Relationships between streamwater nitrogen and primary uptake compartments: an isotopic approach

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 (? 15N; in ‰), both natural abundance (Chapter one, two and three) and 15N labelling techniques (Chapter four) First, we examined the spatial variability of ? 15N natural abundance of PUC types, and related this variability to ? 15N values of dissolved inorganic species (DIN, ammonium and nitrate) across streams differing in nutrient availability. We found that the variability of ? 15N-PUC was mostly explained by location within the fluvial network, and was related to ? 15N of DIN species for PUCs living within the stream channel. The prediction power for ? 15N-PUC was improved by stream nutrient concentrations and stoichiometry, indicating the relevance of stream nutrient environment to understand ? 15N values of PUCs. Second, we analyzed the temporal variability of ? 15N 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 ? 15N variability of epilithic biofilms in different stages of development under contrasting stream nutrient concentrations. We observed that ? 15N variability of early-stage biofilm (colonizing artificial substrates) was lower than late-stage biofilm (attached to stream cobbles). Except at the low-nutrient stream, ? 15N of early-stage epilithon was lower than that of late-stage biofilm. Moreover, during biofilm colonization, ? 15N 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 ? 15N 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.