Jean-Michel Jaquet

A GIS approach of aquatic plant spatial heterogeneity in relation to sediment and depth gradients, Lake Geneva, Switzerland

A georeferenced database was constructed for a section of the littoral zone of Lake Geneva in order to investigate aquatic plant spatial heterogeneity at a community level, by exploring the relationships between species distribution and: (i) depth and sediment characteristics; and (ii) plant traits. This database includes information layers on vegetation, sediment and bathymetry. Vegetation was mapped from a digital photo interpretation coupled with field observation of species distribution and cover index. An abundance index was calculated for each species by multiplying the surface area of each stand by the species relative percentage and its cover index. Bathymetry was established from echosounder profiles and sediment maps were obtained from point interpolation between sediment core samples. Measured sediment characteristics include texture, nutrient and organic matter contents. Multiple regression models were developed in a related article (Lehmann et al., 1997) in order to estimate species biomass and maximum shoot length from depth and sediment characteristics. These models are applied in this article between the corresponding GIS layers to give a spatial estimation of these plant traits for three species of Potamogeton. Macrophyte stands, wherein a given species was dominant at more than 80%, were selected by a spatial selection together with the underlying depth and sediment characteristics and the estimated plant traits. When compared, P. lucens L. appeared to have the best competitive ability in relation to the observed plant traits, but it was dominated by a species more tolerant to wave disturbance in shallower depth (P. pectinatus L.) and a species more tolerant to the stress of light attenuation and nutrient availability in deeper sites (P. perfoliatus L.).

The manganese cycle in Lac Léman, Switzerland: the role of Metallogenium

The manganese pathways in Lac Leman have been investigated on the basis of chemical analyses undertaken on water, suspended solids, bottom sediments and interstitial water samples.

Contribution of GIS to submerged macrophyte biomass estimation and community structure modeling, Lake Geneva, Switzerland

Abstract This works is part of a study that aimed to develop a method for qualifying the littoral ecosystem of lakes, through the application of a Geographical Information System (GIS). It is centered on the biomass of three dominant species of submerged aquatic plant in Lake Geneva. The first part deals with changes in shoot biomass during the growing period (April to October 1991), using sampling quadrats of 0.25 m 2 . Potamogeton lucens L. presented the longest vegetation period (198 days), followed by Potamogeton perfoliatus L. (171 days) and Potamogeton pectinatus L. (127 days). The maximum biomass was reached by P. lucens with a dry weight of 325 gm −2 on the 136th day, followed by P. perfoliatus (196 g m −2 , 118th day) and P. pectinatus (150 g m −2 , 95th day). The relationship between the index of vegetation density and biomass appears to be linear for the three species. However, the relationship between depth and biomass can be fitted by a parabola with observed maxima of 215 g m −2 at 2.6 m depth for P. pectinatus , 359 g m −2 at 3.1 m for P. lucens , and 228 g m −2 at 3.8 m for P. perfoliatus . These results, together with aerial photography interpretation and echosounder survey were, used to estimate the spatial and temporal distribution of biomass by means of the GIS. Finally, this work present a three-dimensional model of the littoral zone, based on a prespective view of GIS information layers draped over a digital elevation model. It gives a mean to visualize the architectural changes that occurred throughout the vegetation season. These changes are caused by the growth, blooming and death of the plants, and result in important modifications in the ecosystem spatial structure which, in turn, influences the physical and chemical balance and life cycle of many animals and plants. It appears that GIS represents an interesting and powerful tool for the study and management of lake littoral ecosystems.

BASIC CONCEPTS AND ASSOCIATED STATISTICAL METHODOLOGY IN GEOCHEMICAL STUDY OF LAKE SEDIMENTS

On the basis of data collected in Lac Leman (Switzerland), we discuss a few of the fundamental concepts that underlie geochemical investigations: the carrier substance, the natural background level, the grain-size effects and the areal variability of geochemical associations.

Multi-scale thermal pattern monitoring of a large lake (Lake Geneva) using a multi-sensor approach

The applicability of satellite imagery products from different sensors (AVHRR‐derived multi‐channel sea surface temperature (MCSST), MODIS sea surface temperature (SST) products 5‐Min L2 Swath 1 km and Landsat TM band 6 thermal signature) for the comprehensive monitoring of temperature and its temporal patterns over a large lake is tested in this study. The coverage of cloud‐free satellite data for Lake Geneva is reported throughout a year and, more specifically, during a 13 day period in summer 2003. In a second step, we demonstrate the feasibility of the AVHRR/MODIS imagery to discern day and night temperature patterns, by generating day and night climatologies and various spatial statistics over the 13 day period. The different day and night surface thermal patterns observed by satellite imagery could be linked to the thermal structure existing in deeper water using the concept of the diurnal decoupled layer. The forcing of the persistent patterns, two warm cores divided by a saddle‐shaped cold anomaly, is explained by wind periodicity and insolation conditions. The patterns can be matched to features postulated by findings of different limnologists in the past. Other surface temperature related phenomena such as water upwelling and downwelling and the occurrence of plumes are related to meteorological and hydrological events. The lakewide average lake surface water temperature (LSWT) trends for day and night during the study period are roughly parallel. A sudden loss of stored heat can be explained by episodes of long fetch, synoptic wind (bise) that interrupted the predominant breeze regime.

Contribution of mathematical modeling to lake ecosystem understanding: Lake Bourget (Savoy, France)

This paper demonstrates how mathematical modeling can contribute to improve understanding of lake behavior. Since the 60s Lake Bourget, one of the largest in France, had been suffering from eutrophication which was checked in 1980 by the diversion of the main sewers entering the lake. A research program was implemented between 1987 and 1990, including an on-site sampling campaign conducted concurrently with thermal and biogeochemical modeling of lake behavior. The model helped provide a better understanding of the ecosystem, displaying some processes hitherto misunderstood: (1) Winter overturn does not reach the bottom of the water column when the weather is mild. This leads to a incomplete reoxygenation of the hypolimnion and to redox conditions inducing the release of orthophosphate from the sediment, (2) Grazing by herbivorous zooplankton is getting more important in the control of spring algal growth as eutrophication of the lake regresses, (3) Settling of particulate phosphorus seems a complex and very important process in Lake Bourget, showing high sedimentation rates for particulate mineral phosphorus.

Marine and freshwater micropearls: Biomineralization producing strontium-rich amorphous calcium carbonate inclusions is widespread in the genus Tetraselmis (Chlorophyta)

Unicellular algae play important roles in the biogeochemical cycles of numerous elements, particularly through the biomineralization capacity of certain species (e.g., coccolithophores greatly contributing to the “organic carbon pump” of the oceans), and unidentified actors of these cycles are still being discovered. This is the case of the unicellular alga Tetraselmis cordiformis (Chlorophyta) that was recently discovered to form intracellular mineral inclusions, called micropearls, which had been previously overlooked. These intracellular inclusions of hydrated amorphous calcium carbonates (ACCs) were first described in Lake Geneva (Switzerland) and are the result of a novel biomineralization process. The genus Tetraselmis includes more than 30 species that have been widely studied since the description of the type species in 1878. The present study shows that many other Tetraselmis species share this biomineralization capacity: 10 species out of the 12 tested contained micropearls, including T. chui, T. convolutae, T. levis, T. subcordiformis, T. suecica and T. tetrathele. Our results indicate that micropearls are not randomly distributed inside the Tetraselmis cells but are located preferentially under the plasma membrane and seem to form a definite pattern, which differs among species. In Tetraselmis cells, the biomineralization process seems to systematically start with a rod-shaped nucleus and results in an enrichment of the micropearls in Sr over Ca (the Sr/Ca ratio is more than 200 times higher in the micropearls than in the surrounding water or growth medium). This concentrating capacity varies among species and may be of interest for possible bioremediation techniques regarding radioactive 90Sr water pollution. The Tetraselmis species forming micropearls live in various habitats, indicating that this novel biomineralization process takes place in different environments (marine, brackish and freshwater) and is therefore a widespread phenomenon.

Linking environmental observations and solid solution thermodynamic modeling: the case of Ba- and Sr-rich micropearls in Lake Geneva

Abstract Intracellular inclusions of amorphous Ba- and Sr-rich calcium carbonates – referred to as “micropearls”– have recently been detected in Lake Geneva. These micropearls are formed under conditions of pronounced Ba and Sr undersaturation in the lake waters. Their formation can be explained by the ability of certain microorganisms to preconcentrate these trace elements in tandem with a non-equilibrium solid-solution growing mechanism.