Phosphorus forms with respect to sediment characteristics, such as grain-size, and major chemical elements, were studied in French coastal marine areas (the Bay of Seine and the Loire and Gironde Estuaries). In the three areas, Fe/Al-bound phosphate (Fe/Al-P) and exchangeable phosphate (exch-P) were significantly related to the proportion of fine fraction (<63 μm). The special association of these forms with fine particles, as well as the relationships between exch-P, Fe/Al-P and Fe showed that adsorption processes on Fe oxides had taken place. However, for similar Fe concentrations, Bay of Seine sediments showed Fe/Al-P concentrations which were four to five times higher than those found in the Loire and Gironde Estuaries. This was attributed to differences in sediment nature and processes in these two types of areas. Indeed, the Loire and Gironde Estuaries frequently show hypoxia. Calcium-bound phosphate (Ca-P) was found in all grain size classes with comparable concentrations in the Bay of Seine where there was no correlation between CaP and Ca. This corresponded to the calcium's marine origin (shells) in the Bay of Seine. In contrast, the correlation of CaP with Ca and the fine fraction of the sediment in the Loire and Gironde Estuaries was characteristic of the predominant metamorphic origin of CaP in these areas. Useful information for interpretation of P-forms can be obtained from major sediment characteristics. Thus, for areas where specific relationships have previously been established good estimates of Pforms could be predicted from sediment properties.

The multiannual dynamic of the cyst-forming and toxic marine dinoflagellate Alexandrium minutum was studied over a time scale of about 150 years by a paleoecological approach based on ancient DNA (aDNA) quantification and cyst revivification data obtained from two dated sediment cores of the Bay of Brest (Brittany, France). The first genetic traces of the species presence in the study area dated back to 1873 ± 6. Specific aDNA could be quantified by a newly developed real-time PCR assay in the upper core layers, in which the germination of the species (in up to 17-19-year-old sediments) was also obtained. In both cores studied, our quantitative paleogenetic data showed a statistically significant increasing trend in the abundance of A. minutum ITS1 rDNA copies over time, corroborating three decades of local plankton data that have documented an increasing trend in the species cell abundance. By comparison, paleogenetic data of the dinoflagellate Scrippsiella donghaienis did not show a coherent trend between the cores studied, supporting the hypothesis of the existence of a species-specific dynamic of A. minutum in the study area. This work contributes to the development of paleoecological research, further showing its potential for biogeographical, ecological and evolutionary studies on marine microbes.

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The macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) has been subjected to recurrent annual toxic blooms of Alexandrium minutum since 1988. This study aims to specify the phosphorus dynamics and bioavailability in sediments in order to improve our understanding of Alexandrium occurrences. Sediment-P pools and diffusive phosphate fluxes were studied under similar hydrodynamic conditions, in the intermediate estuary in . The results highlight a decrease in bioavailable phosphorus (iron and organic bound) from the inner part of the estuary seaward. The ratio of iron-bound phosphorus to ironoxyhydroxides is lower in the inner and intermediate estuaries (5-8) than in the outer site (15), suggesting a saturation of sorption sites and greater phosphorus bioavailability in this area. Pools of bioavailable phosphorus in surficial sediments are about eight times higher than the annual net-export of P (7 ton year -1 ). Phosphate releases from sediments are always lower than 5 lmol m -2 d -1 in March. The highest supplies occur in June and August in the intermediate area (up to 400 lmol m -2 d -1 ) where they represent up to 50% of river loadings. These results further suggest that phosphate pulses coincide with occurrences of Alexandrium reported in June.

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