Since 2003, algal blooms have increased in coastal areas around the world. This is shown by a study based on images from NASA’s Aqua satellite. Accordingly, both the area and the frequency of blooms from phytoplankton have increased. The study also shows correlations with rising sea temperatures and changes in ocean circulation. The results can provide a basis for environmental policy decisions.
In seas and lakes, microscopic algae known as phytoplankton form the basis of the food web. However, under certain conditions, they can multiply en masse. These so-called algal blooms can provide other organisms with food and nutrients, but are often associated with negative effects. For example, some phytoplankton species give off toxins that can accumulate in food webs. In addition, there is quickly a lack of oxygen, since the dense carpets of algae dampen the incidence of light and more oxygen is consumed. As a result, so-called dead zones can form in previously intact ecosystems. Increased nutrient input into water bodies, especially through agriculture, promotes the development of algal blooms.
Satellite data as a basis
“The frequency and prevalence of algal blooms is projected to increase with climate change, with some changes having negative impacts on aquatic ecosystems, fisheries and coastal resources,” writes a team led by Yanhui Dai of Southern University of Science and Technology in Shenzhen, China . “However, our current state of knowledge did not allow us to assess whether algal blooms have changed worldwide in recent decades.” The reason for this is that previous records in different parts of the world used inconsistent methods, covered different periods of time and related to different specific ecosystems.
In order to create a unified dataset, Dai and his team have now, for the first time, recorded and analyzed on a global scale how algal blooms have developed in the 21st century. To do this, they evaluated 760,000 images taken by NASA’s Aqua satellite between 2003 and 2020 of the world’s waters. Using the images, the research team was able to determine with an accuracy of one kilometer when and where algal blooms occurred and to what extent. Since the satellite also recorded numerous other data – including, for example, sea temperature – Dai and his team were also able to include possible influencing factors in the analysis.
More frequent and extensive algal blooms
The result: “Of the 153 coastal regions that we analyzed, 126 were affected by algal blooms,” reports the team. The total area affected by algal blooms in 2020 was 31.47 million square kilometers – more than half of it on the coasts of Europe and North America alone, which accounted for the largest share in terms of area. Less extensive but most common, algal blooms occurred around Africa and South America. On average, over the two decades analyzed, the researchers recorded more than six algal blooms per year there, while the global average was 4.3 algal blooms per year.
Over time, Dai and his team noticed a significant increase in algal blooms, both in terms of area and frequency. The area affected in 2020 was 3.97 million square kilometers larger than in 2003 – an increase of 13.2 percent. The global mean frequency increased by 59.2 percent over the observation period. The team saw a significant increase in algal blooms, especially at higher latitudes, while a slight decrease was observed in some tropical and subtropical waters.
Human influences increase algal blooms
Further analyzes provided indications of possible mechanisms: “In many areas at higher latitudes, we found a significant positive correlation between the frequency of algal blooms and the sea surface temperature measured by the satellite,” reports the research team. However, this was not the case in tropical and subtropical waters. Here the researchers found an explanation in changes in ocean circulation affecting how many nutrients are available. If the currents provided additional nutrients, such as around the Canary Islands, there were more algal blooms. If, on the other hand, they reduced the available nutrients, the algal blooms decreased, for example in the area of the California Current, a cold ocean current in the northern Pacific.
“In addition, we examined trends in the use of fertilizers and related them to the frequency of algal blooms,” reports the team. “We found high positive correlations in China, Iran, Vietnam and the Philippines.” Also increasing aquaculture in Finland, China, Algeria, Guinea, Vietnam, Argentina, Russia
and Uruguay have been linked to the increased occurrence of algal blooms. In other countries, on the other hand, the team could not find any significant effects of fertilization and aquaculture – possibly because they were masked by other factors such as sea temperature.
Database for political decisions
To what extent the algal blooms identified in the analysis actually posed a problem for the affected regions – whether they were toxic algal blooms, for example – cannot be determined from the satellite data. In addition, the authors point out that their method of recording potentially still underestimates the frequency and extent of algal blooms, as it is possible that not all algal blooms were recognizable on the automatically evaluated satellite images.
Nevertheless, Dai and his team believe that the results can make a valuable contribution when it comes to making political decisions about controlling nutrient discharges into the water, for example, or taking measures to minimize the effects of harmful algal blooms. The study also provides a basis for better understanding the causes, benefits and risks of algal blooms on a global scale.
Source: Yanhui Dai (Southern University of Science and Technology, Shenzhen, China) et al., Nature, doi: 10.1038/s41586-023-05760-y