Andrea Butturini

Nitrogen Removal by Riparian Buffers along a European Climatic Gradient: Patterns and Factors of Variation

AbstractWe evaluated nitrogen (N) removal efficiency by riparian 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 NO3input, r2 = 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.

Water table fluctuations in the riparian zone: comparative results from a pan-European experiment

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.

Effects of riparian vegetation removal on nutrient retention in a Mediterranean stream

We examined the effects of riparian vegetation removal on algal dynamics and stream nutrient retention efficiency by comparing NH4-N and PO4-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 NH4Cl) and P (as Na2HPO4) 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 PO4-P. Despite a greater increase in PO4-P retention efficiency relative to that of NH4-N following logging, retention efficiency for NH4-N was higher than for PO4-P in both study reaches. The PO4-P mass-transfer coefficient was correlated with primary production in both study reaches, indicating that algal activity plays an important role in controlling PO4-P dynamics in this stream. In contrast, the NH4-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 NH4-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.

Organic matter availability during pre- and post-drought periods in a Mediterranean stream

Mediterranean streams are characterized by water flow changes caused by floods and droughts. When intermittency occurs in river ecosystems, hydrologic connectivity is interrupted and this affects benthic, hyporheic and flowing water compartments. Organic matter use and transport can be particularly affected during the transition from wet to dry and dry to wet conditions. In order to characterize the changes in benthic organic matter quantity and quality throughout a drying and rewetting process, organic matter, and enzyme activities were analyzed in the benthic accumulated material (biofilms growing on rocks and cobbles, leaves, and sand) and in flowing water (dissolved and particulate fractions). The total polysaccharide, amino acid, and lipid content in the benthic organic matter were on average higher in the drying period than in the rewetting period. However, during the drying period, peptide availability decreased, as indicated by decreases in leucine aminopeptidase activity, as well as amino acid content in the water and benthic material, except leaves; while polysaccharides were actively used, as indicated by an increase in β-glucosidase activity in the benthic substrata and an increase in polysaccharide content of the particulate water fraction and in leaf material. During this process, microbial heterotrophs were constrained to use the organic matter source of the lowest quality (polysaccharides, providing only C), since peptides (providing N and C) were no longer available. During the flow recovery phase, the microbial community rapidly recovered, suggesting the use of refuges and/or adaptation to desiccation during the previous drought period. The scouring during rewetting was responsible for the mobilization of the streambed and loss of benthic material, and the increase in high quality organic matter in transport (at that moment, polysaccharides and amino acids accounted for 30% of the total DOC). The dynamics of progressive and gradual drought effects, as well as the fast recovery after rewetting, might be affected by the interaction of the individual dynamics of each benthic substratum: sand sediments and leaves providing refuge for microorganisms and organic matter storage, while on cobbles, an active bacterial community is developed in the rewetting. Since global climate change may favor a higher intensity and frequency of droughts in streams, understanding the effects of these disturbances on the materials and biota could contribute to reliable resource management. The maintenance of benthic substrata heterogeneity within the stream may be important for stream recovery after droughts.

Seasonal variability of dissolved organic carbon in a Mediterranean stream.

The seasonal variability of dissolved organic carbon(DOC) flux in a Mediterranean stream subjected todischarges of wide range of intensities and variabledry period was studied as a function of the hydrologicconditions, and the relationship between surface andsubsurface (hyporheic and groundwater) DOCconcentration. DOC concentration in stream water(2.6 mg l−1 ±1.5 SD) was higher thangroundwater (1.3 mg l−1 ± 1.2 SD) and lower thanhyporheic water (3.8 mg l−1 ±1.7 SD),suggesting that, at baseflow, stream DOC concentrationincreases when groundwater discharges through thehyporheic zone. Storms contributed to 39% of annualwater export and to 52% of the total annual DOCexport (220 kg km−2). A positive relationship wasobserved between Discharge (Q) and stream DOCconcentration. Discharge explained only 40% of theannual variance in stream DOC, but explained up to93% of the variance within floods. The rate of streamDOC changes with discharge change during storms (dDOC/dQ), ranged between 0 and 0.0045 C mgl−1 s l−1, with minimum values during Springand Summer, and maxima values in Fall and Winter.These dynamics suggest that storm inputs ofterrigenous DOC vary between seasons. During floods inthe dormant season, DOC recession curves were alwayssteeper than discharge decline, suggesting shortflushing of DOC from the leaching of fresh detritusstored in the riparian zone.

Nitrogen processes in aquatic ecosystems

Nature of the problem * 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 * This chapter is a review of major processes and factors controlling N transport and transformations for running waters, standing waters, groundwaters and riparian wetlands. Key findings/state of knowledge * The major factor controlling N processes in freshwater ecosystems is the residence time of water, which varies widely both in space and in time, and which is sensitive to changes in climate, land use and management. * The effects of increased N loadings to European freshwaters include acidifi cation in semi-natural environments, and eutrophication in more disturbed ecosystems, with associated loss of biodiversity in both cases. * An important part of the nitrogen transferred by surface waters is in the form of organic N, as dissolved organic N (DON) and particulate organic N (PON). Th is part is dominant in semi-natural catchments throughout Europe and remains a signifi cant component of the total N load even in nitrate enriched rivers. * In eutrophicated standing freshwaters N can be a factor limiting or co-limiting biological production, and control of both N and phosphorus (P) loading is oft en needed in impacted areas, if ecological quality is to be restored. Major uncertainties/challenges * The importance of storage and denitrifi cation in aquifers is a major uncertainty in the global N cycle, and controls in part the response of catchments to land use or management changes. In some aquifers, the increase of N concentrations will continue for decades even if efficient mitigation measures are implemented now. * Nitrate retention by riparian wetlands has oft en been highlighted. However, their use for mitigation must be treated with caution, since their effectiveness is difficult to predict, and side eff ects include increased DON emissions to adjacent open waters, N2O emissions to the atmosphere, and loss of biodiversity. * In fact, the character and specifi c spatial origins of DON are not fully understood, and similarly the quantitative importance of indirect N2O emissions from freshwater ecosystems as a result of N leaching losses from agricultural soils is still poorly known at the regional scale. * These major uncertainties remain due to the lack of adequate monitoring (all forms of N at a relevant frequency), especially - but not only - in the southern and eastern EU countries. Recommendations * The great variability of transfer pathways, buffering capacity and sensitivity of the catchments and of the freshwater ecosystems calls for site specific mitigation measures rather than standard ones applied at regional to national scale. * The spatial and temporal variations of the N forms, the processes controlling the transport and transformation of N within freshwaters, require further investigation if the role of N in infl uencing freshwater ecosystem health is to be better understood, underpinning the implementation of the EU Water Framework Directive for European freshwaters.

Stream Hydrological Fragmentation Drives Bacterioplankton Community Composition

In Mediterranean intermittent streams, the hydrological fragmentation in summer and the successive water flow re-convergence in autumn allow exploring how local processes shape the microbial community within the same habitat. The objectives of this study were to determine how bacterial community composition responded to hydrological fragmentation in summer, and to evaluate whether the seasonal shifts in community composition predominate over the effects of episodic habitat fragmentation. The bacterial community was assessed along the intermittent stream Fuirosos (Spain), at different levels of phylogenetic resolution by in situ hybridization, fingerprinting, and 16S rRNA gene sequencing. The hydrological fragmentation of the stream network strongly altered the biogeochemical conditions with the depletion of oxidized solutes and caused changes in dissolved organic carbon characteristics. In the isolated ponds, beta-Proteobacteria and Actinobacteria increased their abundance with a gradual reduction of the alpha-diversity as pond isolation time increased. Moreover, fingerprinting analysis clearly showed a shift in community composition between summer and autumn. In the context of a seasonal shift, the temporary stream fragmentation simultaneously reduced the microbial dispersion and affected local environmental conditions (shift in redox regime and quality of the dissolved organic matter) tightly shaping the bacterioplankton community composition.

Effects of removal of riparian vegetation on algae and heterotrophs in a Mediterranean stream

The effect of removal of a riparian strip on aquaticautotrophic (algae) and heterotrophic (bacteria,macroinvertebrates) organisms was monitored in aMediterranean stream during the canopy growing period.Community composition, biomass and metabolicactivities were compared with those recorded during apre-riparian removal period and in a forested stretchdownstream. Higher irradiance was associated with Cladophora increase in the logged section. Algalbiomass increased up to ten times, and productivitywas up to four times higher than in the pre-removalperiod and the forested section. Bacterialcommunities showed higher ectoenzymatic activities(β-glucosidase, β-xylosidase) in thelogged section than in forested conditions. Moreoverthe coincidence between the maxima ofβ-glucosidase and chlorophyll-a suggeststhat bacterial activity was enhanced by the higheravailability of high-quality algal material. Themacroinvertebrate community had higher density andbiomass in the logged section than in the forestedsection and in the pre-removal period. Scrapers andfilterers become dominant after riparian removal,while shredders, predators and collectors did not showsignificant changes either between sites or periods.Responses of environmental variables and bioticcommunities indicate that the changes occurring in thestream due to riparian removal could be consideredbottom-up controlled, as increased illumination wasthe main mechanism responsible.

The effects of sediment depth and oxygen concentration on the use of organic matter: An experimental study using an infiltration sediment tank

Water flowing through hyporheic river sediments or artificial recharge facilities promotes the development of microbial communities with sediment depth. We performed an 83-day mesocosm infiltration experiment, to study how microbial functions (e.g., extracellular enzyme activities and carbon substrate utilization) are affected by sediment depth (up to 50 cm) and different oxygen concentrations. Results indicated that surface sediment layers were mainly colonized by microorganisms capable of using a wide range of substrates (although they preferred to degrade carbon polymeric compounds, as indicated by the higher β-glucosidase activity). In contrast, at a depth of 50 cm, the microbial community became specialized in using fewer carbon substrates, showing decreased functional richness and diversity. At this depth, microorganisms picked nitrogenous compounds, including amino acids and carboxyl acids. After the 83-day experiment, the sediment at the bottom of the tank became anoxic, inhibiting phosphatase activity. Coexistence of aerobic and anaerobic communities, promoted by greater physicochemical heterogeneity, was also observed in deeper sediments. The presence of specific metabolic fingerprints under oxic and anoxic conditions indicated that the microbial community was adapted to use organic matter under different oxygen conditions. Overall the heterogeneity of oxygen concentrations with depth and in time would influence organic matter metabolism in the sediment tank.

Hydrological conditions regulate dissolved organic matter quality in an intermittent headwater stream. From drought to storm analysis.

Storms and droughts are an essential driver for the dissolved organic matter (DOM) concentration in headwater streams. However, the relationship between DOM quality and discharge (Q) has not been addressed in depth and the impact of other hydro-climatic or biogeochemical drivers has not been explored. In this study DOM quality variability was explored at seasonal and storm event scales during an intensive 2.5-year-long sampling in a Mediterranean stream characterized by a severe summer drought. DOM quality was described in terms of absorbance and fluorescence properties. Most of the DOM properties were strongly related to discharge revealing the input of allochthonous, degraded, aromatic, humic and increased-molecular-size DOM under high flow conditions. However, these relationships disappeared or reversed during drying and rewetting periods. Each DOM response at the storm event scale (DOM-Q hysteresis) was outlined with two descriptors that summarised its trend (dilution/flushing/chemostasis) and shape (linear/nonlinear response). Multiple linear regression and commonality analysis showed that, in addition to the magnitude of storm episodes, antecedent hydrological conditions, namely pre-event basal flow and the magnitude of the previous storm event, played a significant role in regulating the trends and shapes of DOM-Q hysteresis.