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Incredible satellite images show how toxic algal blooms now grow in at least 12.1 million square miles (31.47 million square kilometres) of ocean, thanks to human activity.
A study from researchers at the Southern University of Science and Technology in China found that the total area affected by phytoplankton growth has increased by 13.2 per cent since 2003.
When nutrients from fertilisers, sewage, air pollution and livestock manure find their way into water sources, phytoplankton will feed on their nutrients and grow.
The researchers also found that rising sea temperatures have increased the frequency of algal blooms over this last two decades, as warmer waters increases the length of the blooming season.
They wrote: ‘Many algal blooms are beneficial, fixing carbon at the base of the food chain and supporting fisheries and ecosystems worldwide. However, proliferations of algae that cause harm, termed harmful algal blooms, have become a major environmental problem worldwide.’
Incredible satellite images show how toxic algal blooms now grow in at least 12.1 million square miles (31.47 million square kilometres) of ocean, thanks to human activity
A study, from researchers at the Southern University of Science and Technology in China, has found that the total area affected by phytoplankton growth has increased by 13.2 per cent since 2003
An algal bloom is a rapid increase in the population of algae in an aquatic system, and can occur in freshwater as well as marine environments.
Blooms can cause discoloration of the water at surface level, turning it yellow, red or bright green, as can be seen in the new satellite images.
They are often the result of eutrophication – when bodies of water become enriched with nutrients like phosphates and nitrates, which provide food for algae and other green plants, allowing for their growth.
The phytoplankton become harmful when they use up all the dissolved oxygen in the water, leading to the suffocation of fish and aquatic insects, which in turn become nutrient sources themselves.
Some types of algae also produce biotoxins which can have severe impacts on wildlife, and contribute to these ‘dead zones’ where aquatic life cannot survive.
For the study, published today in Nature, scientists used 760,000 images taken by NASA satellites from 2003 to 2020 to calculate the global ocean area covered by algae each year and bloom frequency.
They then compared this data to sea surface temperatures (SSTs) as well as how much the temperature changes per metre, or the ‘spatial SST gradient’.
A large difference in temperature between two adjacent areas in the ocean, or large spatial SST gradient, can indicate poor water circulation.
This is because hotter water is less dense than colder water and reduces the sinking in the natural ‘conveyor belt’ motion of the currents.
As a result, nutrients are less evenly distributed through the water column, and phytoplankton is able to grow in areas where they are highly concentrated, generally those closer to the surface.
Large differences in temperature between adjacent areas of ocean can also cause turbulence and bring up more nutrients from deeper waters to further encourage algal growth.
The maps showed that coastal phytoplankton blooms have increased by 1.53 million square miles (3.97 million square kilometres) over the observed period.
In 2020, they covered 8.6 per cent of all global ocean area.
The researchers also found that rising sea temperatures have increased the frequency of algal blooms over this last two decades, as warmer waters increases the length of the blooming season
The maps showed that coastal phytoplankton blooms have increased by 1.53 million square miles (3.97 million square kilometres) over the observed period. Pictured: Spatial distribution of annual average bloom count based on satellite images
The average annual number of blooms observed was also found to increase by 59.2 per cent between 2003 and 2020 – the equivalent of 2.19 per cent each year.
This rising bloom frequency correlated significantly with ocean temperature, suggesting that ‘bloom-favourable seasons in these temperate seas have been extended under warmer temperatures’.
Bloom frequency also increased with the spatial SST gradient in many areas, including California, the Gulf Steam and the Canary Current.
This suggests that the weak ocean currents caused by rising sea temperatures contribute to the growth of algae.
Around China, Iran, Vietnam and the Philippines, the researchers also noted an increase in fertiliser usage over the study period, which likely contributed to the upswing in phytoplankton.
The same could be said for the intensified aquaculture industry in Finland, China, Algeria, Guinea, Vietnam, Argentina, Russia and Uruguay.
The average annual number of blooms observed was also found to increase by 59.2 per cent between 2003 and 2020, the equivalent of 2.19 per cent each year. Pictured: Annual average bloom frequency and total global bloom-affected area from 2003 to 2022
Bloom frequency also increased with the spatial SST gradient in many areas, including California, the Gulf Steam and the Canary Current. This suggests that weak ocean currents, that can be brought on by global warming, contribute to the growth of algae. Left: Global patterns of trends in spatial SST gradient from 2003 to 2020. Right: Global patterns of trends in SST from 2003 to 2020
The researchers hope that their maps and data will help increase understanding the mechanisms that drive and dissipate coastal phytoplankton blooms.
This may inform policymakers assess the risk of harmful algal blooms and that decide on strategies to help reduce their occurrence.
‘Many blooms are beneficial, particularly in terms of their positive effects on ecosystems as well as on wild and farmed fisheries,’ the authors wrote.
‘The results here can also contribute toward policies and management actions that sustain those beneficial blooms.’