- Climate change is driving major shifts in plankton communities and pelagic habitat conditions across UK waters.
- Long-term monitoring shows declines in large phyto- and zooplankton in oceanic waters beyond the continental shelf, while smaller plankton and microbial species are dominating in warmer, stratified shelf waters. Smaller species transfer energy less efficiently, reducing food availability for predators.
- Warming and changes in stratification and currents are impacting phyto- and zooplankton species distribution, with some warm-water species spreading northward and contracting their southward distributions, while others are declining in southern areas.
- The timing of the spring plankton bloom has advanced in many parts of the North Sea, creating risks of mismatch with fish spawning.
- Gelatinous organisms such as jellyfish are increasing in some regions, likely due to warming as well as changes in predator and prey availability; however, in most regions they remain poorly monitored.
- Marine heatwaves and other extreme events may impose increasing stresses on plankton communities although their effects are still in early stages of being explored.
- These climate driven pressures on plankton communities are amplified by eutrophication, changing nutrient ratios, coastal darkening (reduction in water clarity caused by land runoff and climate-driven sediment resuspension), fishing impacts, and offshore energy infrastructure.
High evidence, medium consensus
Long-term monitoring programmes, including the Continuous Plankton Recorder Survey, Western Channel Observatory, Scottish Coastal Observatory, Lorn Pelagic Observatory, inshore Environment Agency monitoring, and Earth observation data, provide extensive temporal and spatial coverage. Although these datasets vary in duration, resolution, and spatial extent, together they present a clear pattern of change, including consistent shifts in phenology, size structure, and trophic pathways, as well as changes in zooplankton communities and an increasing influence of gelatinous taxa. Observational evidence is further supported by mesocosm experiments, trait-based analyses, and multi-trophic studies, which reinforce the mechanisms driving these changes. However, agreement among studies was more moderate, as contrasting patterns between offshore declining trends off the shelf and more variable or increasing trends in coastal and North Sea regions, alongside differences among plankton groups and lifeforms, reduce consensus in the direction and magnitude of change. In addition, while key drivers such as warming, stratification, nutrient stoichiometry, circulation, and light availability are widely understood, their relative importance varies across regions and seasons, limiting agreement on the dominant processes responsible for observed changes.
- Future warming is expected to result in earlier and longer lasting spring blooms in some reasons, farther northward shifts in phyto- and zooplankton, as well as fish species, reduced overall primary productivity by intensifying water-column stratification and depleting surface nutrients, and increased dominance of smaller phytoplankton and protist species (single-celled eukaryotic organisms) adapted to nutrient-poor conditions.
- Climate-driven changes in primary productivity and phyto- and zooplankton community composition are expected to adversely impact marine food web functioning and efficiency, reducing the maximum sustainable yield for key fish species in some regions. Fish larvae are also likely to need more prey to survive as warmer water increases metabolic demands.
- Carbon export to the seafloor is expected to be weakened with declining primary productivity and phytoplankton community shifts towards smaller and slower sinking plankton, reducing the rate at which UK seas remove anthropogenic carbon from the atmosphere.
- Reduced food web productivity from changes in plankton will have wider impacts to the functioning of the UK marine environment and alter the provision of future ecosystem services, including key fisheries.
Medium evidence, low consensus
The evidence base supporting projections of plankton change is moderate, but overall confidence is limited by low agreement among studies. A wide range of modelling and experimental studies have explored future scenarios, including trait- and species-based phenology models, end-to-end ecosystem frameworks, coupled physical-biogeochemical models, and mesocosm experiments examining warming, heatwaves, and nutrient dynamics. These approaches generally agree on broad patterns, such as earlier bloom timing, extended growing seasons, and poleward shifts in species distributions. However, there is considerable variation in the magnitude, timing, and regional expression of these changes, reflecting differences in model structure and how key processes are represented. In particular, major disagreement among models remains for bulk ecosystem properties, such as consumer biomass, and some projections diverge substantially from observed trends, highlighting ongoing challenges in model reliability. Agreement is also restricted by the complexity of multiple interacting pressures, including warming, stratification, nutrient stoichiometry, light availability, and circulation, which are inconsistent across modelling frameworks. Additionally, important components of the plankton community, such as microbial and gelatinous taxa, are often underrepresented in both observations and models, and responses to ocean acidification and combined stressors remain poorly understood outside experimental settings. Overall, these limitations lead to lower confidence in the direction and magnitude of predicted changes, although modelling capability is improving and the evidence base is growing.
1. Attributing multiple interacting pressures and model uncertainty
Attributing changes in plankton communities to specific drivers (e.g., warming/heatwaves, stratification, underwater light, nutrient balance, advection/circulation, and top‑down controls) remains challenging and major gaps remain in our ability to model future changes in pelagic food webs with confidence. Determining causality in complex systems that involve interacting drivers, non-linear responses, and critical tipping points, makes accounting for observed ecosystem shifts difficult, and model limitations and internal variability mean uncertainty in projecting future production, distribution, and food webs remains challenging. Without clearer attribution of plankton responses to environmental change, setting realistic management targets and assessing risk for fisheries and carbon flux remain challenging.
2. Monitoring blind spots and data integration issues for policy indicators
Major UK monitoring programmes, including the CPR Survey, under‑sample small or fragile taxa, including small and gelatinous zooplankton, which are increasing in some regions and are both expected to become more influential in future food webs. The CPR also misses important dynamics in offshore dinoflagellate. Pico- and nanoplankton, which dominate summer biomass in some regions and respond strongly to warming and stratification, are absent from OSPAR or UKMS indicators, despite including pathogenic microbes relevant to food security. Gelatinous zooplankton are inconsistently included in these policy frameworks. Key geographic monitoring blind spots also persist, particularly in western and offshore Scottish waters where important fisheries are concentrated.
Expanding monitoring coverage alone is insufficient; targeted mechanistic studies are also needed to identify processes controlling plankton populations, community structure, and ecosystem function. Without this understanding, predicting responses to warming, stratification, multi-stressor interactions, or altered nutrient regimes remains challenging.
3. Increase in extreme events, shifting circulation regimes, and tipping-points risk
Future climate pressures can be grouped into slow-onset hydrographic changes, sudden onset extreme events (e.g. more marine heatwaves, extreme storms and rainfall, and droughts, with multiple stressors compounding these pressures and slowing recovery) and low probability high impact tipping points through abrupt large scale ocean circulation changes.
Slow‑onset hydrographic changes influence nutrient availability and primary productivity, as well as prey fields and recruitment windows for forage fish such as herring and sandeel, increasing risk of mismatched phenology with their zooplankton prey and making fisheries harder to manage and predict.
Marine heatwaves have become more frequent, intense, and prolonged since the 1980s, disrupting plankton phenology and pushing key zooplankton toward thermal limits, with collapses observed in warm years. Other extremes (droughts, floodwater discharge, storms, and storm surges) are projected to intensify and they can resuspend toxins, darken coastal waters, decrease salinity, deliver pulsed nutrient loads, and cause plankton mortality. The short duration and sporadic nature of these events make sampling challenging, thus, impacts on plankton remain poorly understood. More field research at finer scales using developing imaging technologies will likely improve mechanistic understanding.
Low‑probability but high‑impact tipping points are also important to consider. For example, the Atlantic Meridional Overturning Circulation (AMOC), an important component of North Atlantic heat transport, is very likely to weaken this century. Strong weakening could substantially reduce phytoplankton biomass and cause ~30% declines in higher trophic level biomass. Even without full collapse, major weakening would reduce primary production, impact commercial fish stocks, and reshape the UK’s climate.
Image credit: Aaron Sanders