Fig. 6. Cells of Mesodinium major. 13. Different views of living cells. 46. Representation of pigment dynamics during Mesodinium cell degradation: transition from phycoerythrin-dominated red coloration to chlorophyll-dominated green coloration following cell lysis. All images to scale. accompanied by unpleasant odours, can generate concern among the public and stakeholders. During this event, local management responses included a municipal decree in Tréguennec temporarily prohibiting bathing and nautical activities as a precautionary measure in response to the observed water discoloration and associated uncertainties. These restrictions were subsequently lifted once no health risk was identified. Such measures illustrate how visually conspicuous but non-toxic blooms may nevertheless trigger short-term access limitations and management interventions. While posing no direct threat to human health or marine life, the presence and degradation of these blooms can disrupt local economies, affecting tourism and recreational and professional maritime activities, and shellfish fisheries. This underscores the importance of rapid diagnostic capacity and clear communication, which are essential for distinguishing harmless blooms from harmful algal blooms involving toxin-producing species and for guiding appropriate management responses. Beyond their visual impact, blooms of Mesodinium spp. are increasingly recognised as important indicators within early warning frameworks for harmful algal blooms. As a key prey for diarrheic toxin-producing Dinophysis species, their presence and abundance can provide advance signals of conditions potentially favourable for subsequent toxic events [15]. However, recent studies emphasise that high Mesodinium densities alone are not sufficient predictors of Dinophysis blooms, as successful 16 development depends on the spatial and temporal coupling of predator and prey populations, as well as appropriate water column structure facilitating encounter rates [15]. This highlights the need for integrated monitoring approaches combining biological observations with physical oceanographic context. In this perspective, conspicuous Mesodinium blooms such as the one described here may serve as valuable early indicators within coastal surveillance systems, particularly when combined with high-frequency observations and emerging imaging technologies capable of resolving trophic interactions in situ. This event shows the importance of combining citizen observations, monitoring observations, targeted sampling, microscopy, and satellite imagery to rapidly identify and interpret coastal discoloration phenomena. Such integrative approaches are increasingly valuable for coastal monitoring programmes and for improving our understanding of transient and visually striking plankton dynamics. Acknowledgements We thank the participants of the PHENOMER network for reporting the discoloration events. References 1. Siano R et al. 2020. Mar Policy 117, 103039. https://doi.org/10.1016/j.marpol.2018.01.022. https://www.phenomer. org/ 2. Ifremer 2026. Diagnostic de lorigine des eaux colorées observées en baie dAudierne. Expertise 26-013. Ifremer, France. 3. Kim M & Park MG 2019. Sci Rep 9, 14081. https://doi.org/10.1038/s41598-01950659-2 4. Gernez P et al. 2023. RSE 287, 113486. https://doi.org/10.1016/j. rse.2023.113486 5. Yoo YD et al. 2017. Harmful Algae 68, 105117. https://doi.org/10.1016/j. hal.2017.07.012 6. Moeller HV et al. 2025. J Eukaryot Microbiol 72, e13066. https://doi. org/10.1111/jeu.13066 7. Nishitani G & Yamaguchi M 2018. Sci Rep 8. https://doi.org/10.1038/s41598-01835629-4 8. Park MG et al. 2008. J Phycol 44:1154 1163. https://doi.org/10.1111/j.15298817.2008.00579.x 9. González-Gil et al. 2010. Aquat Microb Ecol 58:273286. https://doi. org/10.3354/ame01372 10. Pochic V et al. 2024. J Plankton Res 46, 100116. https://doi.org/10.1093/ plankt/fbad061 11. Crawford D et al. 1997. ECSS 45(6), 799812. https://doi.org/10.1006/ ecss.1997.0242 12. Johnson MD et al. 2013. J Plankton Res 35(4), 877893. https://doi. org/10.1093/plankt/fbt028 13. Neaud-Masson N et al. 2023. Procédure nationale pour la mise en œuvre du réseau dobservation et de surveillance du phytoplancton et de lhydrologie dans les eaux littorales (REPHY). Ref. Document de prescriptions. Version 2 de janvier 2023. ODE/VIGIES/23-01. Ifremer. https://doi.org/10.13155/50389 14. Zingone A & L Escalera 2025. Non-toxic animal-killing microalgal species In: Lundholm N et al (Eds) (2009 onwards). IOC-UNESCO Taxonomic Reference List of Harmful Microalgae. Accessed at https:// www.marinespecies.org/hab on 31-032026. doi:10.14284/362 15. Díaz PA et al. 2025. J Plankton Res 47(2), fbae068, https://doi.org/10.1093/ plankt/fbae068 Authors Kenneth N Mertens, Anne Doner, Amélie Derrien, Aouregan Terre-Terrillon, Audrey Duval, Morgan Le Moigne, Galyna Terenko, Nicolas Chomérat & Jean-Côme Piquet, Ifremer, LERBO, COAST, Concarneau, France Victor Pochic & Pierre Gernez, Nantes Université, ISOMER, UR2160, F-44000 Nantes, France Mathilde Schapira, Ifremer, LERMPL, COAST, Concarneau, France Email corresponding author: kenneth.mertens@ifremer.fr https://doi.org/10.5281/zenodo.20583341 HARMFUL ALGAE NEWS NO. 83 / 2026 Harmful Algae News An IOC Newsletter on Toxic Algae and Algal Blooms No. 83 June 2026 https://hab.ioc-unesco.org/ Long and Winding Sea-lanes for Fish-Killing Algal Events An ancient idiom dead fish rot (or stink) from the head down possibly attributable to Turkish or Persian fishers but the orig Fig. 2. Programme for the Advanced International Colloquium and Technical Workshop on fish killing marine algae and their effects. blooms. The WG also decided to revise the classic but outdated Cooperative Research Report [2] on HAB effects on mariculture and marine fisheries published in 1992 for Chilean government, through CORFO and cooperation of CREAN-IFOP (reported in HAN 63 [3]) (Fig. 2). The colloquium convenors invited international experts to Puerto Varas, Chile in 2019 to review disciplinary knowledge on all aspects of fish-killing algae and associated mortality events (Fig. 3). A p ins were added during the IPHAB XVII Intersessional (2025-2026), but the total meagre reported ichthyotoxins score (by March 2025) (zero goniodomins, zero prymnesins, one karlotoxin [sterolysin]) has increased dramatically (by February 2026): seven goniodomins, four prymnesins, one karlotoxin, and m (admittedly controversial) explain how toxigenic blooms may directly kill fish in aquaculture operations [e.g., 11]. Access to comprehensive time-series databases on HAB events (HAEDAT, HAIS/ OBIS) has allowed for interpretation of fish-killing events over decades on a regional geographical basis, e Catastrophic marine mass mortalities, shellfish toxicity and human respiratory problems from a Karenia cristata dinoflagellate bloom in South Australia, 20252026 Fig. 1. Satellite chlorophyll image from March 2024 showing widespread offshore diatom blooms in response to a massive upwelling event du from which the species name cristata is derived), and a longer hypocone with the right lobe slightly longer than the left. Scanning electron microscopy revealed that the crest was formed by a slight elevation of the right side of the apical groove. On the dorsal side, the apical groove extended to o Unprecedented bloom of Fibrocapsa japonica on French coasts Fig. 1. Map of the different bays in southern Brittany monitored as part of the REPHY program. Since 1987, the REPHY (French Phytoplankton and Hydrology Monitoring Network in Coastal Waters), operated by IFREMER, has conducted long-term m Fig. 3. Light microscope images of living Fibrocapsa japonica cells. Scale bars = 20 μm. French waters. REPHY data [9] indicate that the previous maximum abundance occurred in 2013 in the Vilaine estuary, reaching 1.9 x 105 cells L1, approximately 25 times lower than values than maximum concentrati First Record of Fukuyoa sp. (Gambierdiscoideae) in the Northeastern Region of Términos Lagoon, Campeche, Mexico Fig. 1. Map of sampling stations at Términos Lagoon, Campeche, Mexico. The station where Fukuyoa sp. was found is circled in red. Términos Lagoon is a region of substantial economic rele Fig. 2. (AB). Fukuyoa sp. in ventral view. (C). Dorsal view. (D). Antapical view showing plates S. d. p., 1, 1. (EF). Ventral view showing plates 1, 1,2, 7, S. d. p., 1,1. (GH). Antapical view showing plates 1p,2,3, 1, 2. Scale bars = 20 μm. in Australia [6] recorded F. paulensis at temperatures of Trophic interactions shape ciguatera risk in a warming ocean Ciguatera is one of the most widespread marine poisonings worldwide, caused by the consumption of fish that bioaccumulate ciguatoxins (CTXs) produced by dinoflagellate species belonging to the genus Gambierdiscus. Its expansion into non-en drivers such as temperature variability and habitat disturbance. Calibration and validation with empirical data on cell densities, toxin concentrations, and ecosystem dynamics will be essential to generate robust predictions. The inclusion of spatial structure and coupling with human health risk mod Red and green waters in southern B rittany (France) in March 2026 linked to a bloom of Mesodinium spp. Fig. 1. Map of reported water discoloration events in southern Brittany (1012 March 2026) based on PHENOMER observations and direct reports. Locations include the Bay of Audierne and the Bay of C Fig. 3. Sentinel-2 satellite image (11 March 2026) showing the spatial extent of the bloom in southern Finistère, with burgundy-red patches indicative of high Mesodinium surface concentrations. are more stable and impart a green coloration. This transformation can occur within minutes, explaining t Fig. 6. Cells of Mesodinium major. 13. Different views of living cells. 46. Representation of pigment dynamics during Mesodinium cell degradation: transition from phycoerythrin-dominated red coloration to chlorophyll-dominated green coloration following cell lysis. All images to scale. accompanied Who turned on the light? First report of extensive bioluminescent blooms of the heterotrophic dinoflagellate Noctiluca scintillans with low abundance of bioluminescent bacteria in the Gulf of Nicoya, Costa Rica Historically, the Gulf of Nicoya has experienced recurrent algal blooms, including events Fig. 2. Dominant dinoflagellate of the Noctiluca scintillans algal bloom around Cedros Island. (A) Bloom of N. scintillans. (B) Ciliate fed on by Noctiluca, Strombidium sp. (C) Culture of bioluminescent bacteria. (D, E) bioluminescence observed at night on Cedros Island and surrounding areas (photog estuaries supporting fisheries, tourism, and biodiversity. Monitoring these processes allows differentiation between benign and harmful blooms and supports the preservation of ecosystem integrity. Ultimately, such phenomena not only expand scientific understanding but also underscore the resilience International Conference on Molluscan Shellfish Safety (ICMSS) Dear colleagues, We wanted to draw your attention to the upcoming International Conference on Molluscan Shellfish Safety (ICMSS), taking place 611 September 2026 at the University of Exeter, UK. Further details, including registration (w DART in action: Scientists launch regional effort against toxic diatoms Scientists from across Asia have come together to tackle the growing threat of toxic diatoms that produce neurotoxin Domoic Acid (DA). On 1718 March 2026, researchers from China, Malaysia, and Singapore gathered in Qingdao, Chin Fig. 2. Kick-off meeting and research presentations. (A) Nancheng Chen, the lead PI, delivers the opening address. (BD) PIs from participating countries presenting their national research plans. (EF) Selected presentations delivered during the workshop. molecular tools can greatly improve our abilit IOC-FAO Intergovernmental Panel on Harmful Algal Blooms (IPHAB) Extraordinary online Session 27 October 2026 and 18th Session (IPHAB-XVIII), 1820 March 2027, FAO, Rome The Intergovernmental Panel on Harmful Algal Blooms (IPHAB) was established in 1992 to strengthen the scientific, managerial, and fi