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 (photographs by Pablo Novoa Villegas). seas and contributes significantly to microbial recycling and the regeneration of dissolved and particulate organic matter, thereby sustaining ecosystem productivity [6]. For this reason, it is often used as an indicator of microbial recycling, as well as the balance between primary and bacterial production. Although the Gulf of Nicoya may present relatively oligotrophic conditions in certain areas and certain seasons, mainly during the dry season (DecemberMarch), it is not a typical oligotrophic system. The estuary is characterized by strong tidal influence and significant riverine inputs, especially during the rainy season (MayNovember), resulting in a highly dynamic estuarine mixing system [3]. Its high primary productivity in the inner basin is strongly influenced by tidal cycles and riverine input. These conditions favour the development of dinoflagellate blooms associated with strong trophic variability. In this context, the extensive N. scintillans bloom did not depend solely on dissolved nutrient concentrations, as this species typically feeds on smaller plankton and benefits from stable surface waters where it accumulates. In addition, high concentrations of Strombidium sp. may have contributed indirectly to the formation of blooms throughout the area. Bacterial counts ranged from 48 to 840 CFU mL-1, while bioluminescent bacteria did not exceed 2 CFU mL-1. This low abundance suggests relatively oligotrophic conditions 18 in areas adjacent to Cedros Island. This condition may contribute to the persistence of bioluminescent events, with episodic intensification driven by nutrient pulses associated with oceanographic dynamics such as wind-induced mixing. It has been shown that nutrient availability controls the abundance and structure of marine bacterial communities, with lower densities observed in oligotrophic environments [7]. Although further research is required to understand microbial interactions in this system, the low abundance of bioluminescent bacteria suggests that the observed luminescence is primarily associated with the trophic dynamics of N. scintillans. While historical records include bioluminescent events driven by high densities of luminous bacteria over large oceanic regions (Milky Seas), these produce a continuous, homogeneous glow consistent with bacterial emission [8]. In contrast, dinoflagellate bioluminescence is characterized by short flashes or bursts triggered by mechanical stimulation [9], as observed here. Although N. scintillans blooms have previously been reported with reddish coloration in the Gulf of Nicoya, this is the first observation of a dense bloom with intense orange coloration. These blooms consistently exhibit strong nocturnal bioluminescence. An additional previously undocumented feature in this region is the oily or viscous tactile sensation during intense bloom conditions, associated with dense surface ag- gregations and mucilaginous material. At high densities, N. scintillans can produce significant amounts of ammonia and organic compounds, contributing to altered water texture [10]. The bioluminescent blooms of N. scintillans in the Gulf of Nicoya represent ecologically significant phenomena of increasing scientific and touristic interest. These events have contributed to the development of local ecotourism activities based on bioluminescence observation. Although N. scintillans is not considered directly toxic to humans, it may cause indirect ecological impacts through oxygen depletion and fish mortality associated with organic matter degradation and bacterial activity. The occurrence of N. scintillans blooms in this region appears to be influenced by seasonal temperature increases and nutrient inputs associated with wind-driven mixing. Bioluminescence is observable only under mechanical disturbance but can be visible under moonlight or boat illumination. Its extreme luminosity has been proposed to play an adaptive role in predation or intraspecific communication. The recent bloom highlights the sensitivity and ecological complexity of estuarine systems. Such events demonstrate how relatively small variations in climate, circulation, and nutrient inputs can trigger large-scale responses across planktonic and microbial food webs. Understanding these dynamics is essential for assessing productivity, biodiversity, and environmental change in 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