Resumen de Influence of hydrology on dissolvedorganic matter quantity and properties in a temporal Mediterranean stream = Influència de la hidrologia sobre la quantitat i disponibilitat de la matèria orgànica dissolta en una riera temporal mediterrània
The studies presented in this thesis focus on the biogeochemistry of dissolved organic matter (DOM) in an intermittent stream in a Mediterranean climate zone, with great emphasis on extreme hydrologic events: floods and the seasonal drought period. The changes in concentration and molecular weight, and the properties, especially those related to its origin, and bioavailability to the microbial community. The samplings were carried out during a wide span of years in order to characterize the concentration – discharge responses (C-Q) initially, and more intensively during the different periods of the hydrologic year (wet, drought and transitional periods) in order to capture the widest possible picture of the dissolved organic matter dynamics and processing. The last chapter is a review of different fluorescence spectroscopy and UV-vis absorbance methods applied in DOM characterization. The main conclusions from these works are: 1. The most probable C-Q response types represent only a 40% of all cases from the data set. Therefore, the majority of DOC and NO3-Q responses fall within some low probability C-Q response types. 2. The dry-wet transition does not present a typical C-Q response. Instead, it promotes the diversity of DOC-Q responses. The 33% of events comprised during this period contribute to the 41% of the total DOC-Q response diversity. On the other hand, it does not enhance NO3-Q responses. 3. The most important driver for the succession of NO3-Q responses is the periodicity of the magnitude of storms (?Q) is, while the effect of the seasonal temperature change (i.e., Tday), appears more perceptible after the removal of the signal of the weaker and more frequent rain episodes. 4. The occurrence of a severe summer dry period favored the mobilization and transport of labile DOC, as shown by the 56% of retention across the stream – riparian interface, during the following hydrological transition period. Furthermore, the DOC retention during this period was preferential from high to low molecular weight fractions. In the case of no apparent hydrological transition all DOC molecular weight fractions were equally conservative. 5. During both drought and rewetting periods DOC and DON bioavailability are not tied to a MW fraction. During the rewetting period, although there is no clear pattern of bioavailability according to any DOC MW fraction, 50% of cases at which DOCMW release is observed (BDOCMW<0) correspond to the LMW fraction. 6. In general, DOCMW dynamics fit within the framework that storm events enhance the input of larger molecular weight DOC fraction (higher than 1 kDa), but DONMW does not. Furthermore, the hydrologic characteristics of the rewetting period evidence an asynchronous mobilization of DOC and DON. 7. DOC bioavailability is heavily influenced by DON content rather than molecular weight. Furthermore, bioavailability seems to be regulated by the DOC:DON ratio presenting a threshold value of 13. The most remarkable case is that of DOCLMW that shifts from refractory to bioavailable when its DOC:DON ratio is lower than 13. 8. Drought causes a gradual hydrological fragmentation of the fluvial network enlarging the variability of DOM properties, and amplifying the biogeochemical variability of a fluvial system. 9. Sampling sites with lotic water bodies in summer show DOM properties similar to those observed in autumn under base flow hydrological conditions and reflect the prevalence of terrestrial inputs. In contrast, in isolated and lentic water bodies, DOM analyses reveal a supplementary contribution of autochthonous organic matter, originated by in situ microbial processes. 10. BDOC estimated during summer indicates an upper threshold of 40% that is also observed during the rewetting period. This allows asserting that the highest bioavailable DOC concentration can be reached during two ecosystem perturbations of opposite sign.
