Jean-Bernard Lachavanne

Does size matter? The relationship between pond area and biodiversity

Larger areas support more species. To test the application of this biogeographic principle to ponds, we consider the relationship between size and diversity for 80 ponds in Switzerland, using richness (number of species) and conservation value (score for all species present, according to their degree of rarity) of aquatic plants, molluscs (Gastropoda, Sphaeriidae), Coleoptera, Odonata (adults) and Amphibia. Pond size was found to be important only for Odonata and explained 31% of the variability of their species richness. Pond size showed only a feeble relationship with the species richness of all other groups, particularly the Coleoptera and Amphibia. The weakness of this relationship was also indicated by the low z-values obtained (< 0.13). The SLOSS analyses showed that a set of ponds of small size has more species and has a higher conservation value than a single large pond of the same total area. But we also show that large ponds harbour species missing in the smaller ponds. Finally, we conclude that in a global conservation policy (protection, restoration, management), all size ranges of ponds should be promoted.

Relationship between biomass and surface area of six submerged aquatic plant species

Mathematical relationships relating the biomass to the surface area of Elodea canadensis Michx., Myriophyllum spicatum L., Nitellopsis obtusa (Desv.) J.Gr., Potamogeton lucens L., Potamogeton pectinatus L. and Potamogeton perfoliatus L. were developed. The methods leading to these relations are presented for plants from Lake Geneva (Switzerland). Results include three sets of data for each species: leaves, stems and whole plant. Differences were evidenced according to plant species; for a same biomass E. canadensis and M. spicatum offered the highest surface area, P. pectinatus and N. obtusa the lowest. For 1.0 g dry weight of whole plant, the surface area was 1255 cm2 for E. canadensis, 1205 cm2 for M. spicatum, 560 cm2 for N. obtusa, 653 cm2 for P. lucens, 500 cm2 for P. pectinatus, and 762 cm2 for P. perfoliatus. Plants with dissected morphology did not necessarily offer the largest surface area per unit biomass.

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.).

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.

Occurrence of Characeae in Switzerland over the last two centuries (1800–2000)

Some 3100 specimens of Characeae from herbarium collections and recent sampling programmes in Switzerland were redetermined and their provenance analysed, in order to investigate historical changes in frequency, distribution and current status of the species. Altitudinal and geographic distribution patterns of 27 taxa are described in 125 km2 grid squares covering Switzerland, and an analysis was made of species richness of Characeae in different grid squares. Data were available for approximately half (117 out of 262) of the grid squares covering Switzerland, showing that the recording effort had not been uniform. The lowland was better investigated than either the hills or the mountains. And 17 taxa were categorised as rare, on the basis that they were each known from <100 records. Ten taxa were classified as not uncommon, among which Chara vulgaris and C.globularis are the most frequent and widespread species. The distribution of some species shows a significant relationship with altitude, but analysis of the species distributions shows only a weak relationship with altitude of the grid square and biogeographical region, revealing that these two variables are not the main factors explaining the distribution of the species. Over the last 200 years, regression has occurred in the species that were formerly the rarest and expansion has occurred in a few species that were previously the most common. Further investigations are needed to precise the distribution of the declining species, particularly in the under-prospected parts of the country and in poorly known habitats.

Morphological traits and spatial heterogeneity of aquatic plants along sediment and depth gradients, Lake Geneva, Switzerland

Abstract The morphological response and distribution of Potamogeton pectinatus L., P. lucens L. and P. perfoliatus L. were investigated in relation to sediment and depth gradients within the littoral zone of Lake Geneva. Biomass, maximum shoot length, shoot biomass and biomass density were the plant traits analyzed, together with sediment characteristics: texture, water content, organic nitrogen and carbon, and exchangeable phosphorus and potassium. The relationships between (i) depth and sediment characteristics vs. species distribution and (ii) depth and sediment characteristics vs. plant traits were first studied. Regression models for the biomass and the shoot length of each species are proposed. These models are applied in a related article (Lehmann et al., 1997) in order to study the relationship between plant traits vs. species distribution through a Geographic Information System modeling. Besides the fact that P. pectinatus was more often found in sediments with higher organic content, no direct relationships were found between sediment characteristics and species distribution. Neither could the presence of patches of bare substrate be explained by differences in sediment characteristics. The effect of the sediment characteristics on plant growth showed in general a negative influence of organic matter content (except for P. pectinatus), and a positive influence of the fine texture and phosphorus. P. pectinatus grew better in shallower water, P. lucens appeared to have the best competitive abilities and P. perfoliatus performed best at deeper sites.

Influence of water eutrophication on the macrophytic vegetation of Lake Lugano

The influence of water eutrophication on the macrophytic vegetation of Lake Lugano (Lago di Lugano) is demonstrated using two complementary procedures:A comparative study of the flora, floristic diversity and abundance of vegetation of the three main basins (Northern, Southern, and Ponte Tresa) differing in their geographical and limnological (trophic) characteristics. Our findings are briefly compared with the observations made on other Swiss lakes.A description of the qualitative and quantitative evolution of macrophytes based on the comparison of our results of 1980 and 1987 with those of Steiner in 1912. These three studies correspond to different trophic levels and provide trends to link the evolution of vegetation and flora to the phenomenon of eutrophication.

The prediction of macrophyte species occurrence in Swiss ponds

The study attempted to model the abundance of aquatic plant species recorded in a range of ponds in Switzerland. A stratified sample of 80 ponds, distributed all over the country, provided input data for model development. Of the 154 species recorded, 45 were selected for modelling. A total of 14 environmental parameters were preselected as candidate explanatory variables. Two types of statistical tools were used to explore the data and to develop the predictive models: linear regression (LR) and generalized additive models (GAMs). Six LR species models had a reasonable predictive ability (30–50% of variance explained by the selected predictors). There was a gradient in the quality of the 45 GAM models. Ten species models exhibited both a good fit and statistical robustness: Lemna minor, Phragmites australis, Lysimachia vulgaris, Galium palustre, Lysimachia nummularia, Iris pseudacorus, Lythrum salicaria, Lycopus europaeus, Phalaris arundinacea, Alisma plantago-aquatica, Schoenoplectus lacustris, Carex nigra. Altitude appeared to be a key explanatory variable in most of the species models. In some cases, the degree to which the shore was shaded, connectivity between water bodies, pond area, mineral nitrogen levels, pond age, pond depth, and the extent of agriculture or pasture in the catchment were selected as additional explanatory variables. The species models demonstrated that it is possible to predict species abundance of aquatic macrophytes and that each species responded individually to distinct environmental variables.

Influence of submerged aquatic vegetation on size class distribution of perch (Perca fluviatilis) and roach (Rutilus rutilus) in the littoral zone of Lake Geneva (Switzerland)

The abundance of different size classes of perch and roach in the littoral zone of Lake Geneva was compared between submerged aquatic vegetation and unvegetated zones. Samples were taken with gillnets during four periods between June and October 1993. During the vegetation period (June to September), perch ≤9 cm and roach ≤10 cm were more abundant in vegetation whereas roach > 20 cm were more abundant in open water. Perch larger than 18 cm and medium roach were equally distributed in both habitats whatever the period, whereas medium perch distribution fluctuated according to the period. In October, after the decline of the vegetation, no more differences in fish distribution were observed except for small roach, which were always more abundant in the “vegetated sites”.

The M-NIP: a macrophyte-based Nutrient Index for Ponds

In Swiss ponds, eutrophication represents one of the major threats to biodiversity. A biological method to assess the trophic state would, therefore, be particularly useful for monitoring purposes. Macrophytes have already been successfully used to evaluate the trophic state of rivers and lakes. Considering their colonizing abilities and their roles in pond ecosystem structure and function, macrophytes should be included in any assessment methods as required by the European Water Framework Directive. Vegetation survey and water quality data for 114 permanent ponds throughout Switzerland were analysed to define indicator values for 113 species including 47 with well-defined ecological response to total water phosphorus (TP). Using indicator values and species cover, a Macrophyte Nutrient Index for Ponds (M-NIP) was calculated for each site and assessed with both the original pond data set and a limited validation data set. The resulting index performed better when considering only species with narrow responses to TP gradient and was more applicable, but less accurate when including all species. Despite these limitations, the M-NIP is a valuable and easy tool to assess and monitor the nutrient status of Swiss ponds and was shown to be robust and relatively sensitive to slight changes in phosphorus loading with a validation subset.