Pierre Marmonier

A Perspective on the Permeability of the Surface Freshwater-Groundwater Ecotone

The ecotone concept is used to consider exchanges of matter and energy between groundwater and surface water systems. Ecotones control these exchanges by acting as a combination of three types of filter: the contrast between permanent darkness and day-night alternation produces a photic filter; the matrix of interstitial underground systems creates a mechanical filter which slows down water velocity; biological and chemical processes, which often occur simultaneously, define a biochemical filter. The permeability of these filters changes according to water velocity. Thus, for ecotones between a surface system and a porous aquifer with large substrate pore size, dynamics of permeability are governed by hydrology, whereas between a surface system and a partially clogged aquifer with small substrate pore size, they are governed by aerobic and anaerobic processes. According to the degree of permeability, ecotones can be classified as either permanent or temporary sinks for the two adjacent systems. In the latter case, matter fluxes are retained in the ecotone, often undergoing transformation before being released to one of the ecosystems.

Biodiversity in ground waters

Despite the importance of ground waters in the global water cycle, their ecology and biodiversity have only recently received attention. Three areas are currently being studied: (I ) the origin and colonization ground ground waters, (2) the adaptation of animals to the subterranean environment, and (3) the role of ecotone between surface and ground waters. There are still several gaps in our knowledge of groundwater biodiversity (at the genetic level, the species level, the functional group level and the ecosystem level) to which future research must be directed.

Distribution of Dissolved Organic Carbon and Bacteria at the Interface between the Rhône River and Its Alluvial Aquifer

To understand the efficiency of interstitial habitats in the elimination of organic matter as it moves from surface water to groundwater (bank filtration), we studied spatial and temporal variations of sediment organic matter concentration, biodegradable (BDOC) and refractory (RDOC) fractions of the dissolved organic carbon, bacterial abundances, and microbial enzymatic activity in the first metre of sediment of the Rhône River immediately downstream of a large city. The study area was fed most of the year by the surface water inflow (downwelling area), because of groundwater pumping wells located ∼ 80 m from the river. Decreasing gradients from surface water to deep sediments and from the river to the shore were observed in most of the cases for the four variables. The decrease in RDOC concentrations did not vary seasonally (this decrease is probably due to physical process, such as adsorption on fine mineral particles), whereas decreases in BDOC concentrations only occurred when microbial enzymatic activities were high; BDOC is rapidly assimilated by microbial communities. Physical and biological processes together make this first metre of sediment an efficient filter for organic matter.

Effects of intense agricultural practices on heterotrophic processes in streams.

In developed countries, changes in agriculture practices have greatly accelerated the degradation of the landscape and the functioning of adjacent aquatic ecosystems. Such alteration can in turn impair the services provided by aquatic ecosystems, namely the decomposition of organic matter, a key process in most small streams. To study this alteration, we recorded three measures of heterotrophic activity corresponding to microbial hydrolasic activity (FDA hydrolysis) and leaf litter breakdown rates with (k(c)) and without invertebrates (k(f)) along a gradient of contrasted agricultural pressures. Hydrolasic activity and k(f) reflect local/microhabitat conditions (i.e. nutrient concentrations and organic matter content of the sediment) but not land use while k(c) reflects land-use conditions. k(c), which is positively correlated with the biomass of Gammaridae, significantly decreased with increasing agricultural pressure, contrary to the taxonomic richness and biomass of Trichoptera and Plecoptera. Gammaridae may thus be considered a key species for organic matter recycling in agriculture-impacted streams.

Effects of spates on interstitial assemblages of the Rhône River. Importance of spatial heterogeneity

The dynamics of interstitial assemblages, after a spate and during low discharge, was studied in a regulated channel (Miribel Canal) of the Upper Rhône River, France. Using a Bou-Rouch pump, three stations were samples: 1) Station IIA, a site fed by superficial water infiltrations, 2) Station IIC, a site fed by riparian phreatic water, and 3) Station IA, a site fed by both surface and phreatic waters. The spate greatly influenced the interstitial assemblages, their dynamics were different according to the hydrology of the given site. At Station IIA, the spate had a wash-out effect on the assemblages (reduction in abundance and diversity), whereas during low discharge the interstitial layer received a continuous influx of epigean organisms (benthic and limnophilous). At Stations IIC and IA, the spate introduced numerous limnophilous and benthic invertebrates into the interstices, which function as an organismic trap. However, these sites appear to be more isolated from surface waters during low discharge. Stygobites decreased or disappeared after the spate. They appear highly sensitive to hydrologic perturbations in the surface waters.

Effects of spates on the vertical distribution of the interstitial community

The effect of hydrological variations on the vertical distribution of interstitial organisms was studied in a by-passed channel of the Rhône River(France) in terms of the distortion and recovery of community structure over a 480-day period that included 9 spates. The hypogean and epigean components of the interstitial assemblages were studied, the latter at four depths (50, 100, 150 and 200 cm) within the substratum, during a succession of artificially regulated spates and periods of low flow. Interstitial fauna were obtained with a Bou-Rouch sampler through permanent standpipes at intervals of one, seven and seventeen days after each spate. A strong relationship was demonstrated between disturbance intensity (amplitude and duration of spates) and the magnitude of changes in the vertical distribution of the fauna. Both hypogean and epigean organisms were displaced vertically after each spate, resulting in an increase in the numbers of epigean organisms in interstitial layers. Recovery of vertical distribution of the interstitial fauna to pre-spate conditions, consisted of a decrease of epigean and an increase of hypogean fauna with the substratum, mainly at intermediate sampling depths (100, 150 cm). The effect of spates on community structure and rate of recovery varied in relation to 1) the combination of spate amplitude and duration (i.e. the shape of the discharge pattern), 2) the disturbance regime (i.e. the recent hydrological past) and 3) season (temperature and biological cycle).

Seasonal dynamics of nutrient and biofilm in interstitial habitats of two contrasting riffles in a regulated large river

Abstract: To describe nutrient and biofilm dynamics in interstitial habitats of riffles (in a downwelling - upwelling sequence), oxygen, nitrate, dissolved organic carbon (DOC) contents, bacterial abundances, hydrolytic and deshydrogenasic activities of biofilms were studied for 18 months in two geomorphologically contrasting riffles of a regulated channel of the Rhône river (a 7th-order stream). The first one was located in an section of the channel affected by river bed incision, whereas the other section has only been slightly impacted by river bed incision. During the transit of surface water through river bed sediments, oxygen consumption, DOC immobilization, and nitrate production were observed especially during the warm seasons when biofilm activity was high. Organic matter decomposition by interstitial biofilms may induce nitrate release by mineralization and nitrification. When the oxygen content decreased strongly during summer, exceptional nitrate depletion occurred in the sediments due to denitrification. A comparison with the Maple River, a 3rd-order stream (Hendricks, 1993) highlights that differences were mostly linked to the origin of groundwater, to differences in stream order (high quantities of nutrients), and to local geomorphological characteristics (heterogeneity in fine sediments and total organic matter - TOM - deposition). River bed incision might induce (1) weak biofilm development and activities in the sediment, (2) changes in vertical distribution of bacteria and biofilm activities, (3) marked temporal variations of biofilm activities, and (4) decrease of sediment efficiency in oxygen consumption, DOC immobilization, and nitrate production.

15N-Nitrate signature in low-order streams : effects of land cover and agricultural practices

Many studies have shown that intensive agricultural practices significantly increase the nitrogen concentration of stream surface waters, but it remains difficult to identify, quantify, and differentiate between terrestrial and in-stream sources or sinks of nitrogen, and rates of transformation. In this study we used the delta15N-NO3 signature in a watershed dominated by agriculture as an integrating marker to trace (1) the effects of the land cover and agricultural practices on stream-water N concentration in the upstream area of the hydrographic network, (2) influence of the in-stream processes on the NO3-N loads at the reach scale (100 m and 1000 m long), and (3) changes in delta15N-NO3 signature with increasing stream order (from first to third order). This study suggests that land cover and fertilization practices were the major determinants of delta15N-NO3 signature in first-order streams. NO3-N loads and delta15N-NO3 signature increased with fertilization intensity. Small changes in delta15N-NO3 signature and minor inputs of groundwater were observed along both types of reaches, suggesting the NO3-N load was slightly influenced by in-stream processes. The variability of NO3-N concentrations and delta15N signature decreased with increasing stream order, and the delta15N signature was positively correlated with watershed areas devoted to crops, supporting a dominant effect of agriculture compared to the effect of in-stream N processing. Consequently, land cover and fertilization practices are integrated in the natural isotopic signal at the third-order stream scale. The GIS analysis of the land cover coupled with natural-abundance isotope signature (delta15N) represents a potential tool to evaluate the effects of agricultural practices in rural catchments and the consequences of future changes in management policies at the regional scale.

Optimizing a sampling strategy for assessing hyporheic invertebrate biodiversity using the Bou-Rouch method: Within-site replication and sample volume

To determine the optimal combination of sample volume and replication required to assess hyporheic taxa richness and abundance, 10 consecutive one-L Bou-Rouch pump samples were collected from each of 9 wells inserted in a 3 × 3 m grid at two sites on the Rhone River, France. Variable and non-linear relationships between sample volume and numbers of taxa and individuals demonstrate that hyporheic densities cannot simply be expressed per L for comparison among studies that have collected different sample volumes. Our results imply that an optimal sampling design for rapid assessment of hyporheic biodiversity and abundance would need at least five samples of 3-5L to provide reasonable levels of precision (20 %). At both sites, 6-7 replicates of 5 L sufficed to be able to detect a 50 % change in mean taxa richness with a power of 0.90 and α = 0.10. Although this sampling exercise should be repeated whenever accurate population estimates are needed, it appears that the spatial variance of the hyporheos at a fine scale (<9m 2 ) is great enough to obscure detection of potentially large differences when replication or sample volume is small.

Potential impact of invasive amphipods on leaf litter recycling in aquatic ecosystems

The impact of biological invasions on local biodiversity is well established, but their impact on ecosystem functioning has only been sketchily documented. However, biological invasions may impede services provided by aquatic ecosystems, such as, for example, the decomposition of organic matter, a key process in most small streams. To address this question, we experimentally quantified the leaf litter breakdown activity of native and invasive amphipod species, which are keystone species in aquatic ecosystems. The breakdown rate of each species was used to estimate the potential leaf litter recycling in the Rhône and Meurthe Rivers in sites occupied solely by native species and sites dominated by invasive species. We found that invaders were not able to compensate for the activity of native species and that the replacement of native species led to a decrease of at least 66% in the rate of leaf litter recycling. Our approach provides empirical evidence of the functional impact of non-indigenous species on leaf litter recycling, using standard protocols and literature data.