Mechanisms underlying drought responses
Over the 17 year record, summer low flow hydrology varied considerably with mean daily minimum discharge ranging several orders of magnitude between the driest and wettest summers (0.0003 to 0.13 mm day-1). The most pronounced summer low flows occurred in 2006 and 2018 with 62 and 41 days where daily discharge was below the 0.1 mm day -1 threshold (Figure 1). This inter-annual variability in hydrology had clear consequences for DOC concentrations, which declined at each site as the severity of summer drought conditions increased. For all catchments, summers with more than 40 days of low flow conditions had the largest percentage decrease in DOC concentrations from the 17-year average (20-55% in small to larger catchments, p<0.05), respectively; Figure 2a). These concentration declines as drought severity increased are consistent with reduced hydrological connectivity to more surficial, organic-rich riparian soils as the groundwater table drops and intersect deeper horizons that store less organic matter 19,20. As further support for these mechanisms, DOC concentrations measured in near stream wells also declined as the drought severity increased (Figure 5). Here, DOC concentrations in riparian wells were reduced by 75% compared to pre-drought values (r2=0.72, p<0.05), whereas concentrations in nearby mire wells showed only minor changes (r2=0.11, p>0.1). By comparison, the wettest years in the record were associated with elevated DOC concentrations in streams, increasing from 10-20% relative to the long-term mean. Collectively, these inter-annual differences highlight the overwhelming role of hydrological connectivity between streams and near-surface soils (Supplementary Figure S1) that supply DOC to aquatic systems 21. Importantly, while warmer temperatures have the potential to increase DOC supply from riparian soils 22 and peat 14,23, our results suggest that potential changes in hydrology, including greater drought frequency, could fundamentally shift the seasonality of DOC in boreal aquatic ecosystems (Figure 3).
Drought episodes also directly influenced DOC's character, yet these effects were more variable and in some cases subtle. For example, the CN ratio declined at all sites as drought severity increased (Figure 4c, Table 1, r2>0.17, p<0.05 for 12 out of 13 sites). We attribute the decrease in this ratio to a shift in the contribution to stream DOM from organic-rich, near-surface soils to deeper strata, where soils are more biologically processed and tend to have lower CN ratios 24,25. At greater soil depths, there are also higher levels of reduced inorganic nitrogen (ammonium) which can contribute to the lower CN ratio as calculated here 26,27. By contrast, the LMW DOC, changed only marginally (p<0.05 for nine out of thirteen sites, Table 1), with bidirectional responses to drought across catchments (Figure 4a), suggesting that the direct effects of drying are less systematic for the character of DOC as represented by this index. Regardless, the decreasing trends for both indexes were mirrored by observations in the riparian and mire wells, with the largest declines observed in years with the longest summer droughts (Figure 5 c, e, respectively, riparian wells r2=0.82, mire wells r2=0.81, p<0.1). Lower groundwater LMW DOC and CN ratio were synchronous with lower quantities of DOC observed in the streams during drought periods compared to pre-drought conditions. Thus, droughts limit the mobility of organic carbon across landscape types, reduce stream DOC concentrations, and alter key characteristics of DOC across the stream network. Such changes to stream chemistry could have cascading consequences for downstream aquatic ecosystems in the future, should extreme drought events increase in frequency and severity.
Post-drought recovery of stream biogeochemistry to summer droughts
Periods of low flow conditions, which cause longitudinal fragmentation and lateral flow disconnections can also lead to increased production of DOC in upper soil horizons that may be mobilized when dry periods are terminated 28. Consistent with this, the rewetting periods following summer drought were associated with high DOC concentrations in streams across the Krycklan network, with the largest increases observed during years where rewetting followed the most severe dry periods. Indeed, the driest summers resulted in 100-150% increases in DOC concentration after rewetting across all catchments (Figure 2b, p<0.05, Table 1). As above, these episodes of DOC flushing in streams were also mirrored in observations from riparian (50% increase, r2=0.31, p<0.05) and mire wells (150% increase r2=0.79, p<0.05) (Supplementary Figure S1 b). In addition to the effects on concentration, DOC properties were also influenced by the severity of the preceding drought during rewetting periods. Specifically, the LMW DOC increased linearly in 12 of the 13 catchments (p<0.05), while the CN ratio increased in 11 out of 13 sites, from pre-drought conditions in the study streams (Figure 5b, d, Table 1). Patterns in groundwater LMW DOC and CN ratio during these rewetting periods were also similar to stream observations where both showed increasing trends in riparian and mire wells (p<0.1) (Figure 5 d, f). Thus, the changes in both DOC quantity and quality parameters suggest that seasonal low flow periods mobilize accumulated solutes during the following rewetting period to an extent that is proportional to the severity of the drought across all landscapes.
Elevated DOC concentrations following the drought periods are in line with the reconnection of near-surface organic-rich soil horizons. Yet, the increases in the DOC concentration, as well as changes in DOM properties, as summer droughts became more severe suggests that biogeochemical processes strongly influenced the soil DOC pools during dry periods. Several mechanisms have been linked to small-scale increases in soil organic matter mineralization and DOC production in response to drying/rewetting cycles. For instance, droughts have been found to decrease phenolic microbial inhibiter compounds in wetlands resulting in increased organic matter decomposition and an increase in carbon loss in peats 29. Additionally, droughts increase the temperature and degree of aeration of soils that are normally inundated, upregulating organic matter decomposition 30,31. Finally, rewetting these exposed soils also trigger the physical and microbial processes that promote organic matter mineralization [Borken and Matzner, 2008]. While we cannot resolve amongst these mechanisms, the observed patterns suggest that processes in seasonally-exposed organic soils support a large pulse of DOM upon rewetting, including a greater fraction of LWM forms that are more bioavailable for aquatic organisms 32. In fact, based on prior studies in the Krycklan catchment 32, the changes in LMW DOC we observe in response to rewetting (ca.0.2- 0.5 units) in both surface and groundwaters could correspond to as much as a 50% increase in microbial growth efficiency in streams. In addition to these implications for energy mobilization by aquatic microbes, increased autumn pulses of DOC may also have implications for the fate and transport of a variety of hazardous pollutants such as mercury 8, and, when combined with longer-term browning trends, contribute to poorer water quality and higher water treatment cost 33.
Network Scale responses
Responses to drought at different time intervals were notably variable across the river network, reflecting differences in the sensitivity to low flow disturbance. Yet, the importance of the catchment size as a mediator of these responses differed depending on the time frame considered. For example, larger catchments showed both the greatest decline in DOC concentration in response to drought (Figure 6a) and the largest increases in DOC upon rewetting during the post-drought period (Figure 6b). Strong drought responses in the larger catchment likely relate to their greater distance to near-surface organic DOC sources that feed headwaters (Figure 6, Table 1). Isolation from these sources is exacerbated by the increasing influence of deeper and DOC-poor groundwater as catchment size increases 12. As a result, even small losses in connectivity to more DOC-rich headwaters during drought may cause the chemistry of larger rivers to shift abruptly towards the character of deeper groundwater sources. Upon rewetting, these larger streams and rivers have such low DOC concentrations that the sudden reconnection to upstream headwaters creates a strong chemical response (Figure 6b, p<0.05 for 10 of the 13 catchments, Table 1). By comparison, headwaters are seldom supported by these deeper groundwater sources 34, and hence their responses to drying and rewetting events are more attenuated. In this sense, larger river systems may be less prone to complete water loss during drought than headwaters, but nonetheless, show stronger biogeochemical responses to such events. Thus, while larger rivers may sustain aquatic communities during low flow periods, dramatic increases in DOC after rewetting may lead to a host of ecological and societal challenges 33 including greater stress for organisms in cases of rapid pH declines 6 and ultimately higher cost for the production of potable water 35.
While catchment size clearly plays an overarching role in regulating stream chemistry following prolonged dry periods, it is not possible to exclude the influence of land cover effects. For instance, several small catchments are dominated by peat-forming mires and display the lowest response to droughts (Figure 6b). Contrastingly, the stronger response by larger, forest-dominated catchments could also be an indication of the degree to which they dry during more severe droughts, as reduced mire cover is linked to lower water storage capacity, and greater evaporative losses, and thus potentially weakened ecohydrological resilience to drought 36. Conversely, catchments with greater peat coverage, showed the weakest post-drought response, suggesting these landscape elements confer resilience to such events, likely by acting as important water storage zones 37 with the potential to dampen effects of long-term environmental change 38.
Conceptualizing drought impacts on stream chemistry
Integrating long-term monitoring data of biogeochemistry and hydrology with modeling techniques provide a more comprehensive understanding of how climate extremes feedback on the mobilization and biogeochemical cycling of soil organic carbon across temporal and spatial scales in boreal catchments. Observed seasonal variation in amplitude of DOC, LMW DOC, and CN ratio demonstrate differential responses in catchment biogeochemical processes to droughts, such that stream water quality is not only affected by reduced soil water supply, but also by declines in the leaching and availability of organic resources important for aquatic microbial processes. Further, the combined responses of the three variables suggest that, while drought effects on stream water chemistry are direct and immediately result in lower than average DOC responses (Figure 7a), the indirect, lagged effects are magnified and extend beyond the duration of the disturbance itself (Figure 7b). Overall, increases in the intensity of seasonal drying/rewetting cycles have the potential to shift the seasonality of DOC in boreal streams by reducing summer peaks in concentration while causing anomalously high concentrations during periods of hydrological reconnection later in the autumn (Figure 7b). Therefore, how recipient aquatic ecosystems cope with such changes during and following droughts, remains a key question.
Almost two decades of monitoring data show that inter-annual variation in summer low flows shape the seasonality in DOC quantity and quality in boreal streams more than is currently appreciated. While much emphasis is currently placed on the direct effects of climate warming at high latitudes39, our study indicates that potential hydrological changes will likely be another important driver of carbon mobilization and water chemistry change. In light of the current climate projections of an increase in drought frequency in Scandinavia40, our results suggest the immediate declines in the quality and quantity of organic carbon in streams during summer followed by lagged increases during rewetting. Despite the variations in landscape properties, all catchments showed similar responses to droughts, however, the magnitude of these responses was more pronounced in the largest catchments. Changes to both the quantity and quality of carbon across the stream network can potentially have vital implications for the aquatic ecosystems that rely on the seasonal balance of DOC production and mobilization from catchment soils 41. Overall, these results highlight the importance of integrating responses at multiple temporal and spatial scales and present a step forward in establishing a unifying theory of drought impacts in boreal biogeochemistry.