Coastal Management

How will climate change impact harmful algal blooms?

Nogueira, E., Bravo, I., Montero, P., Diaz-Tapia, P., Calvo, S., Ben-Gigirey, B., Figueroa, I., Garrido, J. L., Ramilo, I., Lluch, N., Rossignoli, A. E., Riobo, P., Rodriguez, F. (2022). HABs in coastal upwelling systems: Insights from an exceptional red tide of the toxigenic dinoflagellate Alexandrium minutum. Ecological Indicators, 172.

What are Harmful Algal Blooms?

Harmful Algal Blooms (HABs) are excessive growths of toxic microorganisms that impact marine ecosystems as well as recreation and aquaculture activities. They usually occur in semi-enclosed areas, such as estuaries, harbors, or lagoons, where higher temperatures, limited circulation of water, and accumulation of resting cysts (dormant gametes of the microorganisms) lead to favorable conditions for the sudden proliferation of phytoplankton that is characteristic of algal blooms. They most commonly occur from spring to autumn when seasonal upwelling events bring large amounts of nutrients from deeper ocean waters up to the surface.

Although some algae found in blooms are not harmful, those that make up HABs can be life-threatening to people and other animals, so it is important that blooms are monitored, and warnings are sent out when necessary. A recent study by scientists in Spain focused on a type of phytoplankton called Alexandrium minutum, which is an extremely harmful dinoflagellate. This dinoflagellate leads to “red tide” HABs, which turn the water red. Blooms of A. minutum can cause paralytic shellfish poisoning (PSP) in people who consume shellfish that grew in an area where a toxic bloom occurred. Paralytic shellfish poisoning can cause muscular paralysis, neurological symptoms, and sometimes even death.

An image of A. minutum taken with a scanning electron microscope. Photo Credit: US Fish and Wildlife Service, public domain

The researchers of this study focused on an outbreak of PSP that occurred from May to July in 2018 in shallow bays off the NW region of the Iberian Peninsula. This event lasted longer than most harmful algal blooms (around 1 month) and was particularly intense. Because of its unique characteristics, the researchers hoped to analyze the conditions that led to its onset and development to inform better prediction of future bloom events.

Additionally, the researchers investigated the potential that climate change could lead to more frequent HABs in the future. Since HABs occur in response to environmental conditions like warmer temperatures or changes in precipitation, climate change could have an impact on their frequency or severity.

Studying A. minutum bloom development

Since 1994, an institute in Galicia, Spain has recorded a weekly time series of A. minutum cell abundance at the study site. The institute also monitors the temperature and salinity profiles of the study site, which were used in this study to infer hydrographic conditions during the bloom. Meteorological data, including air temperature, rainfall, daily irradiance, and wind direction and intensity, as well as data for upwelling, were also considered. Water samples were collected to measure phytoplankton in the water column, and sediment samples were collected to study cyst distribution.

Satellite image of the Iberian Peninsula. Photo Credit: NASA, public domain.

The time series (since January 1994) indicated that low to moderate abundances of A. minutum were the norm at the study site until the 2018 bloom. Since then, higher values of A. minutum have been observed during 2019 and 2020, reaching a maximum concentration in June of 2020. This indicates that the past several years have seen higher-than-normal toxin concentrations. It is abnormal for blooms to lead to several years of elevated toxins.

The 2018 bloom caused prolonged shellfish harvesting closures due to the presence of the toxins that cause PSP, which resulted in economic loss. After the bloom, resting cysts were observed in water and sediment samples at high concentrations. Cysts are dormant gametes of the phytoplankton, and they turn into phytoplankton cells once conditions are favorable. The presence of cysts following the 2018 bloom likely contributed to the higher-than-normal concentrations of A. minutum during 2019 and 2020 and the subsequent shellfish harvesting closures.

After analyzing the bloom conditions alongside meteorological conditions, the researchers found that the weather conditions during the 2018 bloom differed from climatic averages. Precipitation and runoff values were found to be higher than normal directly before the bloom and were then followed by a sudden rise in air temperature and daily irradiance for two weeks. Additionally, there was high upwelling intensity directly before the bloom occurred. The researchers hypothesize that these environmental conditions led to the abnormal 2018 bloom and the high concentrations of A. minutum during 2019 and 2020.

The observed meteorological conditions, namely a heavy rain event followed by high temperatures, are projected to become increasingly common as climate change continues. Although it is not possible to say with certainty that the 2018 HAB event was caused by climate change, it is reasonable to assume that climate change could lead to these events becoming the new norm for the NW Iberian Peninsula.

This study contributes to a better understanding of the environmental factors that trigger A. minutum blooms and can hopefully lead to better predictions of bloom events in the future. HABs can be extremely dangerous to people and wildlife, so it is important that we gather a better understanding of their causes in order to improve prediction and warning systems. With the possibility that HABs could become more common or more intense as a result of climate change, studying HABs in the context of our changing world is extremely important.


Cover photo: HABs as viewed from above. Photo credit from left to right: Landsat-8 (NASA/USGS satellite); NOAA

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