Vigo, 22 April 2021. A scientific team from the Spanish Institute of Oceanography (IEO, CSIC), the Technological Institute for the Control of the Marine Environment of Galicia (INTECMAR) and the Institute of Marine Research (IIM-CSIC) has published a study analysing the unusual conditions that gave rise to a major bloom of a dinoflagellate that produces paralytic toxins in 2018. The study will help improve monitoring systems for events that scientists expect to become more frequent and more intense.
Between May and July 2018, a toxic bloom of Alexandrium minutum, a dinoflagellate that produces paralytic toxins, occurred in the rías of Vigo and Pontevedra.
It was an exceptional event that caused water discolouration for a month and prolonged closures of shellfish harvesting. The first sighting took place on Samil beach, in the Ría de Vigo, and in the following days the most intense patches were located in areas with limited water circulation, such as the Vigo port dock.
“The weather conditions during that period, with rainfall above the average and a subsequent rise in temperature, together with a pattern of shifting winds, created the optimal conditions for the development of this extensive and massive toxic proliferation,” explains Enrique Nogueira, IEO researcher and first author of the paper.
Alexandrium minutum is a common species in the phytoplankton of the Galician rías, but it is usually more abundant in sheltered estuarine areas and it is not common for it to reach densities or a distribution as broad as those observed in the 2018 episode.
In the study, recently published in the journal Ecological Indicators, simulations were carried out using a particle-tracking model to reconstruct the phases of the bloom: its onset and development within a small inner bay, transport within the surface layer from these sites towards the inner ría, and dispersal throughout the bay. “These models improve our overall understanding of harmful algal bloom dynamics,” notes Nogueira.
Months after the red tide, cysts were detected in the sediment in several areas on the southern coast of the Ría de Vigo, especially in the port area. This may have favoured blooms of Alexandrium minutum in the following two years, particularly in 2020. The subsequent presence of cysts and the likely recurrence of the climatic scenario that enabled this unusually intense event “could lead to an increase in both the frequency and the intensity of Alexandrium minutum blooms in the area, as well as their regional expansion,” the scientist points out.
The findings of this work “are of interest not only at local or regional scale, but also for other geographic areas with similar characteristics,” Nogueira concludes.