Contamination of planktonic food webs in the Mediterranean Sea: Setting the frame for the MERITE-HIPPOCAMPE oceanographic cruise (spring 2019).

This paper looks at experiential feedback and the technical and scientific challenges tied to the MERITE-HIPPOCAMPE cruise that took place in the Mediterranean Sea in spring 2019. This cruise proposes an innovative approach to investigate the accumulation and transfer of inorganic and organic contaminants within the planktonic food webs. We present detailed information on how the cruise worked, including 1) the cruise track and sampling stations, 2) the overall strategy, based mainly on the collection of plankton, suspended particles and water at the deep chlorophyll maximum, and the separation of these particles and planktonic organisms into various size fractions, as well as the collection of atmospheric deposition, 3) the operations performed and material used at each station, and 4) the sequence of operations and main parameters analysed. The paper also provides the main environmental conditions that were prevailing during the campaign. Lastly, we present the types of articles produced based on work completed by the cruise that are part of this special issue.

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.

A Bayesian network approach to determine environmental factors controlling Karenia selliformis occurrences and blooms in the Gulf of Gabès, Tunisia.

A Bayesian Network modeling framework is introduced to explore the effect of physical and meteorological factors on the dinoflagellate red tide forming Karenia selliformis in various sampling sites of the national phytoplankton monitoring program. The proposed models took into account the physical environment effects (salinity, temperature and tide amplitude), meteorological constraints (evaporation, air temperature, insolation, rainfall, atmospheric pressure and humidity), sampling months and sites on both Karenia selliformis occurrences and blooms. The models produced plausible results and enabled the identification of the factors that directly impacted on the species occurrences and concentration levels. The sampling sites dominated the species occurrences. The models show that the relationship between salinity and Karenia selliformis is more apparent when the species concentrations are focused on and that the bloom occurrences can be predicted based on salinity. Concentrations up to 105 cells L-1 were recorded when salinity exceeded 42.5 and dominated the shallow and weak water renewal areas.

Long-term microphytoplankton variability patterns using multivariate analyses: ecological and management implications.

Microphytoplankton data along the southern Tunisian coasts have been monitored weekly since 1995. This study used the data collected during the period 2000-2007 to determine the temporal variability patterns and to characterize the geographical structures of microphytoplankton populations. The methodological approach consisted in isolating the temporal variability common to all sampled sites and in analyzing the "residual" site-specific component. Multivariate ordination methods were used to determine the seasonal and interannual variability. The temporal pattern shared by all sites was highlighted and the seasonal cycle of the most frequent phytoplankton communities contrasted the diatoms winter-spring species to the dinoflagellates summer-fall species. Multitable comparison allowed the identification of three areas exhibiting similar phytoplankton population variability. The results, despite the coastal location of sampling sites, indicated that hydrodynamic and geomorphologic properties of the different areas as well as anthropogenic activities play a key role in the structure of the phytoplankton communities.