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Séminaire au LOG du Dr. Hans Jakobsen

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Séminaire du Dr. Hans Jakobsen, Senior Researcher at the Department of Bioscience,Aarhus University, in Roskilde, Danemark intitulé :

Titre :

Recovery of estuarine ecosystems after reductions in nutrient loading : responses in primary producers and their grazers

Date :26 septembre 2017 à 16h00

Lieu :Salle de Conférences MREN à Wimereux

Résumé :

Results from large-scale Danish national Action Plans, aimed to improve the water quality and ecosystem health in the Danish coastal waters through reduced loads of nutrients, now show the first significant trends of recovery after more than 25 years of mitigation measures. Theses ecosystems in situated at the mouth of the Baltic estuary and covers very variable salinities and nutrient loading. The presentation will report data from the Danish nation- wide monitoring program starting in the beginning of the 1980’s including loadings of nutrients, and chemical and biological components in the coastal zone. Following several action plans, nutrient loadings from land have been reduced with about 50% for nitrogen (N) and 56% for phosphorus (P) since 1990, presenting a large-scale laboratory for studying biological responses to eutrophication. These reductions gave significant and immediate declines in total nutrient concentrations in freshwater surface locations and in the coastal areas (TN and TP). In addition, chlorophyll a concentration decreased whereas cover of macroalgae vegetation has increased as a respond to reductions in loadings. The depth extension of eelgrass meadows, however, only started to expand within the past five years. The benthic macrofauna biomass was earlier dominated by filter feeding bivalves during the late 8o’ies but in contrast to the increasing biomass of deposit feeders over the last 20 years, the biomass of filter feeders decreased, most likely as an effect of reduced food resources. There were no significant changes in phytoplankton biomass or oxygen concentrations from 1981 to 2013 suggesting that multiple factors counteracted the expected positive effects of reduced nutrient loadings. Partial recovery towards pristine like conditions were observed for water clarity, chlorophyll a and at the seafloor for eelgrass maximum depth limits and macroalgae cover. The coastal ecosystem components demonstrated several evident signs of improvements suggesting that at least partial recovery is achievable. During the presentation I will show examples of the yearlong set of monitoring data are used to understand the coupled responses of physiological and their gazers is linked to the altered nutrient loadings