Daily variations of Ostreopsis cf. ovata abundances in NW Mediterranean Sea

Ostreopsis cf. ovata is a benthic dinoflagellate very common in tropical and temperate coastal areas, particularly in the Mediterranean Sea. This species is also found in the plankton, i.e. swimming in the water column or in aggregates floating at the sea surface. The potential links between the planktonic and benthic populations influencing their relative distribution in the water column and attached to the benthic substrate are poorly understood. To shed light on this question, a high-frequency temporal monitoring was conducted in the Villefranche bay (France) to determine the abundance of (1) epibenthic cells attached to macroalgae, (2) planktonic cells in the water column and (3) cells in aggregates floating at the sea water surface (hereafter, referred to sea surface cells) . This monitoring was realized over 3 consecutive years (2018, 2019 and 2020) and at different phases of the bloom (exponential phase - 2020, peak - 2019 and decline phase - 2018). Strong variations in benthic and planktonic O. cf. ovata abundances were observed over the 24 h sampling cycles conducted in three consecutive years. The three populations, planktonic, benthic and sea surface cells, exhibited the highest numbers during the day (light) hours and lowest values at night in 2018 and 2019. In 2020, however, benthic abundances did not differ significantly between light and dark periods. Moreover, epibenthic cells abundances peaked in the morning, followed by the peak of the cells in the plankton and in the surface aggregates during the afternoon. Monitoring of O. cf. ovata is often based on a single sampling per day without precise indications of sampling time and shows great variability in O. cf. ovata abundances. Our observations of daily variations in cell abundances along the water column clearly indicate that time and water column depth of sampling constitute a great source of variability and have to be considered when designing new monitoring strategies to reduce variability and to harmonize data acquisition and international comparisons.