Simulating the dynamics of epiphytic diatom metacommunity in stream environments contaminated with heavy metals
Résumé
Heavy-metal pollution may drive succession in freshwater diatom communities towards more pollution tolerant species, resulting in an increased tolerance of communities but also in loss of species diversity. Using models is the best way to have a fast and robust estimation of natural-community evolution to study efficiently the microorganism community dynamics in freshwater environments. However until nowadays, no ecotoxicological study dealing with modeling diatom dynamics in heavy-metal contaminated stream environments was established. A means of considering population level in ecotoxicology is the "demographic" methodology. It consists of introducing effects of pollutants on demographic parameters into models of population dynamics. When the study deals with several species belonging to a same group, the community must be considered. And when several communities are considered, some processes emerge like spontaneous migration, non-spontaneous migration or "actions at distance". So modeling species-structured communities with matrix methods can be applied to spatial fragmented populations. The dimensions of these models describing real systems (demography, migrations) increase with the number of spatial patches and the number of species considered, which have dissuaded all scientists to work on stream diatom communities. Our work aimed to integrate pollutant effects in such a multi-region model with two different time scales. We developed a model of a theoretical river network based upon the catchment basin of the Vienne river located in the Limousin region (France) with low autoecological variations. We have selected 8 diatom species adnated to one of the most common macrophyte in the limousin rivers, Myriophyllum alterniflorum (Haloragaceae) and two heavy metals (copper and cadmium) as the toxic compounds. These metals were chosen because they are common pollutants of stream environments and they have a high toxicity associated with a strong potential for bioaccumulation. Furthermore, their persistence in aquatic systems lead to their registration on the European Union's blacklist of harmful substances since many years. Our theoretical river network consists of 6 asymmetric levels and 14 patches. The patch distribution among the levels is based on the geography of the 14 studied in situ localities. This allows a better comprehension of the ecotoxicological impacts of heavy metals on macrophyte-associated diatom communities. Our multi-area model considers toxicity in terms of survival and colonization reduction through dose-response curves established from our in situ data. We hypothesized that the maximum heavy-metal concentrations (measured in situ in each studied locality) are discharged when the pollution occurs. Species composition, spatial distribution, demographic and migration processes were taken into account to estimate community responses to this really realistic scenario of pollutant discharge. As a breakthrough, our results highlight that in cadmium-polluted areas, diatom migrations play here an important role in the survival of three species: Achnanthes microcephala, Cocconeis placentula and Navicula simplex. This study brings important information for health environmental projects which must considered not only the pollution frequency but also the sustainability of the biological substrate for periphyton.