Magali Gerino

The effects of bioturbation on particle redistribution in Mediterranean coastal sediment. Preliminary results

In order to quantify bioturbation processes in a coastal Mediterranean ecosystem, experiments were performed to determine sediment mixing rates resulting from macrobenthos activity. Particle flux was measured in situ for 22 days using luminophores, which are colored sediment particles with sizes ranging from 10 to 200 µm.In sediment depths from 0–5 cm, particle mixing was intensive due to high macrobenthos abundance. A small quantity of luminophores was transported down to a depth of 14 ± 2 cm, where the macrofauna was represented principally by Polychetes. In a control experimental structure — without benthic fauna — no transfer of luminophores into the sediment was recorded.Sediment particle mixing measured in the ecosystem studied is intensive, and is the result of high macrobenthos activity. Different mixing modes occur with scales and rates depending on the organisms present. The luminophore profile resulting from bioturbation processes is explained by an intensive bioadvection sediment mixing added to a biodiffusive mixing with an order of magnitude of 10−6 cm2 s−1. Tracer accumulations between 1 and 2 ± 1 cm and between 4 and 5 ± 1 cm are attributed to bioadvection activity of two or more distinct populations. Studies over a larger time scale have been undertaken to monitor developments in the observed subsurface maxima.

Imaging biofilm in porous media using X‐ray computed microtomography

In this study, a new technique for three‐dimensional imaging of biofilm within porous media using X‐ray computed microtomography is presented. Due to the similarity in X‐ray absorption coefficients for the porous media (plastic), biofilm and aqueous phase, an X‐ray contrast agent is required to image biofilm within the experimental matrix using X‐ray computed tomography. The presented technique utilizes a medical suspension of barium sulphate to differentiate between the aqueous phase and the biofilm. Potassium iodide is added to the suspension to aid in delineation between the biofilm and the experimental porous medium. The iodide readily diffuses into the biofilm while the barium sulphate suspension remains in the aqueous phase. This allows for effective differentiation of the three phases within the experimental systems utilized in this study. The behaviour of the two contrast agents, in particular of the barium sulphate, is addressed by comparing two‐dimensional images of biofilm within a pore network obtained by (1) optical visualization and (2) X‐ray absorption radiography. We show that the contrast mixture provides contrast between the biofilm, the aqueous‐phase and the solid‐phase (beads). The imaging method is then applied to two three‐dimensional packed‐bead columns within which biofilm was grown. Examples of reconstructed images are provided to illustrate the effectiveness of the method. Limitations and applications of the technique are discussed. A key benefit, associated with the presented method, is that it captures a substantial amount of information regarding the topology of the pore‐scale transport processes. For example, the quantification of changes in porous media effective parameters, such as dispersion or permeability, induced by biofilm growth, is possible using specific upscaling techniques and numerical analysis. We emphasize that the results presented here serve as a first test of this novel approach; issues with accurate segmentation of the images, optimal concentrations of contrast agents and the potential need for use of synchrotron radiation sources need to be addressed before the method can be used for precise quantitative analysis of biofilm geometry in porous media.

Functional diversity among 3 detritivorous hyporheic invertebrates: an experimental study in microcosms

Macroinvertebrates living at the sediment-water interface can be classified into different functional groups according to their modes of feeding and/or their bioturbation activities. We compared 3 detritivorous taxa of hyporheic habitats (chironomid larvae, asellid isopods, and tubificids) to test whether they represented distinct functional groups. This hypothesis was tested in experimental conditions using slow filtration gravel-and-sand columns during 20-d experiments. We measured the effects of the 3 taxa on particle redistribution, organic matter processing, nutrient fluxes, and microbial distribution. There were 3 experimental treatments: 1) 50 chironomid larvae (>80% Prodiamesa), 2) 25 asellids (Asellus aquaticus), and 3) 100 tubificids (Tubifex sp.). Although biologically mediated sediment fluxes were weak because of the small proportion of fine sediment in the columns, the 3 species redistributed sediment differently. They also had different effects on microbial activity, and O2 and nutrient transport into the sediment. Chironomids caused a downward flux of sediment that accumulated as far as 4 cm below the sediment surface. Asellids homogeneized sediments at 1 to 3 cm depth in the column. Tubificids generated a biodiffusive mixing of sediment at the sediment surface together with a vertical transport inside the sediment and increased microbial respiration at all depths. Both asellids and chironomids increased O2 penetration and decreased microbial activity in the first 5 cm of the sediment. Deeper in the sediment, tubificids and asellids stimulated anaerobic processes, whereas chironomid larvae had little effect on these processes. The initial hypothesis that the 3 taxa could be usefully classified into distinct functional groups was accepted. Their distinct activities in the sediment, demonstrated by specific modes of mixing, produced different effects on sediment properties and microbial activities.

The deep-sea macrobenthos on the continental slope of the northwestern Mediterranean Sea: a quantitative approach

As part of the ECOMARGE operation (J.G.O.F.S. France), macrobenthic assemblages in the Toulon Canyon were described and quantified on the basis of sampling carried out between 250 and 2000 m depth on the Mediterranean continental slope. Results show that Mediterranean bathyal assemblages are made up mainly of continental shelf eurybathic species. The qualitative and quantitative composition of populations varies with depth on the slope and also varies with station position at equivalent depth, whether on the flanks or in the canyon channel. Various analyses have provided evidence on the factors responsible for this population distribution pattern. No single factor emerges as predominant, but rather a group of factors, which are related to the nature and origin of sediments and more particularly their grain size distribution, geochemical composition and mode of transportation and sedimentation (benthic nepheloid or originating from the water column), act in conjunction to determine the pattern. Comparison with ocean continental slopes shows that in the Mediterranean Sea the absence of tidal current modifies the trophic structure of the macrobenthic assemblages, which are characterized by a dominance of surface and subsurface deposit feeders as compared to a dominance of suspension feeders and carnivores in the upper and median part of the slope in the ocean. Surface dumping of dredge spoil at the canyon head and channelling of waste induces an increase of organic matter and pollutant concentrations in sediment from the upper part of the canyon channel but does not give rise to any marked population degradation.

Herbicide accumulation and evolution in reservoir sediments

The aim of the present study was to understand the effect of reservoir configurations on sediment pesticide fate. Two dams were selected on the River Garonne, in southwest France: Carbonne and Golfech, both with reservoirs subject to accumulation of herbicide-contaminated sediment. They are situated upstream and downstream respectively of an agricultural and urban area: the Mid-Garonne. The results presented include pesticide concentrations and C/N ratios in the smaller sediment particles (<2 mm) and values of oxygenation and herbicide concentrations in the water. The dynamic behaviour of sediment in the reservoirs is discussed. The present study shows that the theoretical lifespan (weak remanence in vitro) and the results actually observed in the sediment are conflicting. Pesticide contamination in Carbonne indicates conservation, even accumulation, of herbicide molecules while in Golfech transformation processes clearly dominate. The hydromorphological position of Golfech reservoir, i.e. located at the junction of two rivers with contrasting hydrological regimes and very different oxygenation conditions, leads to accelerated pesticide desorption or degradation. Unfortunately, this configuration is rare.

Evidence of bioturbation in the Cap-Ferret Canyon in the deep northeastern Atlantic

In the sedimentary column, a combined quantification of burrows and macrobenthic community provides evidence of bioturbation features in the submarine canyon of Cap-Ferret between 2000 and 3000 m depth. An image-processing technique allows accurate quantification of burrow volumes with depth in the sedimentary column. The major bioturbation mode seems to be different in the channel compared to the interfluve. Macrobenthic activity is more inclined to mix the sediment in the channel in response to increased organic matter supplies. Sediment mixing leads to burrow destruction in the upper mixed layer of sediment in the canyon. Burrows are better preserved on the interfluve where mixing is slower. Under the mixed zone, the volume of recorded burrows is higher when sedimentation rate increases, as in the upper canyon. In this transition layer, the burrow volume is estimated to be between 3 and 64% of the total sediment volume depending on the sediment depth. The fill-down of numerous burrows with surface sediment by bioregeneration suggests that anaerobic degradation of fresh organic matter is dominant in this canyon. In the sedimentary column, the negative relationship between carbonate content and macrobenthic abundance confirms that carbonate dissolution is largely influenced by bioturbation.

Effects of bioturbation on cadmium transfer and distribution into freshwater sediments.

To investigate bioturbation effects on cadmium (Cd) fluxes from overlying water to sediments, indoor microcosms were developed. The bioturbating organisms were freshwater tubificid worms. Three experimental conditions were studied during 56 d. The three conditions were contaminated water column ([Cd]: 20 microg/L) with or without worms and uncontaminated water column with worms. Cadmium vertical profiles were determined in the pore water and in the sediments, based on six layers (0-0.5, 0.51, 1-2, 2-3, 3-5, 5-12 cm). Dissolved oxygen, manganese, sulfate, and particulate manganese were measured. Bioturbation was analyzed using conservative fluorescent particulate tracers. Bioturbation increased Cd flux into the sediments by close to a factor of two. Scavenging of Cd was more efficient in the bioturbated sediments because particles and adsorption sites for Cd were renewed at the sediment-water interface. Tubificids also increased the thickness of the Cd-enriched layer. Metals adsorbed on particles at the sediment surface were distributed by bioadvection, which predominated the mixing processes. Bioturbation also modified the vertical profiles of dissolved and particulate manganese and dissolved sulfate but not the profiles of dissolved oxygen. These results indicate that the advective transport of particles by bioturbation and their subsequent modification by redox reactions accelerates the trapping of metals in sediments.

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 non-local 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.

Effects of the interaction between tubificid worms on the functioning of hyporheic sediments: an experimental study in sediment columns

The main objective of this study was to quantify the importance of interactions between two genera of tubificid worms (Limnodrilus and Tubifex) on processes occurring in hyporheic sediments. The importance of interaction between tubificid worms was estimated by comparing, at a same density of 100 worms, the influence of individual genera with the combination of the two genera. We measured the effect of worms on sediment redistribution, dissolved oxygen consumption and microbial distribution within low filtration gravel-sand columns. The results showed that 100 Limnodrilus, 100 Tubifex, and the combination of the two genera (50 Limnodrilus and 50 Tubifex) all stimulated the O 2 consumption and bacterial numbers in the upper layer of sediment (from 5 cm above to 5 cm below the sediment surface). However, the combination of the two genera produced higher sediment redistribution and a higher stimulation of the number of bacteria in the top 5 cm of the sediment than Limnodrilus and Tubifex tested separately at the same density. Furthermore, the combination of the two genera produced lower oxygen consumption than the two genera tested separately in the deep layers of the sediment. Thus, the interactions within the group of tubificids played a significant role in the functioning of the first cm of the sediments and should not be neglected when assessing their roles in subsurface ecosystems.

Examination of the uncertainty in contaminant fate and transport modeling: a case study in the Venice Lagoon.

A Monte Carlo analysis is used to quantify environmental parametric uncertainty in a multi-segment, multi-chemical model of the Venice Lagoon. Scientific knowledge, expert judgment and observational data are used to formulate prior probability distributions that characterize the uncertainty pertaining to 43 environmental system parameters. The propagation of this uncertainty through the model is then assessed by a comparative analysis of the moments (central tendency, dispersion) of the model output distributions. We also apply principal component analysis in combination with correlation analysis to identify the most influential parameters, thereby gaining mechanistic insights into the ecosystem functioning. We found that modeled concentrations of Cu, Pb, OCDD/F and PCB-180 varied by up to an order of magnitude, exhibiting both contaminant- and site-specific variability. These distributions generally overlapped with the measured concentration ranges. We also found that the uncertainty of the contaminant concentrations in the Venice Lagoon was characterized by two modes of spatial variability, mainly driven by the local hydrodynamic regime, which separate the northern and central parts of the lagoon and the more isolated southern basin. While spatial contaminant gradients in the lagoon were primarily shaped by hydrology, our analysis also shows that the interplay amongst the in-place historical pollution in the central lagoon, the local suspended sediment concentrations and the sediment burial rates exerts significant control on the variability of the contaminant concentrations. We conclude that the probabilistic analysis presented herein is valuable for quantifying uncertainty and probing its cause in over-parameterized models, while some of our results can be used to dictate where additional data collection efforts should focus on and the directions that future model refinement should follow.