[ca] L'objectiu general d'aquesta tesi és entendre les relacions biogequímiques entre el nitrogen (N) de l'aigua i els principals compartiments primaris de retenció del nitrogen (PUCs: biofilms microbians, algues, briòfits, macròfits i arbres de la ribera) en els ecosistemes fluvials. Per assolir aquest objectiu, s'ha utilitzat tècniques isotòpiques, mitjançant l'estudi de l'abundància natural dels isòtops del nitrogen i tècniques de marcatge isotòpic. S'ha trobat una gran variabilitat en l'abundància natural d'isòtops (δ15N) dels PUCs en rius que diferint en concentracions de nutrients, explicat pels valors de δ15N en el nitrogen dissolt inorgànic en el riu (NID), com també pel conjunt de nutrients en el riu. La variabilitat temporal de δ15N en els PUCs i el NID, va ser més gran en rius amb més concentració de nutrients, majors afectats per activitats antròpiques. Entre diferents tipus de PUCs les diferències van ser menors que entre rius. L'abundància natural de δ15N en el biofilm microbià dels còdols fluvials va presentar patrons diferents segons el seu estat successional i les concentracions de nutrients. A més, la interacció entre el compartiment biofilm-fullaraca i l'aigua del riu es va estudiar durant el procés de descomposició de fulles de dues espècies de Populus marcades isotòpicament (13C i 15N), amb diferents concentracions de compostos recalcitrants. Canvis en δ15N en el biofilm ens van indicar canvis en la dinàmica de nitrogen de captació, reciclatge i dissimilació durant el creixement en còdols o en la descomposició de diferent tipus de fullaraca. En general, aquesta tesi proporciona informació sobre què controla les relacions biogeoquímiques de 15N, el qual té fortes implicacions per entre la dinàmica de N en els ecosistemes aquàtics i una correcta aplicació dels isòtops estables del N.

[en] The overarching goal of this dissertation was to explore relationships between streamwater nitrogen (N) and the most representative primary uptake compartments (PUCs) in stream ecosystems (e.g. microbial biofilm, algae, bryophytes, macrophytes). In particular, environmental factors driving these biogeochemical relationships along a strong anthropogenic gradient were explored and differences among and within PUC types were compared. To elucidate the factors controlling these relationships, we used N stable isotopes (δ15N; in ‰), both natural abundance (Chapter one, two and three) and 15N labelling techniques (Chapter four) First, we examined the spatial variability of δ15N natural abundance of PUC types, and related this variability to δ15N values of dissolved inorganic species (DIN, ammonium and nitrate) across streams differing in nutrient availability. We found that the variability of δ15N-PUC was mostly explained by location within the fluvial network, and was related to δ15N of DIN species for PUCs living within the stream channel. The prediction power for δ15N-PUC was improved by stream nutrient concentrations and stoichiometry, indicating the relevance of stream nutrient environment to understand δ15N values of PUCs. Second, we analyzed the temporal variability of δ15N natural abundance in PUC types and DIN species in four streams with different nutrient concentrations. Our results did not show isotopic temporal patterns over a year. However, among streams, the highest variability was found in the urban stream and, among PUC types, temporal variability tended to be higher in PUCs submerged in streamwater with faster turnover rates, such as filamentous algae. Third, we studied the δ15N variability of epilithic biofilms in different stages of development under contrasting stream nutrient concentrations. We observed that δ15N variability of early-stage biofilm (colonizing artificial substrates) was lower than late-stage biofilm (attached to stream cobbles). Except at the low-nutrient stream, δ15N of early-stage epilithon was lower than that of late-stage biofilm. Moreover, during biofilm colonization, δ15N increased with biomass accrual. Changes between successional stages were more pronounced at the high-nutrient stream. These results suggested successional stage as a relevant factor controlling δ15N variability of epilithic biofilm at the local scale. Fourth, N and C biogeochemical interaction between the biofilm-litter compartment and streamwater during litter decomposition was evaluated by using double-labeled (15N and 13C) leaves of two Populus species (P. fremontii and P. angustifolia). These species differed in their concentration of recalcitrant compounds (i.e. tannins) and were expected to influence the microbial decomposer community dependency to streamwater. Litter type strongly affected biomass and stoichiometry of microbial assemblages growing on litter, but the proportion of N and C derived from streamwater was not different. Gross immobilization of N from the streamwater was higher for the low-tannin litter, probably as a consequence of higher microbial biomass, contrasting to C fluxes which were higher for the high-tannin litter, suggesting C limitation. Overall, this dissertation provides insights into what controls 15N biogeochemical relationships between PUC types and water in fluvial ecosystems. This has implications for the use of N stable isotopes in ecological and environmental studies in aquatic ecosystems, and can help to develop successful management strategies to mitigate N excess in fluvial systems.