Vendredi 13 mars 2015 de 10h à 11h, Valentini Maliaka (Society for the Protection of Prespa & Institute for Water and Wetland Research, Radboud University Nijmegen) présentera à la Tour du Valat (en grande salle) un séminaire intitulé “Society for the Protection of Prespa & Institute for Water and Wetland Research, Radboud University Nijmegen“.
L’accès à ce séminaire est libre et sans inscription.
Résumé :
Eutrophication drives numerous lakes worldwide to a deteriorated state with undesirable effects on aquatic communities and the stability of their ecosystem. Recent findings in Lake Prespa in Greece – which includes the deep Lake Great Prespa and shallow Lake Lesser Prespa – underlined that the cumulative nutrient inputs have resulted in an increased primary productivity during the warm period. In the meantime the causes and processes which result in the excess nutrient enrichment within the lake system and their watershed remain rather unknown.
These concerns initiated a long-term water-quality monitoring since 2013 which aims to unravel the processes which determine the lake water quality under eutrophication stress. Multiple biogeochemical parameters are measured regularly at the two lakes, the tributary streams, the drainage tiles from agricultural lands and discharging groundwater. In this way the lake systems are studied as an integral part of their landscape.
The monitoring results so far show that notable losses of phosphorus and nitrogen from the over-fertilized agricultural lands are polluting the drainage canals which carry nutrients into the tributary system and eventually into the lakes. Findings in the lakes show a building up of organic and nutrient rich sediments which may induce internal eutrophication problems while the loss of oxygen near the bottom waters and the relatively high temperatures can accelerate the nutrient recycling within the lakes. As a result, the proliferation of algal growth with high densities of cyanobacteria is observed during the summertime and early autumn which occasionally provokes the production of cyanobacterial toxins (microcystin) at harmful levels particularly in Lake Lesser Prespa. The susceptibility of Lake Lesser Prespa to nutrient enrichment and warming was also studied in a controlled experiment where nitrogen addition seemed to stimulate cyanobacteria and the production of microcystins whereas higher temperatures stimulated further the nitrogen effect.
At present, much is yet to be understood concerning the interactions that may occur between nutrients along the stream-reedbed-lake system at the Greek part of Prespa Lakes. Apart from the routine monitoring measurements, additional research is currently under progress which studies the expansion patterns of Typha and Phragmites species in response to the nutrient availability at the reedbed soils around Lake Lesser Prespa.
The biogeochemical knowledge is expected to improve our understanding of the functioning of the Lake Prespa system and the extent of the eutrophication problems and can therefore be a helpful tool to guide small or large-scale management schemes which curtail the nutrient input to the lakes.
