Francesc Sabater

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.

Drought and postdrought recovery cycles in an intermittent Mediterranean stream: structural and functional aspects

Abstract The effects of the intensity of seasonal droughts on stream ecosystems were studied in the Fuirosos, an intermittent forested Mediterranean stream. Macroinvertebrate community structure and stream ecosystem metabolism were measured during seasonal summer droughts in 2001, 2002, and 2003. Ecosystem metabolism was profoundly affected by stream intermittency. Organic matter that accumulated during the dry period enhanced ecosystem respiration during the postdrought recovery. Highest biotic diversity was found at low water levels as the stream dried and contracted. Macroinvertebrate community response to drying was stepped and apparently defined by thresholds of transition from drying to cessation of flow and from the dry phase to restoration of flow. Environmental conditions changed markedly with cessation of flow, causing large changes in community structure during 2001 and 2003 (dry years). Drying caused an increase in macroinvertebrate density that peaked in isolated pools soon after flow ceased, but then decreased rapidly because of the physicochemical changes associated with fragmentation of the watercourse. The macroinvertebrate community at the end of the summer dry phase (when flow resumed) differed from the community that had been present before drying began. Differences in community structure during the summer dry period were not as marked in 2002 (a wet year) as in 2001/2003. The influence of drought on the macroinvertebrate community differed across substrata. Drying led to significant changes in density on cobbles and leaves, but not on sand. Few taxa resisted drying, and resilience to drying was the dominant response to disturbance in the Fuirosos.

HIGH VARIABILITY IN TEMPORAL AND SPATIAL NUTRIENT RETENTION IN MEDITERRANEAN STREAMS

Nutrient retention studies were conducted in two second-order streams located north of Barcelona (Spain) that differed in watershed lithology, soil type, and vegetation. La Solana drains a calcareous watershed, whereas Riera Major drains a siliceous watershed. Within each stream we defined two reaches (50 m) differing in channel form and canopy cover : bedrock and sand-cobble. The bedrock reaches were characterized by a dominance of bedrock outcrop and a sparse canopy, while the sand-cobble reaches had sand, cobble, and boulder substrata and a dense canopy. Phosphate and ammonium uptake length and uptake rate were measured in each reach by performing several short-term additions (2 h) of phosphate and ammonium. Releases were conducted from summer 1990 to spring 1992 to encompass seasonal changes in environmental conditions. We examined temporal trends in nutrient uptake length to test the influence of watershed geomorphology and local channel form on nutrient retention in these two Mediterranean streams. Because both nutrients were added simultaneously, we were able, by using the ratio of each nutrients uptake length, to compare ammonium and phosphate retention in each reach. Results of a two-way ANOVA indicated that both stream and reach type had significant effects on phosphate and ammonium uptake lengths. However, channel form had a different effect depending on the stream. In La Solana, shortest phosphate uptake lengths (highest efficiency in phosphate retention) were measured in summer, whereas shortest ammonium uptake lengths were measured in winter in the two reaches. In this stream, both nutrients were retained with greater efficiency in the bedrock reach than in the sand-cobble reach (mean uptake length was 59 ± 12 m for phosphate and 59 ± 14 m for ammonium in the bedrock reach, means ± 1 SE, n = 13 for both nutrients ; and 119 ± 19 m for phosphate and 182 ± 51 m for ammonium in the sand-cobble reach, n = 12 for each nutrient). In Riera Major, only the bedrock reach exhibited a seasonality in nutrient uptake length that was similar to that found in La Solana reaches. The sand-cobble reach in Riera Major showed higher efficiency in ammonium retention than the bedrock reach (85 ± 16 m, n = 14 ; and 189 ± 28 m, n = 10, respectively) ; but phosphate uptake lengths did not differ between reaches to a statistically significant extent (163 ± 26 m in the sand-cobble reach, n = 14 ; and 191 ± 32 m in the bedrock reach, n = 10). When the two streams were compared, La Solana exhibited higher nutrient retention efficiencies (shorter uptake lengths) than Riera Major. However, nutrient uptake rates for both nutrients were higher in Riera Major because of greater nutrient loadings. Mean phosphate uptake rates at ambient levels for the whole study period were 45.4 ± 10.8 μg P.m -2 .min -1 in La Solana (n = 25) and 175.5 ± 68.6 μg P.m -2 .min -1 in Riera Major (n = 24) ; and mean ammonium uptake rates at ambient levels were 103.3 ± 27.4 and 186.7 ± 48.6 μg N.m -2 .min -1 , respectively. Finally, nutrient uptake lengths from this study are within the range reported for other streams of similar order. Overall results indicate that differences in ammonium retention efficiency in these streams may be tightly related to in-stream processes. In contrast, major between-stream differences in phosphate retention efficiency appear to be a function of overall phosphate availability.

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.

Meteorological and riparian influences on organic matter dynamics in a forested Mediterranean stream

Abstract Organic matter inputs, transport, and storage, ecosystem metabolism, and organic C turnover length were measured in a forested Mediterranean 3rd-order stream (Fuirosos) over a period of 3 y. Meteorological patterns influenced the organic matter dynamics of Fuirosos through 2 pathways: summer weather and flood pulses. Summer rains affected the timing of litter fall and the extent of flow intermittency, which gave rise to accumulations of organic matter in the streambed in dry years. With the onset of flow, these organic matter accumulations were the energy sources for considerable ecosystem respiration (30 g O2 m−2 d−1). Interflood periods punctuated by flood pulses determined cycles of steady accumulation and abrupt removal of stored organic matter. During the interflood periods, the efficiency of organic matter processing increased continuously. The seasonal changes in the riparian forest influenced the ecological consequences of the flood pulses. Resilience of the ecosystem, measured in terms of gross primary production and ecosystem respiration, strongly depended on the supply of benthic organic matter and light availability, both of which were seasonally variable in the deciduous riparian forest.

Quantification of metabolically active transient storage (MATS) in two reaches with contrasting transient storage and ecosystem respiration

a deep alluvial deposit. The MATS zones measured 0.002 m 2 in the bedrock reach (37% of transient storage) and 0.291 m 2 in the alluvial reach (100% of transient storage). The effective rate coefficient of Raz transformation in the MATS of the bedrock reach was approximately 16 times that of the alluvial reach. However, when we take into account the contribution of the MATS zone to overall metabolic activity, Raz transformation in the MATS zone was 2.2 times slower in the bedrock reach than in the alluvial reach. The difference was similar to the difference in ecosystem respiration, which was 1.8 times lower in the bedrock reach than in the alluvial reach, suggesting that the MATS zones were important contributors to ecosystem respiration. Results indicate that the quantification of MATS can improve our understanding of the role that transient storage zones play on stream metabolic processes and demonstrate the utility of Raz as a “smart” tracer that provides new information on metabolic activity at a whole‐reach and at smaller scale. Citation: Argerich, A., R. Haggerty, E. Marti, F. Sabater, and J. Zarnetske (2011), Quantification of metabolically active transient storage (MATS) in two reaches with contrasting transient storage and ecosystem respiration, J. Geophys. Res., 116, G03034, doi:10.1029/2010JG001379.

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.

Hydrological extremes modulate nutrient dynamics in mediterranean climate streams across different spatial scales

Hydrology is a key factor in the terrestrial landscape that connects upland and riparian ecosystems to streams. Hydrological connectivity through the catchment and along the stream network influences not only the timing and the magnitude of terrestrial inputs to stream ecosystems, but also regulates stream metabolism and biogeochemical cycling. In mediterranean climate regions (med-regions), there is a marked seasonal alternation of dry and wet periods, and streams experience a wide range of hydrological conditions from floods to droughts. Moreover, nutrient dynamics in med-streams are highly variable within the year and between years compared to temperate streams. In this review, we explore how seasonal changes in hydrological connectivity and hydrological extremes affect nutrient cycling and transport at different spatial scales in med-catchments. We show that during the dry period, the stream network contracts more severely in med-catchments than in temperate catchments. During the contraction phase, upland-stream disconnection decreases nutrient supply from terrestrial ecosystems, while stream fragmentation increases the spatial variation of stream nutrient concentrations. Moreover, the precipitation regime typical of med-regions (with irregular distribution of precipitation and episodic large rainfall events) decreases the ability of terrestrial and aquatic biota to retain nutrients and favours nutrient export to downstream ecosystems. Altogether, this hydrological setting confers to med-streams a characteristic temporal pattern in stream nutrient dynamics that is analogous to a particular biogeochemical heartbeat with higher amplitude, variation and unpredictability than that observed in temperate streams.

Effects of the Dry-Wet Hydrological Shift on Dissolved Organic Carbon Dynamics and Fate Across Stream-Riparian Interface in a Mediterranean Catchment

A bstractThe stream–riparian interface, characterized by a dynamic and complex hydrology, is an important control point for nutrient fluxes and processing between terrestrial and aquatic systems. Predicted alterations in the discharge regime in Mediterranean climate regions make it necessary to understand the effects of abrupt hydrological transition between dry and wet conditions on the transport and fate of dissolved organic carbon (DOC) across the stream–riparian interface. In this study, the concentrations and fate of total DOC (TDOC) and a subset of four molecular weight fractions (<1 kDa, 1–10 kDa, 10–100 kDa, >100 kDa) were investigated in stream water and riparian groundwater during autumn of 2003 and 2004. The two study periods were characterized by contrasting antecedent hydrological conditions: the streamflow was interrupted in summer 2003 but was permanent in summer 2004. Comparison of the two study periods indicates that an abrupt dry–wet hydrological transition amplifies the water exchange across the stream–riparian interface and favors retention of up to 57% of the TDOC that flows across the interface. Furthermore, the efficiency of DOC retention across the stream-riparian interface also varies greatly depending on DOC molecular size. More than 70% of DOC fractions higher than 10 kDa were retained, whereas the smaller fraction (less than 1 kDa) was nearly conserved. Consequently, our study helps to clarify the effects of extreme hydrological events on DOC transport in running waters in Mediterranean regions.

Algal biomass in a disturbed Atlantic river: water quality relationships and environmental implications

Benthic algal biomass was determined at 38 sites along the Oria, a river with intense human activity on its watershed. Water temperature, total suspended solids and nutrient concentration affected algal biomass in different ways. Lower values of algal biomass [(chlorophyll-a concentration and ash-free dry weight (AFDW)] were found in the headwaters and forested tributaries, but also at sites receiving high deposition of solids (due to the outflow from paper mills and quarries). Higher values of algal biomass (both chlorophyll-a and AFDW) were common both in the main stretch of the river and in some tributaries receiving urban sewage outflows. Averages and ranges of temperature and dissolved oxygen concentration were compared for two continuously monitored stations. One of them (forested, nutrient-poor) had low algal biomass, while the second (open, nutrient-rich) showed high chlorophyll-a concentration. Accumulation of algal biomass had environmental implications both in water quality and in the fish diversity of the Oria. Diel variations of dissolved oxygen were much higher (and reached hypoxia) at the site with higher biomass accumulation. The abundance of a fish community (dominated by cyprinids) more tolerant to hypoxia at that site can partly be attributed to the influence of algal biomass accumulation.