Marine Heatwaves Emerge as Key Driver of Alexandrium catenella Blooms in Patagonia

Marine heatwaves may play a key role in driving harmful algal blooms (HABs) caused by the toxic dinoflagellate Alexandrium catenella in Chilean Patagonia, according to a new international study examining two decades of oceanographic and bloom data.

These blooms pose a major environmental and socioeconomic challenge in southern Chile, affecting shellfish aquaculture, benthic fisheries and public health through the accumulation of paralytic shellfish toxins, particularly in filter-feeding bivalves.

The study, “Alexandrium catenella blooms driven by extreme thermal anomalies in a fjord system,” was conducted by researchers from Belgium, Chile and Spain. The team included Dr. Patricio A. Díaz, researcher at INCAR² and Centro i~mar at Universidad de Los Lagos. The scientists analyzed the relationship between A. catenella blooms and marine heatwaves in northwestern Patagonia from 2003 to 2023.

Extreme Warming Linked to More Intense Blooms

The results showed a strong association between positive sea-surface temperature anomalies and bloom development. Some of the most intense events occurred during the summers of 2009, 2016 and 2018, coinciding with major marine heatwaves.

The highest abundance recorded during the study period occurred in March 2009, reaching 340,000 cells per liter, while concentrations above 50,000 cells per liter were observed during all three high-bloom years.

According to the researchers, marine heatwaves can intensify and prolong environmental conditions favorable to A. catenella growth. Statistical analyses also suggest that persistent positive thermal anomalies reduce the likelihood of bloom absence and may help define a temperature threshold above which high cell densities become more likely, particularly in the Aysén region.

Regional differences were significant. Southern northwestern Patagonia, especially Aysén, experienced both the most intense blooms and the longest-lasting marine heatwaves. By contrast, the summers of 2019 and 2020, characterized by weaker or negative temperature anomalies, showed relatively low A. catenella abundance and reduced bloom activity.

Temperature Is Only Part of the Picture

The researchers caution, however, that warming alone cannot explain every bloom. Some marine heatwaves were not followed by major A. catenella proliferations, suggesting that nutrient availability, salinity and biological interactions also influence bloom dynamics.

The findings indicate that persistent and intense thermal anomalies are an important factor influencing bloom magnitude. As ocean temperatures continue to rise under climate change, the risk of more intense or prolonged HAB events could increase across the fjords and channels of Chilean Patagonia.

Understanding this relationship could help strengthen monitoring, early-warning and risk-management systems in a region where aquaculture, fisheries and coastal communities are closely dependent on marine ecosystem health.

Read the study: Alexandrium catenella blooms driven by extreme thermal anomalies in a fjord system
https://www.sciencedirect.com/science/article/pii/S2772735126001721