Spatio-temporal dynamics and biogeochemical properties of green seawater discolorations caused by the marine dinoflagellate Lepidodinium chlorophorum along southern Brittany coast

Blooms of the marine dinoflagellate Lepidodinium chlorophorum cause green seawater discolorations affecting the recreational use and the tourism economy along southern Brittany (NE-Atlantic, France). Hypoxic conditions associated with phytoplankton biomass recycling are suspected to cause fauna mortalities. An in situ monitoring was performed in 2019 to characterise the seasonal variability of L. chlorophorum. This species was observed from May to November, with a maximum abundance in June-July. Specific bloom sampling demonstrated a domi-nance of L. chlorophorum within microphytoplankton, and documented its vertical distribution. Satellite obser-vation was used to compute the surface extent of the bloom and to highlight the importance of small-scale temporal variability, with tidal currents being a primary driver of surface distribution of the bloom. Stratification contributed to promoting the bloom of L. chlorophorum. High concentrations of phosphate and ammonium, together with transparent exopolymer particles (TEP), were recorded within the bloom. Bacterial stimulation, leading to nutrient remineralisation or mucus facilitating mixotrophy, is suggested to sustain bloom develop-ment. Hence, TEP production might provide an ecological advantage for the dinoflagellate, conversely causing negative effects on the environment and biological resources through hypoxia. These first insights constitute a baseline for further studies in other ecosystems impacted by this species.

Automated summary

Blooms of Lepidodinium chlorophorum, a marine dinoflagellate, cause green seawater discolorations in southern Brittany, affecting recreational activities and tourism economy. The species is most abundant from May to November, with peak abundance in June-July. Tidal currents and stratification play important roles in the distribution of the bloom, while high concentrations of phosphate, ammonium, and TEP are recorded within the bloom. Bacterial stimulation and TEP production contribute to the sustainment of the bloom, with potential negative effects on the environment and biological resources through hypoxia.