Etude globale sur les cyanobactéries dans la rivière Erdre Travaux de recherche en laboratoire. Rapport final Janvier 2004
Cyanobacteria proliferation and the potential health risk related with the release of the associated toxins have lead the local association EDEN to initiate a comprehensive study on cyanobacteria in the river Erdre. Within the consortium in charge of the project, the Berlin Centre of Competence for Water (KWB) realised lab-scale research in cooperation with the German Federal Environmental Agency (UBA), on the species Planktothrix agardhii which predominates in the river Erdre, and the associated toxin microcystin. The objective was to determine the influence of key factors such as nutrients (nitrogen, phosphorus), light, flow velocity and sediments on cyanobacteria growth and competition as well as microcystin release from the Planktothrix population in the river Erdre. Results from the lab-scale cultures proved that nutrient-limited conditions lead to a decrease of cyanobacteria biomass and may favour some genotypes with reduced needs among the Planktothrix population. Given the current state of scientific knowledge, no differences in competition between toxic and non-toxic Planktothrix strains can be established. Nutrient limitation favours microcystin release from cells, however the global decrease of cyanobacteria biomass induces a decrease of the total quantity of released toxin. These results can be applied in a water body where nutrients concentrations are very low (below 50 µg/L for total phosphorus). In the river Erdre, as long as external nutrients inputs remain considerable, light is the limiting factor. Internal nutrient recycling from the sediments is globally negligible in comparison with external inputs. Culture experiments in a flow simulation flume proved that flow velocity had substantial impact neither on Planktothrix growth nor on microcystin release. Only a short transition phase with negative effects was observed. Overwintering of Planktothrix in Erdre-sediments could be proved by the detection of a substantial population using fluorescence analysis. This inoculum should be large enough for initiation of Planktothrix development in the next vegetation period. However, the high adsorption capacities of the analysed sediments from the river Erdre allow to put aside a potential risk of microcystin release from sludge. While providing innovative results on the species Planktothrix agardhii, this project contributes to the comprehensive study initiated by the EDEN association in order to preserve the values associated with environment, health and tourism in the river Erdre.