Temporal variability of nitrate transport through hydrological response during flood events within a large agricultural catchment in south-west France

The temporal variability of nitrate transport was monitored continuously in a large agricultural catchment, the 1110 km2 Save catchment in south-west France, from January 2007 to June 2009. The overall aim was to analyse the temporal transport of nitrate through hydrological response during flood events in the catchment. Nitrate loads and hysteresis were also analysed and the relationships between nitrate and hydro-climatological variables within flood events were determined. During the study period, 19 flood events were analysed using extensive datasets obtained by manual and automatic sampling. ThemaximumNO3 − concentration during flood varied from 8.2 mg l−1 to 41.1 mg l−1 with flood discharge from 6.75 m3 s−1 to 112.60 m3 s−1. The annual NO3 − loads in 2007 and 2008 amounted to 2514 t and 3047 t, respectively, with average specific yield of 2.5 tkm−12 yr−1. The temporal transport of nitrate loads during different seasonal flood events varied from 12 t to 909 t. Nitrate transport during flood events amounted to 1600 t (64% of annual load; 16% of annual duration) in 2007 and 1872 t (62% of annual load; 20% of annual duration) in 2008. The level of peak discharge during flood events did not control peak nitrate concentrations, since similar nitrate peaks were produced by different peak discharges. Statistically strong correlations were found between nitrate transport and total precipitation, flood duration, peak discharge and total water yield. These four variables may be the main factors controlling nitrate export from the Save catchment. The relationship between nitrate and discharge (hysteresis patterns) investigated through flood events in this study was mainly dominated by anticlockwise behaviour

Assessment of hydrology, sediment and particulate organic carbon yield in a large agricultural catchment using the SWAT model

The Soil and Water Assessment Tool (SWAT, 2005) was used to simulate discharge and sediment transport at daily time steps within the intensively farmed Save catchment in south-west France (1110 km2). The SWAT model was applied to evaluate catchment hydrology and sediment and associated particulate organic carbon yield using historical flow and meteorological data for a 10-years (January 1999–March 2009). Daily data on sediment (27 months, January 2007–March 2009) and particular organic carbon (15 months, January 2008–March 2009) were used to calibrate the model. Data on management practices (crop rotation, planting date, fertiliser quantity and irrigation) were included in the model during the simulation period of 10 years. Simulated daily discharge, sediment and particulate carbon values matched the observed values satisfactorily. The model predicted that mean annual catchment precipitation for the total study period (726 mm) was partitioned into evapotranspiration (78.3%), percolation/groundwater recharge (14.1%) and abstraction losses (0.5%), yielding 7.1% surface runoff. Simulated mean total water yield for the whole simulation period amounted to 138 mm, comparable to the observed value of 136 mm. Simulated annual sediment yield ranged from 4.3 t km−2 y−1 to 110 t km−2 y−1 (annual mean of 48 t km−2 y−1). Annual yield of particulate organic carbon ranged from 0.1 t km−2 y−1 to 2.8 t km−2 y−1 (annual mean of 1.2 t km−2 y−1). Thus, the highest annual sediment and particulate carbon yield represented 25 times the minimum annual yield. However, the highest annual water yield represented five times the minimum (222 mm and 51 mm, respectively). An empirical correlation between annual water yield and annual sediment and organic carbon yield was developed for this agricultural catchment. Potential source areas of erosion were also identified with the model. The range of the annual contributing erosive zones varied spatially from 0.1 to 6 t ha−1 according to the slope and agricultural practices at the catchment scale

Influence of nontrophic interactions between benthic invertebrates on river sediment processes: a microcosm study

The main objective of this study was to measure the impact of benthic invertebrate diversity on river sediment processes. We quantified the effects of interactions between three taxa (asellids, chironomid larvae, and tubificid worms). The impacts of different taxa richness treatments were measured on sediment reworking, O2 concentrations, bacterial abundances, and numbers of active bacteria in slow filtration sand–gravel columns. The coefficients of sediment reworking measured in multitaxa treatments were lower than those predicted from one-taxon treatments. The interactions among invertebrates also significantly reduced O2 concentrations in sediments. These results were probably due to interactions between the different sediment structures produced by each taxon (tubes, macropores, and fecal pellets) that modified water flow and associated microbial activities in the interstitial habitat. The stimulation of aerobic microbial processes with two- and three-taxa treatments, whereas one-taxon treatments could increase or decrease O2 consumption in columns, indicates that interactions among invertebrates limited the variability of the system functioning. We suggest that, beyond a small number of detritivorous taxa, a threshold effect on bioturbation process and microbial activities was produced by animals in the experimental system. Finally, the interactions between taxa played a significant role in microbial processes in the system studied

Bioturbation experiments in the Venice Lagoon

Short experiments (14–21 days) were carried out during autumn 1998 and spring 1999 at one selected site of the Venice Lagoon to measure bioturbation activities and mixing rates, as well as to obtain quantitative information on benthos functionality. Fluorescent sediment particles (luminophores, 63–350 μm) were introduced as pulse inputs at the sediment surface. The concentration–depth profiles of the tracer were simulated with a new advection– diffusion–non local model applied under non-steady state conditions. This allowed the quantification of the mixing parameters associated with different mechanisms: biodiffusion (Db), bioadvection (W) and non-local mixing (Ke,z1, z2). A parameter RS (removed sediment) was also calculated to account for the flux of sediment due to nonlocal transport. Results show that bioturbation was dominated by biodiffusion in autumn and by bioadvection in spring. Mean mixing parameters Db, W, and RS changed from 3.09 to 0.87 cm2 y−1, from 0.93 to 15.50 y−1 and from 5.85 to 7.79 g cm−2 y−1, respectively

On modeling chronic detachment of periphyton in artificial rough, open channel flow

Periphyton communities, which are native to river beds, serve as a functional indicator of river health but remain one of the least-studied communities despite the significant increase in the examination of aquatic microbial communities in recent years. In this study, we tested the relevance of three formulations of the chronic detachment term in a simple model for the biomass dynamics of periphyton. Numerical simulations of the periphyton biomass dynamics were performed by using three different descriptors for the flow conditions: the discharge Q, the friction velocity u⁄, and the roughness Reynolds number k+ = u⁄ks/m (where m is water kinetic viscosity and ks is the Nikuradse equivalent sand roughness). Comparisons of numerical simulation results with experimental data from literature revealed chronic detachment to be better simulated by taking the roughness Reynolds number as the external variable of detachment. These results support the idea that transport phenomena that occur in the nearbed layer, e.g. chronic detachment of periphyton matter or vertical transport of nutrients and pollutants in submerged aquatic canopies, are not related to a single turbulence descriptor such as the friction velocity u⁄. Its description requires at least two descriptors, here the friction velocity u⁄ and the Nikuradse equivalent sand roughness ks, which depend on the initial form and dimensions of the colonized substratum, and its changes owing to the thickness, resistance, and composition of the epilithic matter

Cadmium transport in sediments by tubificid bioturbation: An assessment of model complexity

Biogeochemistry of metals in aquatic sediments is strongly influenced by bioturbation. To determine the effects of biological transport on cadmium distribution in freshwater sediments, a bioturbation model is explored that describes the conveyor-belt feeding of tubificid oligochaetes. A stepwise modelling strategy was adopted to constrain the many parameters of the model: (i) the tubificid transport model was first calibrated on four sets of microspheres (inert solid tracer) profiles to constrain tubificid transport; (ii) the resulting transport coefficients were subsequently applied to simulate the distribution of both particulate and dissolved cadmium. Firstly, these simulations provide quantitative insight into the mechanism of tubificid bioturbation. Values of transport coefficients compare very well with the literature, and based on this, a generic model of tubificid bioturbation is proposed. Secondly, the application of the model to cadmium dataset sheds a light on the behaviour of cadmium under tubificid bioturbation. Cadmium enters the sediment in two ways. In one pathway, cadmium enters the sediment in the dissolved phase, is rapidly absorbed onto solid particles, which are then rapidly transported to depth by the tubificids. In the other pathway, cadmium is adsorbed to particles in suspension in the overlying water, which then settle on the sediment surface, and are transported downwards by bioturbation. In a final step, we assessed the optimal model complexity for the present dataset. To this end, the two-phase conveyor-belt model was compared to two simplified versions. A solid phase-only conveyorbelt model also provides good results: the dissolved phase should not be explicitly incorporated because cadmium adsorption is fast and bioirrigation is weak. Yet, a solid phase-only biodiffusive model does not perform adequately, as it does not mechanistically capture the conveyor-belt transport at short time-scales

Interaction between local hydrodynamics and algal community in epilithic biofilm

Interactions between epilithic biofilm and local hydrodynamics were investigated in an experimental flume. Epilithic biofilm from a natural river was grown over a 41 day period in three sections with different flow velocities (0.10, 0.25 and 0.40 m s-¹ noted LV, IV and HV respectively). Friction velocities u* and boundary layer parameters were inferred from PIV measurement in the three sections and related to the biofilm structure. The results show that there were no significant differences in Dry Mass and Ash-Free Dry Mass (g m-²) at the end of experiment, but velocity is a selective factor in algal composition and the biofilms' morphology differed according to differences in water velocity. A hierarchical agglomerative cluster analysis (BrayeCurtis distances) and an Indicator Species Analysis (IndVal) showed that the indicator taxa were Fragilaria capucina var. mesolepta in the lowvelocity (u* = ¼ 0.010e0.012 m s-¹), Navicula atomus, Navicula capitatoradiata and Nitzschia frustulum in the intermediate velocity (u* = ¼ 0.023e0.030 m s-¹) and Amphora pediculus, Cymbella proxima, Fragilaria capucina var. vaucheriae and Surirella angusta in the high-velocity (u* = ¼ 0.033e0.050m s-¹) sections. A sloughing test was performed on 40-day-old biofilms in order to study the resistance of epilithic biofilms to higher hydrodynamic regimes. The results showed an inverse relationship between the proportion of detached biomass and the average value of friction velocity during growth. Therefore, water velocity during epilithic biofilm growth conditioned the structure and algal composition of biofilm, as well as its response (ability to resist) to higher shear stresses. This result should be considered in modelling epilithic biofilm dynamics in streams subject to a variable hydrodynamics regime

Dynamics of Nitrogen loads in surface water of an agricultural watershed by modelling approach, the Save, Southwest France.

Agriculture is known to have a great impact of nutrients enrichment on continental water resources. In south-West of France (Gascogne region), water resource are essentially surface water and shallow aquifer. Nitrogen dynamic in river is complex and highly variable throughout season and year, depending on hydrology, landuse, removal in stream. In this context, agricultural impacts on nitrogen concentration are a matter of concern for agricultural decision-maker. In order to introduce sustainable land use concepts in this hilly, clayey and agricultural shallow soil context, the hydrological simulation model SWAT2005 has been tested as a valuable tool to evaluate the consequences of such land use changes on water and nutrient balance components. This semi-distributed hydrological model coupled with agronomical model EPIC is able to simulate the impact of each agricultural landuse at the outlet of the Save catchment (1100 km2). Hydrological parameters model are calibrated based on 14-year historical record (1994–2008). Nitrogen losses have been measured during 2 years (2006-2008) at the outlet and are used to validate the model calibration. Agricultural data at communal scale coupled with Spot image analyses have been used to evaluate agricultural distribution and pressure in SWAT. The aim of this modelling exercise is to simulate nitrogen cycle in whole agricultural Hydrological Response Units (HRU), depending on plant growth and culture rotation, to simulate accurately nitrate load in river. The ability of SWAT to reproduce nitrogen transfert and transformation at this scale and in this agricultural context will be evaluated by a discussion of importance of each nitrogen cycle process in nitrogen losses. SWAT could be a useful tool to test agricultural scenario to improve the nitrogen management in river

Occurrence of metolachlor and trifluralin losses in the Save river agricultural catchment during floods

Rising pesticide levels in streams draining intensively managed agricultural land have a detrimental effect on aquatic ecosystems and render water unfit for human consumption. The Soil and Water Assessment Tool (SWAT) was applied to simulate daily pesticide transfer at the outlet from an agriculturally intensive catchment of 1110 km2 (Save river, south-western France). SWAT reliably simulated both dissolved and sorbed metolachlor and trifluralin loads and concentrations at the catchment outlet from 1998 to 2009. On average, 17 kg of metolachlor and 1 kg of trifluralin were exported at outlet each year, with annual rainfall variations considered. Surface runoff was identified as the preferred pathway for pesticide transfer, related to the good correlation between suspended sediment exportation and pesticide, in both soluble and sorbed phases. Pesticide exportation rates at catchment outlet were less than 0.1% of the applied amount. At outlet, SWAT hindcasted that (i) 61% of metolachlor and 52% of trifluralin were exported during high flows and (ii) metolachlor and trifluralin concentrations exceeded European drinking water standards of 0.1 µg L−1 for individual pesticides during 149 (3.6%) and 17 (0.4%) days of the 1998–2009 period respectively. SWAT was shown to be a promising tool for assessing large catchment river network pesticide contamination in the event of floods but further useful developments of pesticide transfers and partition coefficient processes would need to be investigated

Longitudinal transformation of nitrogen and carbon in the hyporheic zone of an N-rich stream: A combined modelling and field study

A combined modelling and field study approach was used to examine biogeochemical functioning of the hyporheic zone in two gravel bars in an N-rich fourth-order stream (River Hers, south-west France). Surfacewater and interstitial water were sampledmonthly (August 1994–January 1995), the latter in a network of 29 piezometers in the first gravel bar and 17 in the second. In both gravel bars, the hyporheic zone was created only by advected channelwater without any connectionwith groundwater. Longitudinal chemical profiles of Dissolved Organic Carbon (DOC), nitrate (NO3–N), ammonium (NH4–N) and Dissolved Oxygen (DO) were established for both gravel bars. Ambient and potential denitrification weremeasured in the laboratory during the same period using the acetylene inhibition technique. Factors limiting denitrification were also examined by testing the separate effects of nitrate or nitrate + carbon additions. A 1D reactive-transport model was used to simulate longitudinal transformation of nitrogen in the hyporheic zone, and to estimate the role of organic matter (DOC and POC) in the biogeochemical functioning of the hyporheic zone. Denitrification measurements with nitrate and nitrate + carbon additions both showed increased denitrification, suggesting that denitrification might not be C-limited at this site. Observations and model results showed the hyporheic zone to be a sink of DOC and nitrate, but DOC consumption appeared insufficient to explain nitrate depletion measured in the two gravel bars. Field data were better modelled when an additional DOC source from the POC fraction degraded by anaerobic respiration was included in the model