International audienceWe evaluated nitrogen (N) removal efficiency by ri-parian buffers at 14 sites scattered throughout seven European countries subject to a wide range of climatic conditions. The sites also had a wide range of nitrate inputs, soil characteristics, and vegetation types. Dissolved forms of N in groundwater and associated hydrological parameters were measured at all sites; these data were used to calculate nitrate removal by the riparian buffers. Nitrate removal rates (expressed as the difference between the input and output nitrate concentration in relation to the width of the riparian zone) were mainly positive, ranging from 5% m 1 to 30% m 1 , except for a few sites where the values were close to zero. Average N removal rates were similar for herbaceous (4.43% m 1) and forested (4.21% m 1) sites. Nitrogen removal efficiency was not affected by climatic variation between sites, and no significant seasonal pattern was detected. When nitrate inputs were low, a very large range of nitrate removal efficiencies was found both in the forested and in the nonforested sites. However, sites receiving nitrate inputs above 5 mg N L 1 showed an exponential negative decay of nitrate removal efficiency (nitrate removal efficiency 33.6 e 0.11 NO 3 input , r 2 0.33, P 0.001). Hydraulic gradient was also negatively related to nitrate removal (r 0.27, P 0.05) at these sites. On the basis of this intersite comparison, we conclude that the removal of nitrate by biological mechanisms (for example, denitrification, plant uptake) in the riparian areas is related more closely to nitrate load and hydraulic gradient than to climatic parameters

[1] Storm events have major implications for biogeochemical cycles at local and regional scales and they provide an excellent opportunity to study the hydro-biogeochemical functioning of catchments. However, concentration-discharge (C-Q) responses have only been studied in detail for short periods or a few selected events. In consequence, it is difficult to quantify the diversity of C-Q responses in a hydrological system and impossible to assess whether the succession of forms of C-Q responses follows a predictable sequence or not. Bearing in mind these shortfalls, the variability of dissolved organic carbon (DOC) and nitrate (NO 3 ) pulses during storms is analyzed in a detailed 4-year series from an intermittent Mediterranean stream. In this study, each DOC and NO 3 -Q response is synthesized by two descriptors that summarize its trend (DC; dilution/ flushing/no change) and shape (DR; linear/nonlinear response). We observe that C-Q responses are widely distributed along the two-dimensional DR versus DC continuum. Furthermore, the temporal succession of forms of DOC and NO 3 -Q responses follow a random pattern, and only the dynamics of the DR (NO3) descriptor show periodicity. The long-term data set reveals that it is impossible to predict with reasonable precision the full properties of DOC and NO 3 -Q responses. Thus, a ''typical'' C-Q response does not really exist at our study site, and this apparent diversity of responses has to be handled with a probabilistic approach that allows synthesis of the complexity of the hydrobiogeochemical functioning of a specific catchment.Citation: Butturini, A., M. Alvarez, S. Bernal, E. Vazquez, and F. Sabater (2008), Diversity and temporal sequences of forms of DOC and NO 3 -discharge responses in an intermittent stream: Predictable or random succession?,

No abstract

Soil saturation is known to be of crucial importance to denitrification and other nitrogen cycling processes within the riparian zone. Since denitrification potential generally increases towards the soil surface, water table elevation can control the degree to which nitrate reduction is optimised. Given their topographic location and sedimentary structure, most floodplains are characterised by high water tables. However, detailed field data on water table levels, hydraulic gradients and flow patterns within the riparian zone are generally lacking. This paper presents data collected as part of a pan-European study of nitrate buffer zones, the Nitrogen Control by Landscape Structures in Agricultural Environments project (NICOLAS). An identical experimental design was employed at each site, allowing riparian zone hydrology and nitrogen cycling processes to be explored across a wide range of temperate climates; only the hydrological data are discussed here. A grid of dipwells at 10-metre spacing was installed at each site and manual measurements made at least once a month for a minimum of one year. In addition, at least one dipwell in each grid was monitored continuously using a data logger. All the riparian zones studied displayed a clear annual cycle of water table elevation, although other factors seemed equally important in influencing the range of variation. Where the riparian zone was flat, the water level in the adjoining river or lake proved more significant in controlling water table levels within the riparian zone than was originally anticipated. q

[1] Stream aquifer hydrology and nitrate removal were studied, over a period of 2 years, in an unsaturated riparian zone, bounded by an intermittent Mediterranean stream, (Fuirosos, northeastern Spain). The riparian groundwater system is characterized by drastic hydrological changes and by mixing of stream water with hillslope groundwater. The hillslope groundwater flowed through a medium with low hydraulic conductivity (9.6 10 À3 < k s < 0.1 m d À1 ) and low specific discharges (1.7 10 À3 < q hll < 15 10). In contrast, stream water infiltrated through the near stream porous medium with relatively high hydraulic conductivity (4.8 < k s < 19 m d À1 ) and variable specific discharges (i.e., 0.03 < q st < 1.5 m d À1 ). An intense and short stream discharge period occurred in autumn, when stream water infiltrated a maximum of 10 m into the riparian zone. Nitrate concentration and nitrate removal spatial rates (h NO3 ) showed wide spatial heterogeneity. Higher nitrate concentrations (3.4 NO 3 -N mg L À1 ) and effective nitrate removal (h NO3 = 0.098 ± 0.04 m À1 ) were found in the deep groundwater of hillslope zone associated to low water fluxes. In contrast, in the stream edge zone (with higher water fluxes), nitrate release predominated over depletion (h NO3 = À0.13 ± 0.04 m À1

Abstract. We examined the effects of riparian vegetation removal on algal dynamics and stream nutrient retention efficiency by comparing NH 4 -N and PO 4 -P uptake lengths from a logged and an unlogged reach in Riera Major, a forested Mediterranean stream in northeastern Spain. From June to September 1995, we executed 6 short-term additions of N (as NH 4 Cl) and P (as Na 2 HPO 4 ) in a 200-m section to measure nutrient uptake lengths. The study site included 2 clearly differentiated reaches in terms of canopy cover by riparian trees: the first 100 m were completely logged (i.e., the logged reach) and the remaining 100 m were left intact (i.e., the shaded reach). Trees were removed from the banks of the logged reach in the winter previous to our sampling. In the shaded reach, riparian vegetation was dominated by alders (Alnus glutinosa). The study was conducted during summer and fall months when differences in light availability between the 2 reaches were greatest because of forest canopy conditions. Algal biomass and % of stream surface covered by algae were higher in the logged than in the shaded reach, indicating that logging had a stimulatory effect on algae in the stream. Overall, nutrient retention efficiency was higher (i.e., shorter uptake lengths) in the logged than in the shaded reach, especially for PO 4 -P. Despite a greater increase in PO 4 -P retention efficiency relative to that of NH 4 -N following logging, retention efficiency for NH 4 -N was higher than for PO 4 -P in both study reaches. The PO 4 -P mass-transfer coefficient was correlated with primary production in both study reaches, indicating that algal activity plays an important role in controlling PO 4 -P dynamics in this stream. In contrast, the NH 4 -N mass-transfer coefficient showed a positive relationship only with % of algal coverage in the logged reach, and was not correlated with any algal-related parameter in the shaded reach. The lack of correlation with algal production suggests that mechanisms other than algal activity (i.e., microbial heterotrophic processes or abiotic mechanisms) may also influence NH 4 -N retention in this stream. Overall, this study shows that logging disturbances in small shaded streams may alter in-stream ecological features that lead to changes in stream nutrient retention efficiency. Moreover, it emphasizes that alteration of the tight linkage between the stream channel and the adjacent riparian zone may directly and indirectly impact biogeochemical processes with implications for stream ecosystem functioning.

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO 3 -N and NH 4 -N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km 2 ) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000-2001 was 70 kg km À1 year À1 , of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO 3 -N, DON and NH 4 -N, respectively.

Nature of the problem (science/management/policy) Freshwater ecosystems play a key role in the European nitrogen (N) cycle, both as a reactive agent that transfers, stores and processes • N loadings from the atmosphere and terrestrial ecosystems, and as a natural environment severely impacted by the increase of these loadings. Approaches Th is chapter is a review of major processes and factors controlling N transport and transformations for running waters, standing waters, • groundwaters and riparian wetlands.