Nearly half the Earth’s basic food supply comes from photosynthesis performed by microbes and phytoplankton in the oceans. Since they are the basis of the marine foodweb, changes in these primary producers transfer upwards through to higher trophic levels either directly or through their grazers, the zooplankton. Their success can potentially affect the size of fisheries, and the survival and success of such key species as sharks, turtles, seabirds and marine mammals in UK waters. Microbes and plankton are also sensitive indicators of environmental change at both the regional and global level; through various feedbacks they can both influence, and be influenced by, climate change. However, in spite of their great importance, until recently they have been very poorly understood.

Microbes

Microbes, which include many protists and all archaea, bacteria and viruses, are invisible to the human eye yet make up one of the most important and extraordinarily diverse forms of life on our planet. In the marine environment they exist in complex, interdependent food webs with the rest of the oceanic biosphere. Microbial cyanobacteria account for a substantial proportion of the ocean’s primary productivity, especially in tropical and subtropical waters and even in UK waters are responsible for up to 50% of primary productivity – although this varies by region and season. However, we still lack a fundamental understanding of the complex roles they play in UK waters. There is thus insufficient evidence to assign a current or future health status to
microbes. We do know, though, that because of their large population sizes and quick generation times, microbes can evolve rapidly in response to a changing environment. UK waters are important custodians of microbial diversity; we need to improve our knowledge of the environmental pressures that affect this key dynamic community.

Plankton

Through their production, plankton sustain almost all other marine organisms and determine the carrying capacity of ecosystems around the British Isles. In particular they regulate larval fish development and survival, and thus the success or failure of recruitment to the adult fish stocks. This free-floating life of the sea is divided into phytoplankton (plants) and zooplankton (animals). Copepods, the dominant zooplankton group in the North Atlantic, are small crustaceans, generally between 0.5 mm and about 8 mm long.

Plankton also modulate climate change through a range of feedbacks, especially by exporting carbon to the deep ocean via what is known as the ‘biological pump’. A reduction in the carbon exported by this pump would lead to an increased build up of atmospheric greenhouse gases and accelerated global warming. This role played by plankton in ameliorating or increasing the rate of global warming is probably great, but as yet unquantified. There is clear evidence for large and extensive changes in the composition, abundance and spatio-temporal occurrence of both phytoplankton and zooplankton in waters adjacent to the UK and in the North Atlantic. However, we do not have enough information to determine plankton variability and its contribution to atmospheric levels of carbon dioxide (CO₂) at a UK, regional and global scale, and this is one of the key missing factors in modelling global climate change. Large-scale changes have occurred to the distribution of plankton in UK waters. However, based on the large amount of data gathered on plankton from long-term observations, we consider that the plankton as a whole are healthy and subject to few direct anthropogenic pressures. The ‘traffic light’ status allocated to plankton has been graded amber because of the consequences to ecosystems and fisheries of the observed changes in the plankton due to rising sea temperatures and because it is still unclear to what extent natural variability, climate change and cascading effects from fishing may be contributing to change.

Phytoplankton

Photosynthesis by phytoplankton makes up at least 50% of the primary production in UK waters. Based on data from the Continuous Plankton Recorder (CPR - see photo) survey, there has been a large increase in phytoplankton biomass over the past two decades in offshore waters around and to the west of the British Isles and in the past decade in the subpolar gyre. These changes probably reflect increased phytoplankton production, but there are no field measurements of production rates to confirm this. While the seasonality of the peak spring bloom of phytoplankton has remained relatively stable in time, the marine growing season has lengthened, particularly in the summer months and to a lesser extent during winter, due to rising sea temperatures.

Recent studies based on CPR phytoplankton surveys suggest that climatic variability and water transparency are more important than nutrient concentration to phytoplankton production at offshore regional scales, at least for the North Sea. Despite the overriding influence of climate, elevated nutrient and chlorophyll levels are sometimes of concern in localised areas around the British Isles.

Toxic blooms

A number of phytoplankton found in UK waters are harmful or toxic to some marine life. They tend to reoccur often in areas where they have previously been recorded, but there is no recognisable pattern in their timing. These events are closely monitored and warnings issued when needed. Higher sea temperatures may favour an increase in the intensity and frequency of bloom events for some harmful and/or toxic species in the future.

Mass blooms of jellyfish may at times (rarely) cause serious economic damage to tourism and aquaculture. (Jellyfish are classed as plankton as they rely totally on tides, waves and currents for large-scale movement; species (such as fish) that can determine their own large-scale distributions are known as Nekton.) While there are some suggestions that jellyfish abundances may be increasing, we do not understand when and why blooms occur.

Zooplankton

As a result of changing climate, many phytoplankton taxa have begun to bloom sooner in the year, putting them out of synchrony with the zooplankton and fish larvae that rely on them for food. This in turn may lead to failure of fish recruitment. Copepods in particular are a key food item for higher marine animals. Their composition and seasonal abundance/timing is crucial for fish recruitment. Since the 1950s, the abundance of total copepods has fallen considerably in UK waters. There has also been a marked shift from a cold boreal community dominated by holoplankton (plankton that spend all their time in the water column) to one characterised by warm temperate species. There has been a large increase in the abundance of meroplankton (planktonic larvae of benthic animals living on the bottom) in the North Sea since the mid-1980s but the causes are not clear. The net result of overfishing and changes in the hydroclimate of the North Sea appears to be a change in the balance of productivity in the pelagic environment and on the seabed.

There has been a progressive shift northward in warmer water zooplankton and a retreat to the north of colder water species over the past 50 years. The relative proportions of the cold-water indicator copepod Calanus finmarchicus and its warmer water sister species C. helgolandicus have shown a similar northward movement (see Figure 3.2). Such geographic shifts in these dominant species will affect the recruitment, growth and survival of fish and seabirds. For example, the zooplankton now present in the North Sea which have more ‘warmer water’ characteristics are also smaller and have a lower biomass and oil content than the previously dominant cold-water zooplankton, making them less nutritious. Because the new zooplankton community also blooms at a different time of year, there is now a mismatch in its availability as a food for fish larvae. There is well documented evidence that these changes have influenced the numbers of cod and other fish as well as seabirds.

If, as predicted, sea temperatures around the UK continue to rise, comparison with more southerly latitudes suggests that planktonic diversity will increase, but that the total carrying capacity will reduce.

As sea temperatures rise the introduction of non-native species could increase. The ranges of a number of existing introduced species are already extending due to rising temperatures and are expected to expand further. These changes will have unknown consequences for biodiversity, ecosystem functioning and living marine resources. The summer melting of Arctic ice, and consequent opening up of links between the Pacific and North Atlantic, is likely to exacerbate this problem. It is important that an adequate monitoring programme is funded to assess the rates of introductions and their impacts.

Ecosystems, especially on the western margins of the UK are strongly influenced by changes in the adjacent ocean. Our understanding of the processes modulating interactions between plankton and hydrodynamics between ocean and shelf regions is poor. Ocean acidification is expected to impact planktonic ecosystems and especially vulnerable calcareous organisms in the future. Research is still at an early stage and the evidence to date is equivocal. However, the potential consequences through impacts on the foodweb and on higher trophic levels, including fish, are serious.

Future work

For this assessment we have gathered as much information as possible about the vital, small and little understood creatures in our seas. However, there is still much to do. In order of priority:

• We need to begin more widespread plankton sampling to address geographical gaps in the coverage of UK waters, particularly to the west of Scotland and in parts of the Atlantic to the west of the UK.
• We need more information about the role of plankton in the carbon cycle and biological pump and its contribution to atmospheric levels of CO₂ at a UK, regional and global level. This will require international partnerships to fund monitoring and research in key areas of the ocean that are currently under-sampled.
• We have no long-term measurements of changes in primary production in UK waters. We need to begin routine in situ measurements at one or more sites to calibrate satellite and modelling output.
• We need to increase our efforts to sample smaller biological groups (such as microbes, nanoplankton and picoplankton) to understand their occurrence and role in UK waters.
• We need to know more about the interactions between microbes and other plankton and higher trophic levels, including fish and seabirds, and how these interactions may change in response to climate change and ocean acidification.
• Greater use of models will help. These could be used to develop scenarios of change for a warming climate, and could eventually produce short-term forecasting and longerterm prediction.
• Finally, since Charting Progress there has been considerable development in the use of new genetic tools to study microbial and planktonic populations and their variability through time and space. We need further efforts in this area to improve our understanding of the diversity and occurrence of the many planktonic organisms that are not sampled or cannot be clearly identified by traditional techniques.