Study provides detailed assessment of shifts in toxin-producing phytoplankton abundance

The University led a study showing how the abundance and distribution of two groups of phytoplankton have changed over the last 60 years

Pseudo-nitzschia are a common component of the phytoplankton community along Europe’s Atlantic coast and the North Sea, and a number of species produce the toxins responsible for amnesic shellfish poisoning

Study provides detailed assessment of shifts in toxin-producing phytoplankton abundance

The University led a study showing how the abundance and distribution of two groups of phytoplankton have changed over the last 60 years

Plymouth Marine Laboratory

Mr Alan Williams

Media and Communications Manager

Corporate Communications (Marketing and Communications)

14 May 2026

Researchers in the UK have shown how the distributions of two phytoplankton groups – known to produce natural toxins that can halt shellfish harvesting – have changed in the North East Atlantic over the last six decades.

Phytoplankton are single-celled microscopic algae that play an important role in the marine ecosystem, converting energy from sunlight into carbohydrates via photosynthesis and forming the base of the marine food web.

However, some phytoplankton can produce biotoxins that can accumulate in the flesh of shellfish via filter feeding, and higher toxin levels can pose a serious risk to human health if consumed, and also to marine mammals and sea birds. To combat this, all shellfish are tested for the presence of these toxins by Food Standards Agency, UK and Food Standards Scotland before being placed on the market to ensure human health is protected.

Pseudo-nitzschia and Dinophysis are common components of the phytoplankton community along Europe’s Atlantic coast and the North Sea. A number of Pseudo-nitzschia species produce the toxins responsible for amnesic shellfish poisoning (ASP), while some Dinophysis species produce the toxins responsible for diarrhetic shellfish poisoning (DSP).

A new study, published in the journal Harmful Algae, has provided updated maps for the North East Atlantic showing how the abundance and distribution of these two groups have changed over the last six decades.

These changes are important as the toxins produced can act as a chronic environmental pressure for sea birds and marine mammals, but the increased knowledge can be used as a way of anticipating shifting risks for shellfisheries and aquaculture.

The researchers also wanted to explore whether changes observed within these types of phytoplankton are representative of overall plankton diversity.

The research was led by scientists from the University of Plymouth and the Marine Directorate of the Scottish Government, in conjunction with Plymouth Marine Laboratory (PML), the Marine Biological Association (MBA), and the Scottish Association for Marine Science (SAMS).

These are two of the most closely monitored groups of harmful algae in Europe.

However, one of the main conclusions from this study is that they cannot reliably represent the wider phytoplankton community. Pseudo-nitzschia and Dinophysis follow distinct seasonal cycles and long-term redistributions, rather than following those of the wider groups to which they belong. It highlights the value of sustained long-term monitoring programmes that consider the full plankton community, and many of which are currently under threat due to funding cuts, resource constraints, and skills shortages.

Dr Matthew Holland
Postdoctoral Research Fellow and the study’s lead author

Studying shifts in Pseudo-nitzschia and Dinophysis

The research focused on observations from two offshore monitoring stations – the L4 station managed by PML in the Western English Channel and the Scottish Coastal Observatory monitoring (SCObs) Stonehaven station, south east of Aberdeen in the North Sea and managed by the Marine Directorate of the Scottish Government.

These observations were then compared with six decades of data from the MBA’s Continuous Plankton Recorder (CPR) Survey, the world’s most extensive survey of marine biodiversity. This comparison allowed researchers to assess changes in both coastal and open ocean environments, as well as periods ranging from individual seasons to several decades.

For Pseudo-nitzschia, the CPR data showed that in the North Sea, abundances rose from the 1970s and peaked between 2000 and 2010 before declining after 2010, with increases in spring along the southern and eastern North Sea and later summer peaks northeast of Scotland.

The results from the coastal stations showed significant variation, with a large peak in abundance in the spring and a smaller one in summer at the L4 station but the opposite – a smaller peak in spring and a larger peak in summer – at the Stonehaven site.

The CPR data showed that Dinophysis were mostly restricted to summer months across decades. In the eastern North Sea they declined after 2000. At the Stonehaven station there was often an earlier seasonal peak than revealed by CPR data in adjacent offshore waters. The researchers say these changes are more closely aligned with physical shifts in the ocean driven by climate change rather than by other human activities, such as fishing practices.

This study highlights the value of joined up, long-term plankton monitoring across UK shelf seas. By linking coastal stations such as L4 and Stonehaven with large-scale Continuous Plankton Recorder surveys, we can see how harmful algal groups are changing over decades. Crucially, monitoring the full plankton community is essential for placing these patterns in context and making more accurate predictions about the future state of UK shelf seas.

Claire Widdicombe
Plankton Ecologist at Plymouth Marine Laboratory

These findings show why long-term plankton monitoring is so important. By combining observations from coastal monitoring stations and the CPR Survey, we can track how harmful algae are shifting through time and across the North East Atlantic, helping us better understand risks to shellfisheries, marine wildlife, and the wider food web.

Dr Clare Ostle
Senior Research Fellow at the Marine Biological Association and Chair of the Global Alliance of CPR Surveys

The Continuous Plankton Recorder device is towed in surface waters and occupies a similar space to a marine mammal (Credit Marine Biological Association)

Marine Biological Association

Why are Pseudo-nitzschia and Dinophysis so interesting and important to monitor?

Pseudo-nitzschia are fast-growing diatoms that can dominate for short seasonal periods, particularly in spring and summer. Some Pseudo-nitzschia species produce domoic acid (causing ASP).

The most serious incident of ASP occurred on Prince Edward Island, Canada, in 1988 and resulted in three human fatalities and 153 cases of acute intoxication after consuming contaminated mussels. In recent years, domoic acid has impacted marine mammals and sea birds on the west coast of the USA, resulting in altered behaviours and, in some instances, mortalities.

Dinophysis is a very complex organism which can both consume prey and photosynthesise, and it steals chloroplasts from smaller prey through a distinct life strategy known as kleptoplasty. Some Dinophysis species produce okadaic acid and dinophysistoxins (the toxins responsible for DSP).

For almost 30 years, frequent monitoring has been carried out in shellfish growing waters off the UK coast and areas are closed to harvesting when legal toxin limits are exceeded. DSP toxins are responsible for the most closures of shellfish harvesting areas in UK waters.

However, despite the UK’s monitoring and closure system protecting human health, the changing distribution of these groups highlights the potential impact of a changing marine environment on the aquaculture industry and marine food web.

The full study – Holland, M. M., Bresnan, E., Edwards, M., Faith, M., Ostle, C., Stern, R., Tett, P., Widdicombe, C., Whyte, C. and McQuatters-Gollop, A. (2026) Changing Pseudo-nitzschia and Dinophysis distributions in the North Sea and Western Approaches (NE Atlantic) and their potential use in biodiversity assessments – is published in Harmful Algae, DOI: https://doi.org/10.1016/j.hal.2026.103113

Mr Alan Williams

Studying shifts in Pseudo-nitzschia and Dinophysis

Why are Pseudo-nitzschia and Dinophysis so interesting and important to monitor?

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