Dissolved organic matter fluctuations in an intermittent headwater streamfrom storm oscillations to decadal hydrological changes = Fluctuacions de la matèria orgànica dissolta en un riu de capçalera intermitent : de crescudes a canvis hidrològics decennals

Résumé

Dissolved organic matter (DOM) is an important source of carbon for aquatic microorganisms and it regulates many biogeochemical processes. Therefore, changes in river DOM concentration and properties could notably affect the functioning of fluvial and coastal ecosystems and alter the global carbon cycle. The DOM in headwater streams is strongly influenced by hydrology, as a consequence of the modification of catchment flow paths with high discharges. During storm events, the catchment hillsides are washed and terrigenous DOM is transported to rivers. In the Mediterranean region, the precipitation regime and evapotranspiration strongly modulate fluvial hydrology, which shows low discharges in summer and even flow disappearance. These dry–wet cycles of conditions affect many ecological and biogeochemical processes. In this thesis, I analyse a long time series of discharge and DOM data from Fuirosos, an intermittent headwater stream in NE Spain. My aim is to examine the relationship between DOM and hydrology at different temporal scales. First, I characterise the hydrological regime of this Mediterranean stream. A decrease in discharge was revealed, although trends in temperature and precipitation were not significant. In contrast, I did not find a clear temporal trend in dry period duration. However, rewetting has been significantly delayed, moving from September to October. The mean magnitude of the storm events that occurred in autumn was lower in the years with El Niño phases in those months. The frequency of storm events decreased over the interval 1998–2015, showing a significant positive relationship with solar activity with a 2-year lag. Dissolved organic carbon (DOC) concentration saw a slight decrease during the study period, which was opposed to that observed in boreal systems. This pattern might respond to a reduction of terrigenous DOC input from forest hillsides as a consequence of the decrease in flushing episodes. The DOC temporal dynamics during the rewetting was regulated by dry period duration. Discharge oscillations explained up to 50% of total DOC variability during the wet period. Notably, this weight of discharge increased significantly over the years. DOM quality was also explored, and described in terms of absorbance and fluorescence properties. Most of the DOM properties were strongly related to discharge, revealing the input of allochthonous, degraded, aromatic, humic and large-molecular DOM under high flow conditions. However, these relationships were altered during drying and rewetting periods. The DOM responses at the individual storm event scale were highly heterogeneous. Multiple linear regression and commonality analyses showed that, in addition to the magnitude of storm episodes, antecedent hydrological conditions, namely pre-event basal flow and the magnitude of the previous storm event, played a significant role in regulating the trends and shapes of DOM–discharge hysteresis. Finally, I identified the differences and similarities in the DOM–discharge relationships between the intermittent Mediterranean stream analysed herein and a perennial Alpine stream with higher mean discharge (Oberer Seebach). The DOM in Fuirosos was significantly more concentrated, more terrigenous, more degraded, more aromatic and more humified. The sign of the global DOM–discharge response was the same in both streams. However, discharge was a more robust predictor of DOM variability in Oberer Seebach. In fact, low flow and rewetting periods in Fuirosos introduced considerable dispersion into the relationship. During snowmelt in Oberer Seebach the sensitivity to discharge also decreased or disappeared. The flushing/dilution patterns were essentially associated with the magnitude of storm events in Fuirosos. In contrast, the DOM quality change was more coupled to basal flow conditions in Oberer Seebach, while the storms were behind the DOC oscillations. This study attests to the importance of generating and analysing long-term and high-frequency biogeochemical series, which allow relationships between DOM and hydrology to be explored in intermittent headwater streams that are subjected to extreme hydrological regimes