Intra‐annual variability of organic carbon concentrations in running waters: Drivers along a climatic gradient

Abstract

Trends in surface water dissolved organic carbon (DOC) concentrations have received considerable scientific interest during recent decades. However, intra-annual DOC variability is often orders of magnitude larger than long-term trends. Unraveling the controls on intra-annual DOC dynamics holds the key to a better understanding of long-term changes and their ecological significance. We quantified and characterized intra-annual DOC variability and compared it with long-term DOC trends in 136 streams and rivers, varying in size and geographical characteristics, across a 1400 km latitudinal gradient during 2000-2010. Discharge, temperature, and month of the year were the most significant predictors of intra-annual DOC variability in a majority of the running waters. Relationships between DOC, discharge, and temperature were, however, different along a mean annual temperature (MAT) gradient. Running waters with low MAT generally displayed positive DOC-discharge correlations whereas the relationships in sites with higher MAT were more variable. This reflected contrasting relationships between temperature and discharge with discharge positively correlated with temperature in cold areas, while it was negatively correlated with temperature in catchments with higher MAT. Sites where flow, temperature, and month were poorly related to intra-annual DOC dynamics were large catchments or sites with extensive upstream lake cover. DOC trends were generally much smaller than intra-annual DOC variability and did not show any north-south gradient. Our findings suggest that DOC in running waters could respond to a changing climate in ways not predictable, or even discernible, from extrapolation of recent interannual trends.

Catchment tracers reveal discharge, recharge and sources of groundwater-borne pollutants in a novel lake modelling approach

“…In these areas, it is therefore important to have temporal sampling following decreases in concentrations and to relate them to lake concentrations during sampling. Lake inter-annual DOC and CDOM changes were 310 generally low in our study with an annual CDOM absorbance at A CDOM (340) = 0.41 cm -1 ± SD 0.05, corresponding to what is observed in larger water bodies where WRT integrates inflowing DOC and CDOM (Winterdahl et al, 2014). Inter-annual DOC and CDOM variations in groundwater from the lake catchment (Fig.…”

“…S1) showed the same tendency as described for nutrients suggesting that sampling should be done at multiple times or in a period without drought or high rainfall. On a broader scale, the variation in DOC is known to be related to hydrology (Erlandsson et al, 2008), mean air temperature 315 (Winterdahl et al, 2014) and the recovery from acid deposition (Evans et al, 2006;Monteith et al, 2007). Sampling from wet areas with standing surface water resulted in high concentrations of most tracers (Table S1).…”

Temporal control on concentration, character, and export of dissolved organic carbon in two hemiboreal headwater streams draining contrasting catchments

“…A positive discharge‐DOC concentration relationship has commonly been observed for various catchments across boreal and temperate regions [ Hinton et al ., ; Raymond and Saiers , ; Laudon et al ., ]. But a strong temperature control on stream DOC concentration has previously only been suggested for catchments with a homogenous soil DOC profile typically found in peatland dominated areas [ Winterdahl et al ., ]. This study showed, however, that a strong temperature control could also be found for a hemiboreal catchment draining mineral soils.…”

Sensitivity of freshwaters to browning in response to future climate change

“…On the spatial scale, DOC has been found to be mainly influenced by land-cover, in particular the percentage of wetland and open water in the catchment and the type of vegetation (e.g., Canham et al 2004;Kortelainen 1993;Larsen et al 2011;Sobek et al 2007;Xenopoulos et al 2003). On the temporal scale, DOC variation has been attributed to variation in hydrology (Erlandsson et al 2008), air temperature (Winterdahl et al 2014) and the recovery from acid deposition (Evans et al 2006;Monteith et al 2007). From the numerous DOC studies it becomes clear that main drivers of DOC, and thereby most likely also of a 420 , are not necessarily coherent between the spatial and temporal scale.…”