Bacterial clogging of porous media: A new modelling approach

A comparison is made between existing mathematical models and experimental data that relate the reduction of the saturated hydraulic conductivity (K) of a porous medium to the porosity reduction caused by microbial growth. The models yielded a realistic prediction of a data set obtained with a model porous medium consisting of millimeter‐size glass spheres, but failed to predict the clogging behaviour observed in smaller‐than‐1‐mm sand. A new modelling approach, semi‐mechanistic in nature, is proposed that gives good predictions of fine sand media as well. It relaxes the assumption about uniformly‐thick biofilms by allowing a second arrangement to occur, i.e. discrete plugs filling the pore lumen. The new model requires input data on two intrinsic properties of the system, which renders it sufficiently flexible as to fit very different data sets. The two model parameters are K_min, the minimum K value when all porosity is filled with microorganisms, and B_c, the biovolume fraction at which most cell detachment from biofilm occurs.     

Decrease of the hydraulic conductivity of sand columns by Methanosarcina barkeri

The extent to which a methanogen can clog sand columns was examined: two permeameters packed with clean quartz sand were sterilized, saturated with water, inoculated with Methanosarcina barkeri and percolated under upward flow conditions. After approx. 5 months, the hydraulic conductivity of the sand had decreased to 3% and 25% of the highest values measured earlier. At that point, gas-filled regions in the sand were clearly visible through the transparent walls of the permeameters, and methane bubbles were continuously released from the columns into the effluent. Scanning electron microscopy observations and biomass assays indicated that cell mass accumulation did not contribute significantly to the observed decrease of the hydraulic conductivity. This decrease was therefore attributed to pore blocking due to the entrapment of methane bubbles.D. Sanchez de Lozada and P. Baveye are with the Department of Soil, Crop and Atmospheric Sciences, Bradfield Hall, Cornell University, Ithaca, NY 13853, USA; P. Vandevivere is with the College of Marine Studies, University of Delaware, Lewes, DE 19958, USA. S. Zinder is with the Department of Microbiology, Rice Hall, Cornell University, Ithaca, NY 14853, USA.     

The use of Bayesian networks to guide investments in flow and catchment restoration for impaired river ecosystems

1.  The provision of environmental flows and the removal of barriers to water flow are high priorities for restoration where changes to flow regimes have caused degradation of riverine ecosystems. Nevertheless, flow regulation is often accompanied by changes in catchment and riparian land-use, which also can have major impacts on river health via local habitat degradation or modification of stream energy regimes.
2.  The challenges are determining the relative importance of flow, land-use and other impacts as well as deciding where to focus restoration effort. As a consequence, flow, catchment and riparian restoration efforts are often addressed in isolation. River managers need decision support tools to assess which flow and catchment interventions are most likely to succeed and, importantly, which are cost-effective.
3.  Bayesian networks (BNs) can be used as a decision support tool for considering the influence of multiple stressors on aquatic ecosystems and the relative benefits of various restoration options. We provide simple illustrative examples of how BNs can address specific river restoration goals and assist with the prioritisation of flow and catchment restoration options. This includes the use of cost and utility functions to assist decision makers in their choice of potential management interventions.
4.  A BN approach facilitates the development of conceptual models of likely cause and effect relationships between flow regime, land-use and river conditions and provides an interactive tool to explore the relative benefits of various restoration options. When combined with information on the costs and expected benefits of intervention, one can derive recommendations about the best restoration option to adopt given the network structure and the associated cost and utility functions.     

The response of invertebrates to a gradient of flow reduction – an instream channel study in a New Zealand lowland river

1. The impacts of low flows on invertebrates were assessed in six experimental channels placed in a lowland stream. Each channel consisted of replicate stacked gravel‐filled baskets (buried 0–7, 8–14 and 15–21 cm deep) from which invertebrates were sampled. Samples were collected before, and 1 and 2 months after flows were reduced by varying amounts. We predicted that invertebrate communities would change most with the lowest flows that persisted longest. The effects of reduced flow on channel and intragravel‐velocities, temperature, dissolved oxygen and algal biomass were also monitored. 2. Mean velocity (17 cm s⁻¹) and depth (20 cm) were similar in all channels prior to flow reduction, but were reduced in five channels by 25% to 98%, a sixth channel serving as a control. Mean intragavel velocity prior to flow reduction (1.7 mm s⁻¹) was also reduced from 17% to 51%. Flow reduction had no effect on temperature, which ranged from 11.2 °C (night) to 19.6 °C (day). Dissolved oxygen was generally high (mean = 94% saturation) but variable (range from 50% to 145%), and did not differ between channels after flow reduction. Periphyton biomass (chlorophyll a and AFDM_algae) increased over time and was positively related to velocity after 1 month of flow reduction. After 2 months, however, this relationship reversed, with higher chlorophyll and AFDM_algae in lower velocity channels. Organic matter biomass (AFDM_total) was significantly higher in channels with lower velocities, and did not change with time. There was no relationship between AFDM_total and velocity in baskets from different depths. 3. Greater than 85% of animals were found in the upper baskets (0–7 cm deep), and flow reduction had no influence on the vertical distribution of invertebrates. After 1 month, invertebrate density had declined roughly in proportion to the magnitude of flow reduction in all channels where flows were reduced more than 25%. Densities recovered to pre‐reduction levels within 2 months. The amphipod, Paracalliope fluviatilis dominated all channels numerically prior to flow reduction, but its density declined markedly during the study. Densities of Ostracoda, Oxyethira albiceps and Cladocera increased dramatically after 2 months of flow reduction, especially in the lowest flow channels. Of the physical variables measured, chlorophyll a biomass and discharge best explained the temporal changes in the invertebrate community. 4. The effects of increased duration and magnitude of flow reduction on invertebrate communities were restricted to changes in the relative abundances of just a few taxa. Our results suggest that invertebrates in this lowland stream were resistant to the experimental flow reduction, presumably because of their broad ecological tolerances. We also found that more prolonged low flows did not result in predictable changes. This finding may have implications in terms of using hydraulic‐habitat models to set minimum flows in lowland streams if invertebrates can persist at flows much lower than their supposed optima.     

The organic content of the surficial sediment: a method for the study of ecosystems development in abandoned river channels

To characterise different abandoned river channels, a simple index of the surface sediment was developed i.e. the content of organic matter (expressed as C × N) plotted in relation to that of CaCO₃. The age of the channels studied ranges from 20 to 300 years. Some of them still contain water; others are silted up. Two types are distinguished. The ecosystems of the first one are closed and show a slow rate of development governed by autogenous processes. Those of the second type are more open and show a fast rate of development mainly controlled by allogenous processes. These distinctions are used in a diagrammatic model of the dynamics of Rhône River alluvial plain to be used in fundamental or applied future research.