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Marine Science: Viewpoint

Harmful algal blooms: a global solution…

Marine Science: Viewpoint

Harmful algal blooms: a global solution

by Professor Keith Davidson, Associate Director, Scottish Association for Marine Science (SAMS)

Phytoplankton are free-floating plants found in marine and freshwater ecosystems that, through their photosynthetic growth, form the base of the aquatic food chain. These microscopic algae also play a major role in global biogeochemical processes, being responsible for about 50% of the photosynthetic activity on the planet. There are many thousands of different species of phytoplankton and a small subset of these may be harmful to human health, or to human use of the ecosystem. The organisms that cause harm are now widely referred to as ‘harmful algae’ and the term ‘harmful algal bloom’ (HAB) is commonly used to describe their occurrence and effects.

There is a common misperception that all HABs are causally linked to elevated nutrient concentrations with an anthropogenic origin. Given sufficient nutrients, phytoplankton can indeed grow rapidly and, in some locations as discussed below, elevated abundance is an indicator of eutrophication (excessive richness of nutrients in water, often due to run-off from the land). However, both worldwide and in Scottish waters, most HABs are natural events that are spatially and temporally variable.

Globally, HAB events are perceived to be increasing in frequency and seriousness, with a recent example being the Pseudochattonella bloom in Chile in 2016, which killed 39 million farmed salmon with a value of US$800 million. In Scottish waters, risk from HABs is two-fold and can be broadly categorised as being related to either “high-” or “low”-abundance species. The former impact fish farms, primarily through dense blooms of the ichthyotoxic dinoflagellate Karenia mikimotoi, or various spiny diatoms such as the genus Chaetoceros, which can cause damage to gills. Human health is threatened by the typically low-abundance shellfish biotoxin-producing genera that are ingested by filter-feeding shellfish. Important genera in Scottish waters are Alexandrium, Dinophysis and Pseudo-nitzschia. The toxins become concentrated within the shellfish flesh, reaching levels that can harm consumers.

Whilst regulatory monitoring of shellfish species and their toxins safeguards human health (at least in Europe), it does not provide early warning of future HAB events and is not relevant to fish-killing species. With the global demand for protein from the sea and the associated rapid increase in aquaculture in the UK and abroad, assessment of the timing, location and magnitude of HABs over a time scale of 1 – 2 weeks is potentially of great value to industry. Such information allows remedial action, such as early or delayed harvesting, moving of fish cages, deployment of physical or bubble curtains around farms or other methods of mitigation.

Achieving such early warning is far from straightforward, as key HAB genera or species exhibit different life cycles and potentially variable toxicity. Variability in local or regional oceanography or hydrography is also critical to bloom location and timing. Expert interpretation of multiple data streams, including phytoplankton and associated environmental drivers, is therefore required to assess the risk of HABS and forecast their occurrence. The EU FP7 project Asimuth (2010–2013) provided the platform for the first HAB early warning system in Europe. Building on these foundations, SAMS now operates a HAB early warning system for aquaculture via the web site This provides a searchable map of current and historic HAB conditions Scotland-wide, with the option to download a detailed weekly HAB assessment for the aquaculture-intensive region of the Shetland Islands.

Improvements to forecasts are ongoing, as we gain a better understanding of the ecology of the HAB species. These will be further enhanced by the application of a range of new technologies, including improved remote sensing algorithms, remote underwater vehicles such as gliders, molecular biological approaches to identify morphologically indistinct species, in situ phytoplankton counters and the better physical/biological coupling of mathematical models.

Given the vital role of phytoplankton in marine ecosystems, their importance has been recognised by their inclusion in regulatory frameworks such as the European Water Framework Directive and the Marine Strategy Framework Directive, as an indicator of ecological status. Quantitative techniques are therefore required to assess the environmental status of the phytoplankton in a water body. One such index is the microplankton index PI(mp), based on the relative abundance of different phytoplankton “life-forms”, which was developed by Professor Paul Tett of SAMS. Funded by the RSE (the Royal Society of Edinburgh), we have recently collaborated with Professor Kedong Yin of Sun Yat Sen University in China to use the PI(mp) to evaluate the health of the phytoplankton community in Tolo Harbour, Hong Kong. This is a large bottlenecked bay which has long been used as an example of the link between anthropogenic nutrients and HABs.

Tolo Harbour water quality was seriously impacted during the 1980s and 1990s, following a rapid population increase and associated nutrient run-off. In response to this heavy eutrophication, the Tolo Harbour Action Plan was completed in 1998, reducing anthropogenic nutrient pollution in the area. However, the quantitative impact of this remedial action on the phytoplankton community has not previously been evaluated. We therefore applied the PI(mp) to the 23-year time series (1991–2013) to evaluate how the reduction in nutrient loading has impacted the biodiversity of the phytoplankton community at two sites, in the inner and the outer regions of Tolo Harbour. We found marked differences in the rate of change in the balance of organisms at the two sites, but they both eventually reached a similar, presumed healthier, final state, providing confidence that the large water-treatment investment has paid dividends.

Given the similarity of HAB issues in different regions globally, their study requires international cooperation to achieve a coordinated approach. This has led to the development of the IOC/SCOR GlobalHAB initiative(1). This 10-year programme, which held its first steering committee meeting at SAMS in 2016, seeks to understand the ecological and oceanographic controls on HABs and to promote research that minimises the harm from HABs to human society in a rapidly changing world.


(1) Berdalet E., Kudela R., Urban E., Enevoldsen H., Banas N.S., Bresnan E., Burford M., Davidson K., Gobler C.J., Karlson B., Lim P.T., Mackenzie L., Montresor M., Trainer V.L., Usup G., Yin K. (2017) GlobalHAB: New Program to Promote International Research, Observations, and Modeling of Harmful Algal Blooms in Aquatic Systems. Oceanography 30:70-81.






"Marine Science: Viewpoint". Science Scotland (Issue Twenty-one)
Printed from on 03/07/20 11:09:29 PM

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