Florian Mermillod-Blondin

Ecosystem engineering: the impact of bioturbation on biogeochemical processes in marine and freshwater benthic habitats

In aquatic ecosystems, invertebrate bioturbation significantly influences microbial activities and biogeochemical processes in sediments by modifying water and sediment fluxes at the water-sediment interface. We apply the concept of ecosystem engineering to develop a qualitative general understanding of the role of bioturbation on microbial processes in different benthic environments. We hypothesized that the effects of the bioturbation mode (sediment reworking, biogenic structure building, bioirrigation) on microbial processes vary between diffusion- and advection-dominated benthic environments because bioturbation does not have the same influences on hydrological exchanges (and the flux of resources for micro-organisms living in sediments) at the water-sediment interface of the two systems. To test this hypothesis, we experimentally compared the influence of three bioturbation modes (fine-sediment reworking, U-shaped structure burrowing, and gallery-network burrowing) in a diffusion-dominated system (fine sediments/low interstitial flow rates) and an advectiondominated system (coarse sediments/advection of water in sediments). Our analysis demonstrated that bioturbation modes in the two systems had different impacts on microbial activities. For instance, U-shaped tube burrowing by animals increased O2 consumption in the diffusion- dominated system but produced the opposite effect in the advection-dominated system. The influence of bioturbation was also negatively related to interstitial flow rate, the bioturbation having a higher influence on O2 consumption in the diffusion-dominated system than in the advection-dominated system. According to our hypothesis, bioturbation modified microbial processes in sediments depend on the hydrological characteristics of the system. In the diffusion-dominated system, invertebrate bioturbation can produce water fluxes at the watersediment interface that may strongly influence microbial processes in sediments. In contrast, in the advectiondominated system, invertebrate bioturbation can only modify the water circulation patterns in sediments, moderately affecting microbial processes. Consequently, it is necessary to use a conceptual framework which takes into account the features of sediment habitats in order to allow a better prediction of bioturbation effects on sediment biogeochemistry. With this aim, the conceptual scheme of ecosystem engineers can be an organizing principle to integrate the complex relationships among physical habitat, bioturbation mode and microbial activity.

The functional significance of bioturbation and biodeposition on biogeochemical processes at the water–sediment interface in freshwater and marine ecosystems

Abstract Benthic invertebrates have important ecosystem engineering functions (bioturbation and biodeposition) in freshwater and marine benthic systems. Bioturbation and biodeposition affect the metabolism of the water–sediment interface through modification of water–sediment fluxes or organic-matter enrichment of sediments by biodeposits. The functional significance of these processes depends strongly on the type of invertebrate activities (the functional traits of the invertebrates) and on the modulation of this activity by environmental conditions. The aim of my article is to propose a common framework for the role of bioturbation/biodeposition in benthic habitats of both marine and freshwater environments. In these ecosystems, hydrological exchanges between the water and sediments (interstitial flow rates) control the microbial activity inside sediments. The ability of ecosystem engineers to influence benthic microbial processes differs strongly between diffusion-dominated (low interstitial flow rates) and advection-dominated (high interstitial flow rates) habitats. Bioturbation/biodeposition may play a role in diffusion-dominated habitats where invertebrates can significantly modify water and particle fluxes at the water–sediment interface, whereas a slight influence of ecosystem engineers is expected in advection-dominated habitats where fluxes are predominantly controlled by hydrological processes. A future challenge will be to test this general framework in marine and freshwater habitats by quantifying the interactions between the functional traits of species and the water–sediment exchanges.

Assessment of the spatial variability of intertidal benthic communities by axial tomodensitometry: importance of fine-scale heterogeneity

At the water–sediment interface of aquatic ecosystems, the presence of biogenic structures produced by benthic invertebrates strongly affects biogeochemical processes. The quantification of these structures and the assessment of the vertical distribution of fauna are essential for determining the impact of communities in sediments. In the present study, computer axial tomodensitometry (CAT-scan) was used to measure the space occupied by an intertidal community of the St. Lawrence estuary. Three cores were sampled at a site that was considered homogeneous according to surface sediments. The vertical distribution of biogenic structures and gravel were measured in the three cores using CAT-scan; the vertical distribution of fauna was also analysed for each core. The biogenic structures were highest at the water–sediment interface and decreased with depth in the three cores. The number of invertebrates also decreased with depth. We observed similar distributions of biogenic structures in cores 1 and 2. However, fewer biogenic structures were observed below 90 mm in core 3. This result was correlated with a high quantity of gravel from 90 to 140 mm in core 3 whereas the other cores had lower quantities of coarse material. We found relationships among the distributions of biogenic structures, fauna, and sediment characteristics (gravel quantity) that can affect species distribution. The vertical distributions of Macoma balthica, Mya arenaria, Nereis virens, and small-sized gallery-producing species (nematodes and oligochaetes) could also be recorded with the CAT-scan method. Thus, CAT-scan is an excellent tool to determine the fine-scale heterogeneity in the space occupied by benthic invertebrates in sediments.

Ecosystem engineering at the sediment-water interface: bioturbation and consumer-substrate interaction.

In soft-bottom sediments, consumers may influence ecosystem function more via engineering that alters abiotic resources than through trophic influences. Understanding the influence of bioturbation on physical, chemical, and biological processes of the water–sediment interface requires investigating top-down (consumer) and bottom-up (resource) forces. The objective of the present study was to determine how consumer bioturbation mode and sediment properties interact to dictate the hydrologic function of experimental filtration systems clogged by the deposition of fine sediments. Three fine-grained sediments characterized by different organic matter (OM) and pollutant content were used to assess the influence of resource type: sediment of urban origin highly loaded with OM and pollutants, river sediments rich in OM, and river sediments poor in OM content. The effects of consumer bioturbation (chironomid larvae vs. tubificid worms) on sediment reworking, changes in hydraulic head and hydraulic conductivity, and water fluxes through the water–sediment interface were measured. Invertebrate influences in reducing the clogging process depended not only on the mode of bioturbation (construction of biogenic structures, burrowing and feeding activities, etc.) but also on the interaction between the bioturbation process and the sediments of the clogging layer. We present a conceptual model that highlights the importance of sediment influences on bioturbation and argues for the integration of bottom-up influence on consumer engineering activities.

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.

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.

Chironomid larvae stimulate biogeochemical and microbial processes in a riverbed covered with fine sediment

Abstract.We determined if the influence of chironomid larvae on biogeochemical and microbial processes in fine surface sediments of a riverbed depended on the characteristics of the sedimentary habitat. The influence of chironomids was measured on sediment reworking, biogeochemical processes and bacterial characteristics in infiltration sediment columns having three different surface sediments (with different organic matter characteristics). The results showed that chironomids exhibited similar bioturbation activities (construction and irrigation of U-shaped tubes in the upper layers of sediments) and significantly stimulated O2 uptake and percentage of active bacteria in all sediment treatments. In contrast, the release of NH4+ in anoxic sediment layers was strongly increased by chironomids in the presence of organicrich sediments, whereas they had no effect on this process with organic-poor sediments. Our results indicated that the influence of chironomids on biogeochemical processes in river bed sediments are linked to the redox conditions of the system. Moreover, when the effects of chironomid larvae are compared to those measured with tubificid worms in the same experimental conditions, we conclude that the mode of bioturbation (sediment reworking, biogenic structure building, burrowing depth, bioirrigation) can also determine the impact of benthic animals on microbial activities and biogeochemical processes in the sediment.

A Comparison of Two Ultrasonic Methods for Detaching Biofilms from Natural Substrata

Two types of ultrasound applications are commonly used in order to remove bacteria from sediment for subsequent direct enumeration: ultrasonic baths and narrow tip ultrasonic generators. By measuring four parameters (total number of bacteria, number of ETS-active bacteria, amount of proteins and weight of fine sediment obtained in sonicated juices), we compared the biofilm removal of the optimal ultrasound exposure time previously obtained using the ultrasonic bath with the removal by a method using a narrow tip ultrasonic generator. To obtain comparable removal efficiencies estimated by protein contents in sonicated juices, the ultrasonic bath method required an ultrasound exposure time more than 10 times that with a narrow tip ultrasonic generator. Furthermore, the two methods provided significantly different bacterial counts because of an alteration of the sediment with the ultrasonic bath. Thus, a narrow tip ultrasonic generator is more suitable than an ultrasonic bath for the analysis of biofilms developed on sand.

Leaf litter recycling in benthic and hyporheic layers in agricultural streams with different types of land use.

Changes in land use and intensification of agricultural pressure have greatly accelerated the alteration of the landscape in most developed countries. These changes may greatly disturb the adjacent ecosystems, particularly streams, where the effects of pollution are amplified. In this study, we used the leaf litter breakdown rate to assess the functional integrity of stream ecosystems and river sediments along a gradient of either traditional extensive farming or a gradient of vineyard area. In the benthic layer, the total litter breakdown process integrates the temporal variability of the anthropogenic disturbances and is strongly influenced by land use changes in the catchment even though a low concentration of toxics was measured during the study period. This study also confirmed the essential role played by amphipods in the litter breakdown process. In contrast, microbial processes may have integrated the variations in available nutrients and dissolved oxygen concentrations, but failed to respond to the disturbances induced by vineyard production (the increase in pesticides and metal concentrations) during the study period. The response of microbes may not be sensitive enough for assessing the global effect of seasonal agricultural practices. Finally, the leaf litter breakdown measured in the hyporheic zone seemed mainly driven by microbial activities and was hence more affected by vertical exchanges with surface water than by land use practices. However, the breakdown rate of leaf litter in the hyporheic zone may constitute a relevant way to evaluate the impact on river functioning of any human activities that induce massive soil erosion and sediment clogging.

Influence of hydraulic conductivity on communities of microorganisms and invertebrates in porous media: a case study in drinking water slow sand filters

Abstract.The impact of reduced hydraulic conductivity on the abundance and diversity of microorganisms and invertebrates was examined in an artificial ecosystem consisting of a slow sand-filter. Sand-filters processed pre-treated lake water under high flow rates and acted as small ecosystems inhabited by a complex community. The first trophic level consisting of microorganisms serves as a food source for a dense community of protists, micro- and macro-invertebrates. The reduction of hydraulic conductivity due to the development of larger bacterial and fungal biomass induced a shift of the microbial community towards anaerobiosis that may increase clogging by carbonate precipitation. The presence of more bacterial prey seems to favour the development of higher trophic levels. Predation and bioturbation by eukaryotes were not able to counteract the reduction of hydraulic conductivity due to prokaryotic clogging.