ED Sciences et environnements
assessment of nutrients and carbon in natural and artificial wetlands at the head of a watershed. case of the carcans-hourtin watershed
by Romane DARUL (Environnements et Paléoenvironnements Océaniques et Continentaux)
The defense will take place at 14h00 - Amphi B INP ENSEIRB-MATMECA, 1 avenue du Dr Albert Schweitzer, 33400 Talence
in front of the jury composed of
- Pierre ANSCHUTZ - Professeur des universités - Université de Bordeaux - Directeur de these
- Cristina RIBAUDO - Maîtresse de conférences - INP-ENSEGID - CoDirecteur de these
- Christine DUPUY - Professeure des universités - La Rochelle Université - Rapporteur
- Edouard METZGER - Professeur des universités - Université d'Angers - Rapporteur
- Mathieu SEBILO - Maître de conférences - Sorbonne Université - Rapporteur
- Juliette ROSEBERY - Directrice de recherche - Université de Bordeaux - Examinateur
Wetlands are subject to numerous threats such as eutrophication, disruption of their hydrology by human activities and climate change. Restoration of natural sites and the creation of artificial wetlands are carried out by territorial managers to preserve the ecosystem services they provide: support for biodiversity, nutrient remediation and carbon storage. Headwater wetlands are particularly important as they influence downstream water quality and the biodiversity of aquatic ecosystems in much of the downstream watershed. The watershed of the lacs Médocains is a highly anthropized territory, dedicated to pine cultivation and agriculture. It is home to several natural wetland environments, including the Carcans-Hourtin and Lacanau lakes, marshes, watercourses and many temporary forest ponds, remnants of the marshy area that existed before the expansion of forestry on the Landes plateau. These environments are at risk of eutrophication and drying up; their number decreases every year. Some Agricultural Artificial Wetlands (AAW) are constructed downstream of agricultural exploitations to reduce nutrient flows from field runoff. The study of these natural and artificial wetlands is thus a strategic point in territorial management. In this thesis work, the physico-chemistry of 12 temporary ponds and 3 AAW was monitored for about 2 years. Sediment cores were taken from the AAW to quantify nutrient fluxes at the water-sediment interface and define nutrients depletion processes. Within temporary ponds, a carbon budget was established through measurements of CO2 and CH4 fluxes at the water-atmosphere and exposed sediment-air interfaces using flux chambers, and through measurements of carbon burial rates using 210Pb dating in sediment cores. Identification of vegetation belts and biomass harvesting were carried out on 6 temporary ponds. The results of this thesis show that AAW are currently too small to effectively reduce nitrate coming from field fertilization. However, denitrification and phosphorus precipitation processes in the sediments are indeed occurring. The enlargement of surface of existing AAW and the creation of new artificial wetlands should be recommended in order to effectively reduce nutrient flows in the future. The temporary ponds studied resulted acidic and mostly oligotrophic. However, nitrification was measured during the re-watering of ponds that had dried up for a long time during the summer, and nitrate contamination was identified in several ponds located downstream of agricultural areas. Sites in good conservation status or having undergone recent restoration works, consisting of bank re-profiling and sediment seed bank conservation, were identified as carbon sinks and had the highest vegetation biomass and diversity. On the opposite, ponds that had been over-excavated in the past, resulted as carbon sources and present less biodiversity. A carbon storage gradient was identified in the ponds: central areas, which were immersed the longest, had higher carbon rates than external areas. The current restoration techniques used for natural wetlands and the planned enlargement works for the artificial ones therefore appear to be effective methods for maintaining good water quality and sustaining wetlands in the Carcans-Hourtin lake watershed. However, it is still necessary to monitor the evolution of these environments in the context of climate change.