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

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Universitat de Barcelona. Departament de Biologia Evolutiva, Ecologia i Ciències Ambientals

# metadata.dc.description.abstract[].value
[eng]  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 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.

# metadata.dc.description.abstract[].value
[cat]   La matèria orgànica dissolta (MOD) és una important font de carboni per als microorganismes aquàtics i regula molts processos biogeoquímics. La MOD en els rius de capçalera està modelada principalment per la hidrologia. A la regió mediterrània, el règim de precipitació i evapotranspiració impacta fortament la hidrologia fluvial, que mostra cabals baixos a l’estiu i pot arribar fins i tot a la seva desaparició.
En aquesta tesi hem analitzat un sèrie temporal a llarg termini de cabal i MOD a Fuirosos, un riu de capçalera intermitent al NE de la Península Ibèrica. El nostre objectiu era examinar la relació entre la MOD i la hidrologia en diferents escales temporals. Primer, vam caracteritzar el règim hidrològic d’aquest riu mediterrani. Al llarg de l’estudi es va revelar una disminució del cabal, no vam trobar una tendència temporal clara en la durada de la sequera i la freqüència de les crescudes va disminuir en l’interval 1998–2015.
La concentració de carboni orgànic dissolt (COD) ha patit una lleugera disminució durant el període d’estudi, cosa que contrasta amb el que s’ha observat en sistemes boreals. La dinàmica temporal del COD durant el període de transició va ser regulada per la durada del període sec. La qualitat de la MOD també va ser explorada, i descrita en termes de propietats d’absorbència i fluorescència. La majoria de les propietats de la MOD van ser clarament relacionades amb el cabal. No obstant, aquestes relacions van ser alterades durant els períodes de fragmentació i de transició. A més de la magnitud dels episodis de crescuda, les condicions hidrològiques prèvies juguen un paper significatiu perquè regulen les tendències i formes de les histèresis MOD–cabal.
Per acabar, vaig identificar les diferències i semblances en les relacions MOD–cabal entre el riu mediterrani intermitent analitzat i un riu perenne alpí amb un cabal mitjà superior (Oberer Seebach). Aquest estudi testifica la importància de generar i analitzar sèries biogeoquímiques de llarga durada i alta freqüència, que permeten explorar les relacions entre la MOD i la hidrologia en rius de capçalera intermitents que estan subjectes a règims hidrològics extrems.

# metadata.dc.source[].value
Tesis Doctorals - Departament - Biologia Evolutiva, Ecologia i Ciències Ambientals

# metadata.dc.title[].value
Dissolved organic matter fluctuations in an intermittent headwater stream: from storm oscillations to decadal hydrological changes = Fluctuacions de la matèria orgànica dissolta en un riu de capçalera intermitent : de crescudes a canvis hidrològics decennals