Our long-term records from monitoring stream biogeochemistry over the past 17 years show changes to both the quantity and the quality of dissolved organic carbon (DOC) in streams in boreal landscapes. Here in northern Sweden, brown streams are typified by high concentrations of terrestrially-derived dissolved organic carbon (DOC) which are important for geochemical, biogeochemical, and ecological functions. For instance, DOC supply from soils influences the transport and bioavailability of heavy metals and anthropogenic organic compounds, represents the main energy source for aquatic food webs and promotes the production of harmful byproducts of chlorine disinfection during drinking water sanitization. The variation in DOC ‘quality’, as represented by shifts in the composition of organic compounds and their degree of biological reactivity, can regulate aquatic ecosystem processes, including rates of microbial metabolism and nutrient transformations.
The sources and controls of DOC in landscapes are driven by the combination of soil biogeochemical processes and the strength and timing of hydrological connections across terrestrial source areas groundwater systems, and stream channels. However, less is known about how the amount, timing, and chemical character of DOC are altered by seasonal drought episodes, which reduce lateral connectivity, but also set the stage for biogeochemical and microbial processes in dry and disconnected soils. We found that summer low flow hydrology varied considerably during this period, 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 below summer discharge of 0.1 mm day -1. This inter-annual variability in hydrology had clear effects on summer DOC concentrations, which declined as drought severity increased in all sites resulting in the lowest DOC fluxes (as much as 100%) coinciding with the driest periods.
During these dry periods, we found declining DOC concentrations in all sites resulting in the lowest DOC fluxes (as much as 100%) coinciding with the driest periods. Similarly, the quality changes in DOC showed systematic declines in LMW DOC and C/N ratio but increases in SUVA254 across the catchments as drought severity increased. These changes reflect the lowering of water table to deeper soil strata characterized by more highly processed organics and the transition to more aromatic DOC in surface waters. We also found that peat dominated landscapes showed 50% higher SUVA254 during low flow periods, whereas these changes were marginal in the forested dominated sites. The loss of DOC quantity and changes to the quality of carbon in streams during these low flow periods can influence aquatic communities and ecosystem processes during a biologically important time of the year.
Upon rewetting, we saw elevated DOC flux and concentrations in all streams and groundwater. Similar effects on the quality of DOC was also observed where LMW DOC and C/N ratio increased while SUVA 254 decreased. The increases in DOC quantity and changes to quality are due to flushing of carbon decomposed and/or accumulated in upper soil horizons during the previously dry periods when soils were disconnected from streams. Consequently, elevated energy mobilization through increased autumn pulses of high-quality DOC is likely to promote aquatic ecosystem respiration; indeed, and may create acute periods of anoxia in lakes that cause fish mortality. DOC pulses may also promote inputs of pollutants to aquatic systems, including a variety of toxic metals that form complexes with organic matter and can cause problems for drinking water treatment. Together, when combined with longer-term browning trends, strong seasonal redistributions of DOC inputs in response to drought may contribute to overall poorer water quality and higher water treatment costs.
The variable stream DOC responses during drought and post-drought across the river network, reflected inherent differences in the sensitivity of the sub-catchments to extremely low flows. These variable responses was best captured by the difference in catchment size rather than landscape characteristics. Larger catchments showed both the greatest decline in DOC concentration during drought and the largest increases in DOC responses upon post-drought rewetting. Stronger responses to drought in the larger catchments likely relate to their greater distance to near-surface organic DOC sources that feed headwaters. Isolation from these sources is exacerbated by the increasing influence of deeper and DOC-poor groundwater as catchment size increases. As a result, even small losses in lateral and longitudinal connectivity to the more DOC-rich headwaters during drought may cause the chemistry of larger rivers to shift abruptly towards the character of deeper groundwater sources. In this sense, although larger river systems are less prone to complete water loss than headwaters during drought, they nonetheless may show stronger biogeochemical responses to drought events and recovery.
The perspectives gained from long-term catchment monitoring help us understand how climate extremes may alter the mobilization of soil organic carbon across spatial and temporal scales in boreal catchments. Observed seasonal variation in amplitude of DOC, LMW DOC, C/N ratio, and SUVA254 suggests that, while drought effects on stream biogeochemistry are direct and immediate, important lagged effects extend beyond the duration of the direct disturbance and are observable across the aquatic network. Overall, increases in the intensity of 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. Such hydrological changes will alter the fundamental properties of boreal aquatic systems as reflected in their seasonal regime of terrestrial DOC supply and could potentially be a more important driver of carbon mobilization and water chemistry change in this region.