, which is constantly exported over the year and gets diluted by upper surface or near-surface export during high discharge events. Therefore, this component may be used as tracer for deep bog porewater, and the observed dilution effect clearly points out that bogs do not primarily drive variations in DOM loads of streams.

Summarizing dynamics during events, DOM quality changes reflect different contributions of DOM pools depending on hydrologic preconditions and season. Fresh and labile DOM was exported during spring events at both sites, especially under wet preconditions at the bog site. Even though aromaticity of DOM in the studied catchment was high, events showed an increase in microbial or strongly microbially altered DOM. However, PARAFAC components show that this assumed microbial component is not only sourced in shallow groundwater, but that there is an additional microbial DOM pool in the upper soil layer that is especially mobilized under dry preconditions. Comparing the two sites, our results demonstrate that not only major dynamics in DOM quantities, but also variability in DOM quality was mainly driven by the forested site, i.e., by shallow peaty soils with stronger variations in water tables and thus hydraulic connectivity of the different layers.