An unprecedented bloom of Lingulodinium polyedra on the French Atlantic coast during summer 2021.

At the end of July 2021, a bloom of Lingulodinium polyedra developed along the French Atlantic coast and lasted six weeks. The REPHY monitoring network and the citizen participation project PHENOMER contributed to its observation. A maximum concentration of 3,600,000 cells/L was reached on the 6th of September, a level never recorded on French coastlines. Satellite observation confirmed that the bloom reached its highest abundance and spatial extension early September, covering about 3200 km2 on the 4th of September. Cultures were established, and morphology and ITS-LSU sequencing identified the species as L. polyedra. The thecae displayed the characteristic tabulation and sometimes a ventral pore. The pigment composition of the bloom was similar to that of cultured L. polyedra, confirming that phytoplankton biomass was dominated by this species. The bloom was preceded by Leptocylindrus sp., developed over Lepidodinium chlorophorum, and was succeeded by elevated Noctiluca scintillans concentrations. Afterwards, relatively high abundance of Alexandrium tamarense were observed in the embayment where the bloom started. Unusually high precipitation during mid-July increased river discharges from the Loire and Vilaine rivers, which likely fueled phytoplankton growth by providing nutrients. Water masses with high numbers of dinoflagellates were characterized by high sea surface temperature and thermohaline stratification. The wind was low during the bloom development, before drifting it offshore. Cysts were observed in the plankton towards the end of the bloom, with concentrations up to 30,000 cysts/L and relative abundances up to 99%. The bloom deposited a seed bank, with cyst concentrations up to 100,000 cysts/g dried sediment, particularly in fine-grained sediments. The bloom caused hypoxia events, and concentrations of yessotoxins up to 747 µg/kg were recorded in mussels, below the safety threshold of 3,750 µg/kg. Oysters, clams and cockles also were contaminated with yessotoxins, but at lower concentrations. The established cultures did not produce yessotoxins at detectable levels, although yessotoxins were detected in the sediment. The unusual environmental summertime conditions that triggered the bloom, as well as the establishment of considerable seed banks, provide important findings to understand future harmful algal blooms along the French coastline.

Trajectories of nutrients concentrations and ratios in the French coastal ecosystems: 20 years of changes in relation with large-scale and local drivers.

Along with their important diversity, coastal ecosystems receive various amounts of nutrients, principally arising from the continent and from the related human activities (mainly industrial and agricultural activities). During the 20th century, nutrients loads have increased following the increase of both the global population and need of services. Alongside, climate change including temperature increase or atmospheric circulation change has occurred. These processes, Ecosystem state changes are hard to monitor and predict. To study the long-term changes of nutrients concentrations in coastal ecosystems, eleven French coastal ecosystems were studied over 20 years as they encompass large climatic and land pressures, representative of temperate ecosystems, over a rather small geographical area. Both univariate (time series decomposition) and multivariate (relationships between ecosystems and drivers) statistical analyses were used to determine ecosystem trajectories as well as typologies of ecosystem trajectories. It appeared that most of the French coastal ecosystems exhibited trajectories towards a decrease in nutrients concentrations. Differences in trajectories mainly depended on continental and human influences, as well as on climatic regimes. One single ecosystem exhibited very different trajectories, the Arcachon Bay with an increase in nutrients concentrations. Ecosystem trajectories based on ordination techniques were proven to be useful tools to monitor ecosystem changes. This study highlighted the importance of local environments and the need to couple uni- and multi-ecosystem studies. Although the studied ecosystems were influenced by both local and large-scale climate, by anthropogenic activities loads, and that their trajectories were mostly similar based on their continental influence, non-negligible variations resulted from their internal functioning.