Phytoplankton geographic spatialization in two ponds in Limousin (France)

Phytoplankton distribution was not homogeneous among Limousin ponds and within each pond, as shown by chlorophyll a and diatoms in two representative eutrophic ponds. On a small cartographic scale, differences depended on geographic location and water temperature. Spring bloom occurred with a time-lag of several weeks in the highland pond. On a large-scale geographic distribution depended on hydroclimatic factors, especially wind direction and inflow of the rivers. The high concentrations of chlorophyll a near the dam of the water bodies roughly coincided with the maximum fetch and the direction of the dominant winds during the investigations. In both ponds the heaviest algae seemed not to be influenced by wind currents and the greater number of diatoms upstream in the highland pond was probably due to inflow of silica from the river. Phytoplankton mapping is more explanatory and rigorous than Secchi disk transparency mapping, it is useful to the regional authorities for zoning activities: bathing should be avoided in bays and along longitudinal shores, fishing location may be less restrictive. In plateau regions in general phytoplankton is not randomly distributed and spatial heterogeneity of ponds is not only vertical but also horizontal. Deep and embanked ponds are complex organisms analogous to great lakes for some features and to shallow water bodies for other ones.     

Comparison of Earth’s magnetospheric magnetic field models in the context of cosmic ray physics

Over the last two decades, models of the Earth’s magnetospheric magnetic field have been continuously improved to describe more precisely the different magnetospheric current systems (magnetopause current, symmetric and partial ring currents, tail currents and field aligned currents). In this paper we compare the different Tsyganenko models and the Alexeev and Feldstein model in the context of cosmic ray physics. We compare the vertical cutoff rigidity and asymptotic direction of vertical incidence obtained with these models for the January 20, 2005, ground level enhancement and for the big magnetic storm of April 6, 2000. For the event of January 20, 2005, we study the impact of the differences in asymptotic direction obtained with the models on the radiation dose computation at aircraft altitude. For the magnetic storm of April 6, 2000, we discuss the importance of the different magnetospheric current systems in causing cutoff rigidity variations. Finally we summarise the advantages and drawbacks of the different models in the context of space weather.     

Distributed hydrologic and hydraulic modelling with radar rainfall input: Reconstruction of the 8–9 September 2002 catastrophic flood event in the Gard region,France

On 8–9 September 2002, an extreme rainfall event caused by a stationary mesoscale convective system (MCS) occurred in the Gard region, France. Distributed hydrologic and hydraulic modelling has been carried out to assess and compare the various sources of data collected operationally and during the post-event field surveys. Distributed hydrological modelling was performed with n-TOPMODELs and assessed for ungauged basins with the discharge estimates of the post-event surveys. A careful examination of the occurrence in time and space of the flash floods over the head watersheds indicates that flooding was controlled by the trajectory of the convective part of the MCS. Stationarity of the MCS over the Gardon watershed (1858 km² at Remoulins) for 28 h was responsible for the exceptional magnitude of the flood at this scale. The flood dynamics were characterized by an extensive inundation of the Gardonnenque plain upstream of the Gardon Gorges resulting in a significant peak flow reduction downstream. One-dimensional unsteady-flow hydraulic modelling was found to be required to reproduce these dynamics. Hydraulic modelling also proved to be potentially useful for the critical analysis and extrapolation of operational discharge rating curves.     

Investigating the transport dynamics and the properties of bedload material with a hydro‐acoustic measuring system

This article deals with the following two questions. Are acoustic measurements in running waters appropriate for a highly resolved investigation of the bedload transport? Which characterizations of the bedload regarding mass and shape are possible via the acoustic signals? The signals were recorded by means of data recorders (Tascam Inc. DAP1 Portable Data Recorder) and hydrophones (International Transducer Corp. ITC‐4001 A). The ITC‐4001 is a shallow water omnidirectional transducer containing a flexural disc transducer utilizing Channelite‐5400 ceramics mounted in a rugged corrosion‐resistant housing. These hydrophones were screwed onto the bottom side of stainless steel plates, serving as a contact surface for the bedload in motion above them. After more than 100 series of tests in the laboratory, which indicated the basic relations between the dimension, shape and weight of the bedload and the resulting signal, field tests of the measuring system were conducted. By artificially produced flood waves in the small brooks Riverisbach, Olewiger Bach and by a winter flood wave in the River Moselle, it is possible to elaborate similar structures of the signal course of the bedload movement. The highest transport rates can be observed at the beginning of the increasing limbs and behind the peaks of the waves. At the beginning of the waves, the increasing transport power of the water and the loose material can be considered as the cause for this result. The high stream velocity behind the wave peaks explains the increase in the bedload transport so that material from the channel beds is unfastened and will be mobilized. The characterization of the bedload regarding the shape and mass is still limited regarding the field measurements and could be solved only for homogeneous grain sizes and single stones under laboratory conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Long-term fluvial archives in NW France: response of the Seine and Somme rivers to tectonic movements, climatic variations and sea-level changes

The Seine and the Somme are the two main rivers flowing from northwestern France into the Channel. During the Pleistocene cold stages both rivers were tributaries of the River Manche which was exporting sediments into the central deeps of the Channel. The River Seine has a very well developed terrace system recording incision that began at around 1 Ma. The same age is proposed for the beginning of the main incision in the Somme Valley on the basis of morphostratigraphy, pedostratigraphy, palaeontology, palaeomagnetism and ESR datings. The uplift rate deduced from analysis of the Seine and Somme terrace systems is of 55 to 60 m/Ma since the end of the Lower Pleistocene. The response of the two rivers to climatic variations, uplift and sea-level changes is complex and variable in the different parts of the river courses. For example, the evolution of the lower Seine system is influenced by uplift and climate changes but dominated by sea-level changes. In the middle Seine the system is beyond the impact of sea-level variations and shows a very detailed response to climatic variations during the Middle and Upper Pleistocene in a context of uplift. The Somme Valley response appears to be more homogeneous, especially in the middle valley, where the terrace system shows a regular pattern in which incision occurs at the beginning of each glacial period against a general background of uplift. Nevertheless, the lower Somme Valley and the Palaeo-Somme in the Channel area indicate some strong differences compared with the middle valley: influence of sea-level variations and probably differences in rates of tectonic uplift between the Channel and the present continent. The differences in the responses of the two river valleys during the Pleistocene are related to differences in the size of the fluvial basins, to the local tectonic characteristics, to the geometry of the platform connected to the lower parts of the valleys and to the hydrodynamic characteristics of each river. Finally, it is shown from these examples that the multidisciplinary study of Pleistocene rivers is a very efficient tool for the investigation of neotectonic activity.     

Forecasting search areas using ensemble ocean circulation modeling

We investigate trajectory forecasting as an application of ocean circulation ensemble modeling. The ensemble simulations are performed weekly, starting with assimilation of data for various variables from multiple sensors on a range of observational platforms. The ensemble is constructed from 100 members, and member no. 1 is designed as a standard (deterministic) simulation, providing us with a benchmark for the study. We demonstrate the value of the ensemble approach by validating simulated trajectories using data from ocean surface drifting buoys. We find that the ensemble average trajectories are generally closer to the observed trajectories than the corresponding results from a deterministic forecast. We also investigate an alternative model in which velocity perturbations are added to the deterministic results and ensemble mean results, by a first-order stochastic process. The parameters of the stochastic model are tuned to match the dispersion of the ensemble approach. Search areas from the stochastic model give a higher hit ratio of the observations than the results based on the ensemble. However, we find that this is a consequence of a positive skew of the area distribution of the convex hulls of the ensemble trajectory end points.     

Phytolith indices as proxies of grass subfamilies on East African tropical mountains

The main objective of this paper is to provide researchers that investigate fossil phytolith assemblages and model/data comparisons a new tool for estimating C₃/C₄ grass composition over time. We tested the reliability of modern soil phytolith assemblages and phytolith indices for tracing the dominance of different grass subfamilies and tree cover density. We analyzed modern soil phytolith assemblages from sites over elevation gradients on Mount Kenya (Kenya), Mount Rungwe and around Lake Masoko (southern Tanzania). These data were compared with available botanical data. A phytolith index named Ic, proved to be an effective proxy of the proportions of Pooideae, Arundinoideae and Bambusoideae grasses (mainly C₃ grasses) versus Panicoideae grasses (mainly C₄ grasses), increasing with elevation in East-Africa. When tropical mountains are covered by open habitats (e.g. grasses and shrublands), Ic should be a reliable proxy of the C₃/C₄ grass composition. These results highlight the value of the phytolith index Ic, when interpreting paleo-environmental records from tropical mountains, to: 1) better understand past local and regional C₃/C₄ grass distributions and associated climatic changes and 2) increase the set of C₃/C₄ data available for model/data comparisons.