Vivianite as a phosphorus sink in estuarine systems: The case study of the Brillantes mudflat, Loire, France

Phosphorus (P) is a key nutrient controlling primary production in aquatic systems. In coastal systems, the P cycle involves dynamic interactions between terrestrial, aquatic and sedimentary compartments. Over the last century, human activities such as deforestation, intensive agricultural practices and the disposal of municipal and industrial wastes have increased P inputs to coastal ecosystems. As a result, this increase in P inputs has led to an increase in the occurrence of algal blooms and higher oxygen demand in estuaries. In the Loire estuary, dissolved oxygen deficits have been a recurrent and worrying issue for several decades despite the improvement of water quality over the last 30 years due to reduced wastewater discharge and better effluent treatment. In this context, the burial of bioavailable P may influence the recovery of waters from eutrophication. The major P burial phases are apatite, organic P and iron-bound P. The results of sequential chemical extraction and pore water analysis carried out over a 5m-long sediment core from the intertidal Brillantes mudflat in the Loire estuary indicated a greater abundance of the iron-bound P compared to other phases. Iron-bound P occurs in two different forms: phosphorus bound to iron oxides and in the iron phosphate mineral known as vivianite. Vivianite is a ferrous iron phosphate mineral formed under reducing and low sulphate conditions in sediments where organic matter serve as electron donor for ferric iron reduction. Results of sequential chemical extraction of freeze-dried sediment samples combined with pore water data and scanning electron microscope–energy dispersive x-ray spectroscopy (SEM-EDXS) on resin-embedded sediment samples indicated that vivianite-type minerals may act as an important sink for P at the studied site. Authigenic vivianite crystals were found below the shallow sulphate/methane transition zone (SMTZ) at 94 cm depth and contain significant amounts of manganese, as observed in freshwater sediments. We therefore hypothesise that anthropogenic over-fertilization of coastal regions in the last century may have increased the importance of vivianite authigenesis in surface sediments. Consequently, vivianite is likely to be an important sink for P in estuarine systems worldwide. This study is part of a PhD financed by the European Project Life REVERS’EAU.