The role of Primary Uptake Compartments on stream Nitrogen cycling = El paper dels Compartiments Primaris en el ciclatge del Nitrogen als ecosistemes fluvials

[eng] The main goal of this thesis is to investigate how (15)N natural abundances can help understanding the role of different primary uptake compartments (i.e., biofilm, filamentous algae, bryophytes, macrophytes; PUCs) on stream nutrient retention. The experimental design of this thesis involved the examination of patterns of variability in the (15)N signatures of Dissolved Inorganic Nitrogen (DIN) and PUCs at three different spatial scales: the watershed/global scale, the reach scale, and the habitat scale; as well as the (15)N transfer between them. At each level of organization, a common goal of examining how intrinsic (i.e., among PUCs) and extrinsic (i.e., environmental) factors influence (15)N signatures of DIN and PUCs was addressed, but quite different tools and experimental approaches were used depending on each scale. At the global scale, results showed that land use in the catchment is a key driver of the variability in (15)N signatures of both DIN and PUCs. In particular, (15)N signatures of DIN and PUC are higher in streams draining catchments with agriculture and urban activities than in those draining forested catchments; whereas within each stream, major among-compartments differences in (15)N signatures are between photoautotrophic and detrital-based compartments. At the reach scale, results for the different functional groups of macrophytes in streams indicate that direct assimilation of stream water DIN is occurring by submersed and amphibious species; while species of macrophytes located at the stream-riparian edge most likely rely on DIN sources other than those provided by stream water DIN. Moreover, in streams influenced by inputs from wastewater treatment plants (WWTP), DIN uptake by submersed and amphibious macrophytes seems to be mainly in the form of N03• On the other hand, results for the (15)N transfer between photoautotrophic PUCs and stream water DIN showed that variation in DIN uptake of photoautotrophic PUCs is principally driven by nutrient concentration and light incidence, rather than by the particular characteristics of each PUc. On the contrary, variation in autotrophic N turnover is more remarkable among photoautotrophic PUCs than among study reaches, suggesting an intrinsic control by the particular characteristics of each PUC on N turnover, and being relatively independent of environmental influences. Finally, results from tracer (15)N additions at the habitat and microhabitat scales revealed that spatial heterogeneity of microbial N uptake at the microhabitat scale is characterized by a mosaic of patches dominated by microhabitats of low N, with hot spots of highly active N uptake accounting for the 20% of the reach coverage. Particularly, spatial variation of epilithon N uptake at microhabitat scale is principally driven by flow velocity, while spatial variation of N uptake by detrital compartments is controlled bya combination of biophysical factors, indicating the relevance of organic matter characteristics on their role of stream water N uptake. Overall, these results indicate that the extent to which spatial variation in microbial N uptake at fine scales is integrated at the whole-reach scale is potentially affected by factors operating at the ecosystem level, such as the degree of canopy cover, which determines the relative abundance of each PUc. Results from this present thesis highlights that changes in the major land uses within the catchment, changes in the degree of riparian vegetation and/or in the DIN loads of stream ecosystems caused by WWTP inputs, or losses of spatial heterogeneity in the stream channel can have a significant incidence on the way that PUCs contribute to N cycling at the reach scale, which ultimately dictates the N dynamics of stream ecosystems