A range of different harmful algal blooms (HABs) are routinely observed in UK waters. A strong regional distribution can be observed in the distribution of these HAB genera, with shellfish toxin producing genera and their impacts being more regularly detected along the Irish South and West coasts and in Scotland. Analysis of phytoplankton time series data from the continuous plankton recorder (CPR) has shown the dynamics of this community to be sensitive to changes in the physical conditions in the water column, with a number of changes identified over the last four decades. These include an extension of the growing period and increase in phytoplankton biomass (associated with warmer sea surface temperatures) as well as a change in the timing of dinoflagellate blooms. In particular the distribution of a selected number of HAB genera in the NE Atlantic has been observed to have changed over this time. In some regions of the North Sea these changes appear to be associated with climatic oscillations e.g. changes in the NAO index.

Analysis of data from a number of coastal monitoring sites over the last two decades have also highlighted a number of changes. Large blooms of the dinoflagellate Karenia mikimotoi, which is associated with fish kills, have been observed in northern waters off Ireland and Scotland. These blooms are thought to develop in the more offshore regions and predictions of climate change that indicate any increase in the duration of stratification of the water column in the future could influence the development of these blooms.

Climate change may also influence the toxicity of some HAB species via processes such as pH, nutrient availability, temperature and irradiance (cloud cover). In Scottish waters the incidence of intoxication of blue mussels (Mytilus edulis) with paralytic shellfish poisoning toxins (PSP toxins), primarily due to Alexandruim spp, have decreased since the 1990s, with no closures of shellfish harvesting areas enforced in two of the last five years. Recent research described the dinoflagellate responsible for producing the azaspiracid (AZA), Azadinium spinosum, from specimens collected from Scottish waters This toxin has been responsible for extensive closures of Irish shellfish harvesting areas over the last eight years.

Increases in sea temperature have the potential to allow harmful species currently not detected in UK watersd to become established as part of the phytoplankton community should they be introduced via shipping activity, drifting debris or natural range expansion. There is a high natural variability associated with HAB events, particularly with respect to local hydrographic conditions, therefore any changes in temperature, salinity, water column stability or precipitation have the potential to impact the dynamics of HABs in UK waters through direct or indirect effects. Changes in UK waters may also be driven by offshore influences.  Changes in the circulation of the subpolar gyre influence the ecosystem structure on the eastern margin of the North Atlantic and hence the Northern North Sea.

The influence of some climate change impacts such as ocean acidification remains unknown.

What is already happening: Medium

What could happen: Low

Coastal monitoring data since the 1990s has shown a high degree of variability in the incidence of HABs around the UK coast. The impact from some HAB genera (e.g. Alexandrium) appears to be decreasing. There is the possibility that species predominance may change in the future and thus it is likely that while there may be an increase in events in some areas, others will experience a decrease.

The top priority knowledge gaps that need to be addressed in the short term to provide better advice to be given to policy makers are:

1. The majority of HAB monitoring is performed in coastal areas. The role of advection from the offshore in seeding blooms in coastal areas requires further attention.
2. Limited work has been performed in modelling different HAB species in UK waters. It is therefore difficult to predict the response of different HAB groups and genera to the influences of climate change.
3. The impact of increasing ocean acidification on UK HAB species has yet to be investigated.
4. Ichtyotoxic species such as Karenia mikimotoi have yet to be investigated to a level that would allow a proper evaluation of their impacts on the marine ecosystem (including critical life phases of exploited species).

Increased HAB events may have a direct detrimental effect on ecosystems and they can often have a direct commercial impact on aquaculture industry as both shellfish and fish farms can be affected by HABs. Predictions of future food consumption indicate that these two sources will be of much greater importance in the future.

Predicted strengthening of shellfish toxin regulatory levels will have a large impact on shellfish harvesting activities.

Bresnan, E., L. Fernand, K. Davidson, M. Edwards, S Milligan, R Gowan, J Silke, S Kröger & R Raine (2010) Climate Change impacts on Harmful Algal Blooms (HABs) in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 10pp. www.mccip.org.uk/arc