Karenia selliformis bloom dynamics and growth rate estimation in the Sfax harbour (Tunisia), by using automated flow cytometry equipped with image in flow, during autumn 2019

A Karenia selliformis bloom event in the Gulf of Gabès (Mediterranean Sea), was monitored over 9 days at high frequency during fall 2019, by using an automated flow cytometer (Cytosense, Cytobuoy b.v.) with an image-in-flow attachment. The instrument recorded the shape of the optical signals that lead to the resolution of six cell groups of pico-, nano- and microphytoplankton, during the Harmful Algal Bloom (HAB). K. selliformis cell dimensions derived from the hourly records, enabled to estimate the daily division rate over the bloom period. Results revealed that K. selliformis was the only bloom-forming species and it reached its highest mean abundance the fourth day of the survey. A shift in the nutrient composition occurred with a potential P limitation during the bloom growth and N limitation during the bloom collapse. The co-inertia analysis revealed opposite patterns for K. selliformis and heterotrophic prokaryotes suggesting trophic interactions and possible mixotrophic behaviour of K. selliformis at the end of the bloom. K. selliformis exhibited low growth rates generally < 1 division day-1, which could not explain the observed high abundance. The tide played a crucial role in the dynamics of K. selliformis at a semi-diurnal scale and at spring-neap tide scale and was probably enhancing K. selliformis accumulation.