Spatial structures may not only result from ecological interactions, they may also play an essential functional role in organizing the interactions. Modeling spatial patterns at multiple spatial and temporal scales is thus a crucial step to understand the functioning of ecological communities. PCNM (principal coordinates of neighbor matrices) analysis achieves a spectral decomposition of the spatial relationships among the sampling sites, creating variables that correspond to all the spatial scales that can be perceived in a given data set. The analysis then finds the scales to which a data table of interest responds. The significant PCNM variables can be directly interpreted in terms of spatial scales, or included in a procedure of variation decomposition with respect to spatial and environmental components. This paper presents four applications of PCNM analysis to ecological data representing combinations of: transect or surface data, regular or irregular sampling schemes, univariate or multivariate data. The data sets include Amazonian ferns, tropical marine zooplankton, chlorophyll in a marine lagoon, and oribatid mites in a peat bog. In each case, new ecological knowledge was obtained through PCNM analysis.

International audienceLac du Bourget, in the Alps, is the largest natural French lake. Following major restoration programmes during the 1970s and early 1980s, involving massive efforts to reduce nutrient loads and pollution in the lake, the water quality has improved over the past two decades. This can be inferred from the increase in the nitrate: phosphate ratio, the intensification of the "clear-water phase" (i.e. the increase in the water column transparency in spring), and the reduction in the total phosphorus and chlorophyll a concentrations. However, blooms of the filamentous, phycoerythrin-rich, non-nitrogen fixing and hepatotoxic cyanobacterium Planktothrix rubescens have occurred since 1996 and have been maintained subsequently, at least during summer and autumn periods. Nutrients (especially phosphorus) are usually thought to be one of the most important factors responsible for cyanobacterial blooms, and so the question is asked if this bloom is a paradoxical outcome of the restoration programs? Using a large set of data taken from surveys of Lac du Bourget, from the literature, and from recent laboratory experiments, and also using field data for the neighboring Lake Geneva, we propose a realistic scenario to account for the population dynamics of the cyanobacterium and the occurrence and maintenance of the bloom in Lac du Bourget. The characteristics of the lake (high water column stability, deepening of the nutrient-depleted layer during the last decade, a long water residence time), local conditions (the nutrient load and charge) and global changes (global warming) all had to be taken into account to explain this bloom. We suggest that the success of P. rubescens in Lac du Bourget is probably due to increased transparency and a longer stratified period following (1) the restriction of other phytoplankton species following reduced phosphorus, which has allowed P. rubescens to make use of organic phosphorus to improve its competitiveness; (2) warmer than average winter/spring periods allowing an earlier water stratification and in fine a competitive advantage to P. rubescens; (3) lower than average surface irradiance, which has also given the low-light preferring P. rubescens an advantage. Finally, this study highlights the importance of long-term data sets in attempting to elucidate the global causes of a major ecological problem (such as this cyanobacterial bloom) and impacts with regard to the function and use of freshwater ecosystems. (c) 2004 Elsevier B.V. All rights reserved

Between 1999 and 2002, a routine survey of water quality in the Lac du Bourget was performed to study the dynamics and microcystin (MC) production of Planktothrix rubescens. Using liquid chromatography coupled to diode array detection and mass spectrometry, we found that two main variants ([D-Asp3] and [D-Asp3, Dhb7] microcystin-RR) were produced. The proportion of these two variants was not influenced by the depth or season of sampling. Expressed in microcystin-LR equivalents, high microcystin concentrations were recorded from August to December each year, reaching values of up to 6.7 microg L-1. A significant correlation was found between the microcystin cell content and the cell densities of P. rubescens. Cellular quotas of microcystins ranged from 0.1 to 0.3 pg cell-1. Simultaneously, laboratory experiments were performed on a strain of P. rubescens isolated from the lake to assess the potential impact of various P-PO4 (3-) concentrations on intra- and extracellular microcystin production. Unlike natural populations, this strain only produced [D-Asp3] MC-RR. The intracellular microcystin content was similarly correlated to the cell density, but the cellular quota was slightly higher (0.3-0.7 pg cell-1) than in the natural population. Again, as in the natural population, a linear relationship was found between growth rate and microcystin production rate. These findings support the hypothesis that environmental factors, such as phosphate concentrations, have no direct impact on microcystin production by P. rubescens, but act indirectly by affecting growth rate.