From flood to drought: Transport and reactivity of dissolved organic matter along a Mediterranean river

Rivers play a key role in the global biogeochemical functioning, as they link the biogeochemical cycles of the terrestrial and oceanic systems. In the framework of the carbon cycle, streams and rivers receive dissolved organic matter (DOM) from a variety of sources which is subsequently transported downstream and delivered to the oceans. However, there is increasing evidence that this step involves not only a relocation of DOM from the land to the seas, but also an important in-stream processing which modifies its quality and properties and, to some extent, outgas it to the atmosphere as CO2. However, up to now it has not been assessed how these modifications occur along a longitudinal perspective and, most importantly, the role that hydrology plays in such downstream patterns of DOM processing. This thesis explores the transport and reactivity of DOM along a Mediterranean river (la Tordera) under a variety of hydrological conditions ranging from flash floods to summer droughts. First, the composition of DOM was determined using 3D fluorescence spectroscopy (emission-excitation matrices) and a novel chemometric approach based on a self-organising maps analysis coupled with a correlation analysis. This step allowed discerning the presence of four DOM moieties: a tyrosine-like, a tryptophan-like, and two humic-like fluorescence components. Next, longitudinal patterns of these components are presented for a range of hydrological conditions. Along the main stem, the river exhibited three reaches with a differentiated DOM character: in the headwaters DOM had an eminent humic-like character derived from the drainage of the surrounding terrestrial catchment (high HIX); in the middle reaches (from Sant Celoni to Fogars de la Selva) there was a predominance of the protein-like component C2 reflecting the effect of direct anthropogenic water inputs (concomitant high nutrient concentrations); and the lowest part of the river was dominated by the protein-like component C1 and high FI, suggesting a predominance of microbially-derived DOM. Hydrology appeared to act as a modulator of such longitudinal patterns: during drought, the spatial heterogeneity of DOM character was maximised, whereas during flood conditions there was an homogenisation over the longitudinal dimension, consisting in a highly aromatic humic-like character. By means of an End-Member Mixing Analysis (EMMA) approach it was observed that in-stream reactive processes were likely to be driving DOM quality, especially during non-flood conditions and at the middle and lower reaches of La Tordera. Following such evidences, the relevance of in-stream transformations over transport and physical mixing were explicitly quantified by performing a mass balance approach at thirteen consecutive downstream segments. Results of the mass balance study showed that flood events, despite their brevity in time, had the capacity to export the largest amounts of DOM, although having undergone little in-stream processing. During baseflow conditions, which have been estimated to occur annually about half of the days, there were moderate efficiencies of bulk DOM retention; however, individual fluorescence components had important in-stream generations, especially the protein-like moieties C1 and C2. In space, this processing occurred in the final segments of the river, hence exhibiting a shift from a conservative to a reactive behaviour. Finally, during drought conditions the river had the highest capacity of DOM retention and exported the least amounts of DOM. In space, retention efficiencies were homogeneous along the mainstem, except for two anthropogenically-impacted sites where the retention capacities were reduced. The mass balance study also revealed that bulk DOC processing was subject to a stoichiometric control with nitrate, even though such control was weaker during drought. The findings of this thesis demonstrate that the riverine passage is a decisive step that defines the quantity and quality of the DOM that is finally delivered to the oceans. Moreover, the observed hydrological seasonality in La Tordera shapes a temporally-changing DOM character which may have complex repercussions for its fate once in the coastal systems.