Harmful algal blooms (HABs), caused by approximately 200 phytoplankton species, pose significant threats to human health, seafood security, coastal economies, and natural systems. These blooms produce toxins responsible for shellfish poisoning (e.g., paralytic, diarrhetic, amnesic, and azaspiracid shellfish poisoning), fish kills, and water quality degradation. While HABs are not a new phenomenon, historical records and recent advancements in detection technologies suggest an increase in frequency and intensity. This has led to hypotheses linking HAB expansion to factors such as eutrophication, climate change, introduction of alien species, and aquaculture developments. However, the lack of a comprehensive, global dataset for assessing temporal trends has hampered robust conclusions. This study aims to address this gap by analyzing global data from HAEDAT and OBIS to determine if there is empirical evidence to support the claim of a global increase in HABs and, if not, identify the contributing factors to the widespread perception of an increase.

Existing literature presents conflicting views on global HAB trends. Some studies suggest an apparent increase, attributing it to increased awareness and improved monitoring. Others question the lack of robust evidence for a global increase. The IPCC's 2019 report projected a continued increase in HAB occurrence, toxicity, and risk due to climate change and rising CO2 levels. This highlights the need for a quantitative, global assessment of HABs, considering their impacts on the 3.3 billion people who rely on seafood for a significant portion of their protein intake, especially given the challenges to terrestrial agriculture.

This study used three databases: the IOC-UNESCO Taxonomic Reference List of Harmful Microalgae, HABMAP-OBIS (providing geographic ranges of harmful species), and HAEDAT (containing information on harmful algal events impacting human society). HAEDAT data, encompassing events leading to management actions or negative socio-economic impacts, were categorized into four types: water discoloration, seafood toxin accumulation, shellfish harvesting bans, and impacts on humans, animals, or other organisms. Freshwater HABs and macrophyte blooms were excluded. Data were partitioned into twelve OBIS provinces. To account for potential biases due to variations in monitoring effort, the study used OBIS microalgal records as a proxy for sampling effort. The number of OBIS grid cells with phytoplankton records was counted per region and year. Trend analyses covered the period 1985–2018, focusing on HAEDAT events impacting human society. Meta-analysis was used to assess overall trends, combining correlation results across regions. Four separate meta-analyses were conducted: (1) on the number of HAEDAT events over years; (2) on the number of geographic grids with one or more HAB events per year; (3) on the ratio of HAEDAT events to OBIS microalgal sampling over years; and (4) on the relationship between HAEDAT events and aquaculture production (using FAO data). Temporal autocorrelation was adjusted for in all meta-analyses using the method of Pyper and Peterman.

As of December 2019, HAEDAT contained 9503 events, with 48% related to seafood biotoxins, 43% to high phytoplankton counts/water discoloration, 7% to mass mortalities, and 2% to other factors. Regional HAEDAT records varied widely. Meta-analysis of the number of HAEDAT events revealed a significant positive trend globally (r=0.37, p=0.003), but individual regions showed mixed trends. Adjusting for sampling effort using OBIS data showed no significant global increase in standardized HAB events (r = 0.35, p=0.18), indicating that the increase observed in HAEDAT events could be significantly related to improved monitoring. Meta-analysis of HAEDAT events against aquaculture production showed a significant positive correlation globally (r=0.43, p=0.0003), suggesting aquaculture expansion as a contributing factor to increased economic and societal impacts from HABs. Case studies on Diarrhetic Shellfish Poisoning (DSP), Amnesic Shellfish Poisoning (ASP), and Paralytic Shellfish Poisoning (PSP) revealed significant increases in OBIS records of causative organisms, paralleled by increases in HAEDAT records of associated toxic events. However, human poisonings did not always accurately reflect the occurrence of causative organisms, highlighting the efficiency of food safety management in many affected countries. Ciguatera Poisoning (CP) showed variable trends across different regions. Finfish aquaculture mortalities due to HABs are a major economic concern, with losses reaching billions of dollars. These mortalities are more often linked to HABs affecting intensive aquaculture operations rather than broader ecosystem-level consequences.

The findings challenge the widely held belief of a global increase in HABs. While increases in HAEDAT events were observed, these appear largely attributable to intensified monitoring efforts linked to increased aquaculture production and other factors impacting reporting. The study’s data adjustment using OBIS observations for sampling effort suggests that the actual increase in HAB events might not be as widespread as previously believed, despite an obvious increase in societal impacts. The strong correlation between aquaculture production and HAB events highlights the significant economic and societal consequences of blooms affecting aquaculture, underscoring the need for improved management strategies, early warning systems, and robust economic modeling to support mitigation actions. The varied regional trends highlight that future research requires species and region-specific investigations.

This study provides robust evidence against a uniform, global increase in HABs over the period 1985–2018. The perceived rise in HABs seems largely explained by increased monitoring and the expanding impacts on aquaculture, emphasizing the need for regionally specific analyses. The findings underscore the importance of improved data collection, integration of multiple data sources, and ecological investigations to explain variability in HAB occurrence and mitigate their impacts. Future research must address ecological mechanisms driving regional variations and species-specific HAB trends, considering both climatic and anthropogenic influences.

The study acknowledges limitations related to data availability and the accuracy of historical HAB records. Variations in reporting practices, monitoring capabilities, and the definition of a 'HAB event' across different regions could influence the results. While OBIS data provided a valuable proxy for sampling effort, it may not perfectly capture all aspects of monitoring intensity. The aquaculture production data used may not fully represent the precise impact on HABs in all regions.