Drastic seaweed growth threatens marine life and fishing — but also offers opportunities

Large blooms of seaweed are increasingly being reported along coastlines globally, from Europe and Asia to the tropics and beyond.

Both native and invasive (non-native) seaweeds are appearing in quantities that are hard to ignore and at unusual or surprising times of year.

As an earth observation and remote-sensing scientist, I track these blooms from space using high-resolution satellite imagery. My research shows that seaweed blooms are getting bigger.

My team’s 2025 study reveals a significant rise in sargassum blooms in the north-eastern tropical Atlantic, with a staggering 2.6 million tonnes washing up in September 2020. This is the first long-term analysis of trends in seaweed blooms from 2011 to 2022 in this region.

These unpredictable tides of seaweed have serious consequences for West African coastal communities and marine ecosystems. Our research shows that warming sea surface temperatures link closely with peaks in seaweed growth. Essentially, warmer temperatures can promote seaweed growth and lead to bloom surges.

Seaweed blooms are not a new phenomenon. But over the past 15—20 years, their scale and persistence have increased noticeably.

Of particular concern are free-floating seaweeds: species that float at the ocean surface, either because they detach from the seabed or because they spend their entire lives drifting. Unlike seaweeds that are anchored to the seafloor, floating seaweed can travel long distances to new territories and accumulate in large mats or wash ashore in huge quantities.

One example I have spent much of my career studying is sargassum. Like something from a sci-fi movie, I’ve seen swathes of sargassum seaweed spreading across the tropical Atlantic, with mats reaching depths of 7 m and spanning hundreds of square miles.

While most sargassum species are anchored to the seafloor, two species — Sargassum natans and Sargassum fluitans — are entirely free floating. They float freely at the surface of the ocean, kept buoyant by small air-filled grape-like sacs called pneumatocysts, which lift them up towards the surface for photosynthesis.

Our study shows that, since 2011, huge blooms of sargassum seaweed have appeared across the tropical Atlantic, piling up on coasts in the Caribbean, Gulf of Mexico and increasingly West Africa. This drifting seaweed makes fishing difficult and causes mayhem for coastal communities.

Seaweed plays an essential role in marine ecosystems, but excessive growth can disrupt them. Large floating mats block sunlight, limiting the growth of seagrasses and corals below. They also alter oxygen conditions in the water, and when seaweed decomposes, particularly in sheltered bays or on beaches, it can create low-oxygen environments that are harmful to marine life.

Some of the most striking consequences are seen on wildlife. In tropical regions, sargassum has accumulated on turtle nesting beaches, with recent studies suggesting that up to a quarter of nesting habitat can be affected. Hatchlings struggle to move through both sand and dense seaweed before eventually reaching the sea, exhausted. This reduces their chances of survival.

Across Europe

Sargassum as an invasive species has actually found its way to UK waters, but sargassum blooms are not nearly as vast as in the tropical Atlantic. Blooms of other types of seaweed are becoming more noticeable in the UK and Europe. For example, ulva, a green seaweed known as sea lettuce regularly forms dense mats on the surface of the sea in places like Poole harbour, Dorset.

In small amounts, ulva is a native and largely harmless part of UK coastal ecosystems. But when it blooms excessively, it can start to cause problems. Thick mats at the surface reduce the amount of sunlight reaching seagrasses and other organisms below, while decomposition can reduce oxygen levels in the water, creating stressful conditions for fish and invertebrates and death of plants and animals as a result.

Across Europe, invasive seaweeds are becoming a growing concern. In the Mediterranean, species such as Rugulopteryx okamurae (originally from the northwest Pacific) have spread rapidly, likely introduced through shipping routes. These seaweeds can attach to the seabed, but then detach, float for long distances, and then reattach elsewhere, allowing them to spread efficiently along coastlines. In parts of Spain and Portugal, large accumulations are now washing up on beaches, with negative effects similar to those seen with sargassum in the tropics.

Even when blooms are smaller or more localised, their effects can still be disruptive. Seaweed accumulation can interfere with recreation, small-scale fishing and coastal tourism — all important parts of the UK’s coastal economy.

Why is seaweed blooming?

Seaweed growth is driven by a combination of triggers and favourable conditions, so there isn’t a single cause.

In the case of sargassum in the tropical Atlantic, one important trigger appears to have been an anomaly in the large scale atmosphere-ocean pattern known as the North Atlantic Oscillation in 2009. This change in atmospheric pressure at sea helped redistribute seaweed from the Sargasso Sea. Once established in new regions, further seaweed growth was fuelled by access to nutrients.

Seaweed growth is limited by the availability of nutrients such as nitrogen and phosphorus. As long as those nutrients are available for them, they will grow. Nutrient-rich runoff from agriculture, rivers such as the Amazon and Congo, and sediment inputs all deliver these nutrients into the ocean — so human-caused pollution also plays a part.

Together, warming waters, nutrient enrichment and changing ocean circulation can create ideal conditions for blooms to persist and expand.

Seaweed blooms, while sometimes problematic, are fundamental to ocean ecosystems. They act as habitats to small fish and crustaceans. They absorb carbon dioxide through photosynthesis and transport it to deeper waters. They are also a valuable resource. They are used to make fertiliser and building materials, pharmaceuticals and potentially biofuels.

With effective monitoring, more accurate forecasting and better management, communities can live alongside seaweed blooms, harnessing their benefits while minimising environmental and economic consequences.

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Yanna Alexia Fidai, Earth Observation and Remote Sensing Scientist, Plymouth Marine Laboratory

This article is republished from The Conversation under a Creative Commons license. Read the original article.