An acanthocephalan parasite increases the salinity tolerance of the freshwater amphipod <Emphasis Type="Italic">Gammarus roeseli</Emphasis> (Crustacea: Gammaridae)

Studies of the influence of parasites on host fitness generally conclude that parasites have a strong negative effect on their hosts. In this study, we have investigated experimentally the role of Polymorphus minutus, an acanthocephalan parasite, on the salinity tolerance of the freshwater amphipod Gammarus roeseli, one of its intermediate hosts. Unexpectedly, P. minutus-infected gammarids were more tolerant to salinity stress than uninfected ones. The mean lethal salt concentrations for 50% mortality of hosts tested were 17.3 (infected) and 9.7 g/L (uninfected). The parasitic load (one or two parasites per host) did not affect the result. The size of hosts had no significant influence on the salinity tolerance of either infected or uninfected gammarids. The mobility of all types of gammarid decreased when the salinity exceeded 9.0 g/L, but there was no significant difference between infected and uninfected gammarids. We discuss the higher salinity tolerance of infected amphipods in relation to O₂ consumption and osmoregulation. Finally, we demonstrate that the salinity tolerance is enhanced in the parasitized amphipod but without a significant change in behavior or an osmoregulatory adjustment.     

Vegetation dynamics after compost amendment in a Mediterranean post-fire ecosystem

On Mediterranean calcareous soils, high fire frequency induces soil impoverishment and the development of stable Quercus coccifera garrigues. Organic amendment could increase soil fertility levels, and could alter the vegetation dynamics and the established dominance relationships. In this study, the plant cover evolution is monitored in an amended burnt shrub during two years. Three treatments are studied: control (D0), 50 t ha⁻¹ (D50) and 100 t ha⁻¹ (D100) of fresh co-composted sewage sludge and greenwastes.First, the spreading process leads to the squashing of the vegetation, whose intensity reaches a threshold on D100 and limits the woody species’ recovery after amendment. Consequently, the dominant herb Brachypodium retusum is favoured. On D50, woody species are favoured compared to herbs, probably due to the space colonization strategy of Q. coccifera after squashing. Thus, compost at both rates favours the two dominant resprouter species until they have colonized all the free space created by squashing.Secondly, compost has some fertilizing effects on Rosmarinus officinalis and Ulex parviflorus, that improves their cover on D50. However, compost also increases U. parviflorus’ sensitivity to drought.At the end of the experiment, a third compost effect appears, as seeder cover becomes greater than D0 on D50. After amendment, compost covers rocky stones, which creates a new territory where species with superficial rooting can establish (mulch effect).     

Laboratory investigation of <Emphasis Type="Italic">Fallopia</Emphasis> × <Emphasis Type="Italic">bohemica</Emphasis> fruits dispersal by watercourses

Seed and fruit dispersal along watercourses favours the long-distance migration of invasive species, not only for aquatic or wetland species, but also for terrestrial wind-dispersed plants, like the Japanese knotweed. The present paper aims at investigating the role of watercourses in the dispersal of the knotweed due to its frequent occurrence on riverbanks and production of fertile achenes (type of fruit of the Japanese knotweed). This dispersal occurs along two steps after the fruits deposit on the water surface: floatation first and then sinking towards the bottom of the watercourse. Regarding the first step, the effects of agitation of the water, temperature, surface tension and luminosity on the achenes floatability are experimentally studied. While no influence of luminosity is observed, an increase of temperature greatly decreases the floating time. Floating time also decreases as the contact between water and the fruit is enhanced (through submersion of achenes, agitation of the water or lower surface tension). Regarding the second step, the fall velocity of the fruits in water at rest is measured and appears to be independent of the seed history (floating time). 3D helical motions are systematically observed with constant tangential velocity with respect to the falling velocity. The trajectory of the fruits in a shear flow is then measured and the evolution of their velocity components along the sinking process is discussed. Finally, the contribution of both steps to the long-distance migration of the seeds is estimated.     

Mixing layer in open-channel junction flows

When two open-channel flows merge in a three-branch subcritical junction, a mixing layer appears at the interface between the two inflows. If the width of the downstream channel is equal to the width of each inlet channel, this mixing layer is accelerated and is curved due to the junction geometry. The present work is dedicated to simplified geometries, considering a flat bed and a \(90^{\circ }\) angle where two configurations with different momentum ratios are tested. Due to the complex flow pattern in the junction, the so-called Serret–Frenet frame-axis based on the local direction of the velocity must be employed to characterize the flow pattern and the mixing layer as Cartesian and cylindrical frame-axes are not adapted. The analysis reveals that the centerline of the mixing layer, defined as the location of maximum Reynolds stress and velocity gradient, fairly fits the streamline separating at the upstream corner, even though a slight shift of the mixing layer towards the center of curvature is observed. The shape of the mixing layer appears to be strongly affected by the streamwise acceleration and the complex lateral confinement due to the side walls and the corners of the junction, leading to a streamwise increase of the mean velocity along the centerline and a decrease of the velocity difference. This results in a specific streamwise evolution of the mixing layer width, which reaches a plateau in the downstream region of the junction. Finally, the evaluation of the terms in the Reynolds-Averaged-Navier–Stokes equations reveals that the streamwise and normal acceleration and the pressure gradient remain dominant, which is typical of accelerated and rotational flows.     

A Method to Assess and Define Environmental Flow Rules for Large Jurisdictional Regions1

Snelder, Ton, Doug Booker, and Nicolas Lamouroux, 2011. A Method to Assess and Define Environmental Flow Rules for Large Jurisdictional Regions. Journal of the American Water Resources Association (JAWRA) 47(4):828‐840. DOI: 10.1111/j.1752‐1688.2011.00556.x Abstract: Hydrological rules of thumb are used across jurisdictional regions to set minimum flows and allocation limits that apply by default (i.e., when more detailed site‐scale studies have not been carried out). Uniform rules do not account for spatial variation in environmental characteristics, resulting in inconsistent consequences for the protection of ecosystems, and the reliability of water resources. We developed a method for assessing hydrological rules of thumb that describes their consequences for protection of the ecosystem (in terms of retention of physical habitat) and the reliability of the water resource. The method links regionalized flow duration curves, at‐station hydraulic geometry, and generalized physical habitat models to make assessments at many locations across a region. The method estimates, for a given set of rules, the retained physical habitat for specified taxa/life stages and the proportion of the time abstractions are restricted. We applied the method to assess a set of rules that are proposed as default minimum flows and allocation limits for New Zealand rivers. The assessment showed that the minimum flow rules had variable consequences. The method could be used to quantify the tradeoff between environmental protection and water resources availability and reliability.     

On the Performance and Applicability of Nonlinear Two-Equation Turbulence Models for Urban Air Quality Modelling

It is well known that the commonly used k- turbulence models yield inaccurate predictions for complex flow fields. One reason for this inaccuracy is the misrepresentation of Reynolds stress differences. Nonlinear turbulence models are capable to overcome this weakness while being not considerably more complex. However no comprehensive studies are known which analyze the performance of nonlinear turbulence models for three-dimensional flows around building-shaped structures. In the present study the predictions of the flow around a surface-mounted cube using three nonlinear two-equation turbulence models are discussed. The results are compared with predictions of the standard k- turbulence model and wind tunnel measurements. It is shown that the use of nonlinear turbulence models can be beneficial in predicting wind flows around buildings.