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 the coexistence of red and green patches within the same coastal system (Fig. 2B, Fig. 56). Due to its distinctive optical fingerprint, Mesodinium red tides can often be detected using high-resolution satellite remote sensing; however, similar spectral signatures may also arise from other phycoerythrin-containing planktonic blooms, including occasional blooms of Dinophysis spp. and cryptophytes such as Teleaulax spp., and thus the signal is not entirely specific [5, 78]. Satellite imagery (Sentinel-2, 11 March 2026) confirmed the presence of an extensive surface bloom in southern Finistère [3], suggesting that the phenomenon extended well beyond the sampled locations (Fig. 3). These observations should be interpreted with caution in terms of pigment-based diagnostics, as similar colour shifts can also occur in other mixotrophic or plastid-bearing protists. In particular, laboratory and field studies on Dinophysis spp. have shown that starvation or physiological stress can lead to altered plastid function and a transition towards a greenish appearance, as demonstrated in experimental cultures of D. caudata and in field observations of D. acuminata populations [89]. This reinforces that green coloration in marine planktonic protists is not necessarily indicative of a single taxonomic or physiological state, but may reflect multiple underlying biological processes, including prey-dependent plastid composition, starvation, or cell degradation. The development of such a bloom likely reflects a combination of favourHARMFUL ALGAE NEWS NO. 83 / 2026 able environmental drivers. Blooms of Mesodinium spp. are commonly associated with stratified conditions, high light availability, and sufficient densities of cryptophyte prey [1012]. In early spring, increasing solar irradiance combined with episodic water column stabilisationoften following periods of calm weathermay promote rapid population growth. Additionally, nutrient-enriched coastal waters, influenced by riverine inputs or sediment resuspension, may indirectly support Mesodinium proliferation by sustaining cryptophyte populations. Normal salinity in Concarneau bay is 35, and it can decrease to 2830 in winter depending on rainfall. Freshwater from the Loire River can reach as far as the Brittany peninsula, and depending on river discharge, it can further enhance desalination, as observed this year with salinity falling to 20 PSU in February. The current bloom was in sync with a significant higher discharge of the Loire river, lowering salinities to 20 psu in February. Hydrodynamic conditions may also have played a key role in shaping the spatial distribution of the bloom. The retention of water masses in semienclosed areas such as the Bay of Concarneau, combined with local circulation patterns along the Audierne coast, could facilitate bloom accumulation and patchiness. The rapid appearance and disappearance of coloured waters suggest a transient event controlled by short-term physical forcing, such as wind-driven mixing or tidal advection. The ecological dynamics of Mesodinium blooms are closely linked to their mixotrophic strategy, which relies on the acquisition and maintenance of functional plastids from cryptophytes. This dependency enables rapid growth under optimal conditions but may also lead to abrupt bloom collapse when prey availability declines or environmental conditions shift [7]. The accumulation of organic material observed in green biomass is consistent with bloom senescence and the release of cellular contents, which may stimulate bacterial activity and influence local biogeochemical cycling. Similar discoloration events attributed to Mesodinium spp. have previously been documented in the region, including a comparable episode in February 2023 [2]. Such recurrence suggests that these blooms may represent a characteristic, though episodic, feature of coastal ecosystem dynamics in southern Brittany. In a broader context, increasing frequency or visibility of such events could be linked to ongoing environmental changes, including shifts in stratification regimes, nutrient inputs, or plankton community composition, although further observations are required to assess long-term trends. From a sanitary perspective Mesodinium spp. are not considered harmful but non-toxic [14], and no direct impacts on human health have been reported. Consequently, the March 2026 discoloration events were not expected to pose a risk to shellfish consumers. Nevertheless, the striking visual appearance of red or green waters, sometimes Fig. 4. Light microscopy images of live Mesodinium spp. cells observed in the red water (Concarneau). Cells exhibit characteristic morphology and pigmentation due to kleptoplastids. Fig. 5. Light microscopy images of degraded Mesodinium cells and organic aggregates observed in green water (La Torche), illustrating pigment loss and cellular disintegration. 15 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