Seasonal and event-based dynamics of dissolved organic carbon (DOC) concentration in a Mediterranean headwater catchment

Hydrological factors are known to contribute to regulate the DOC balance at the reach scale. Interannual, intra-annual (seasonal) and event-based hydrological variability, particularly in headwater streams, affects stream-hillslope organic matter exchanges and river network connectivity, leading to significant space and time variations in sources and processes regulating DOC dynamics.

This paper contributes to the ongoing effort to improve understanding of the related dynamics of streamflow and DOC concentration spatial variability across different timescales. Our investigation focused on a Mediterranean headwater catchment (Turbolo River, southern Italy) characterized by dry and hot summer climate enhancing network intermittency. The catchment was equipped with two multi-parameter sondes providing more than two-year (May 2019 to November 2021) continuous high-frequency measurements of several DOC-related parameters (fluorescent dissolved organic matter - fDOM, streamwater temperature and turbidity). The sondes were installed in two nested sections. The upstream sonde was located in a quasi-pristine sub-catchment, while the downstream sonde was placed at the Fitterizzi outlet, where some anthropogenic disturbances on water quality could be observed. Furthermore, streamflow data were acquired at both sites, while weather parameters were monitored at the catchment outlet. DOC estimates were achieved by correcting the fDOM values through an original procedure that did not require extensive laboratory measurements. Then, DOC dynamics at the seasonal and storm event scales were analyzed for both sites.

At the seasonal scale, results confirmed the climate control on DOC production, with background concentrations that increased in hot and dry summer months. The hydrological regulation proved crucial for DOC mobilization and export, with the top 10^th percentile of discharge being associated with up to 79% of the total DOC yield. The analysis at the storm scale examined 19 events per site using flushing and hysteresis indices. Our results highlighted substantial differences between the two catchments. In the steeper upstream catchment, the limited capability of preserving hydraulic connection in time with DOC sources determined the prevalence of transport as the limiting factor to DOC export. Downstream, transport- and source-limited processes were observed almost equally. The correlation between the hysteretic behaviour and antecedent precipitation was not linear since the process turned to be transport-limited for high accumulated rainfall values. Overall, the study demonstrated the importance of high-resolution measurements to explain DOC dynamics at multiple time scales with a quantitative approach.