Early warning is key to ensuring industry can respond to the growing threat of HABs
HABs and HAB nots
ALGAL blooms often hit the headlines these days, featuring in more than 500 news reports last year in the US alone. This intense media focus reflects the fact that algal blooms are now occurring more often, at greater intensities and across a wider geographic range than ever recorded before.
A paper recently published in the journal Nature probed the changing dynamics of freshwater algal blooms using three decades of global satellite data. Their findings were unequivocal – the intensity of summertime blooms has increased in more than two thirds of lakes examined worldwide.
Similar trends are seen in the marine environment. In 2013, a green algal bloom formed on the coast of China that broke all previous size records in the region, extending an astonishing 28,900 square kilometres.
Algae are a diverse group of aquatic, photosynthetic organisms that range from microscopic, single-celled, phytoplankton to large multi-cellular seaweeds. When the density of an algal population increases rapidly, it is described as a bloom.
Blooms can occur naturally, driven by a complex interplay of factors such as the availability of nutrients and light, water temperature and currents.
The rise in algal blooms over the past several decades, however, likely reflects anthropogenic changes to the environment, such as nutrient pollution, climate change and the increased transport of algae in the ballast water of ships.
Algal blooms that have an adverse effect on the aquatic ecosystem, economy or human health are described as ‘harmful algal blooms’ or HABs.
The type of harm caused by HABs can vary. Some HAB forming microalgae produce toxins that kill nearby animals or bio-accumulate within them, causing illnesses such as paralytic shellfish poisoning (PSP) in humans if eaten.
Others are non-toxic and instead inflict physical harm. The HAB forming microalga Chaetoceros, for instance, bears a set of formidable silica spines that can damage the gills of fish, in some cases leading to suffocation.
Many HABs alter the environment around them so severely that it becomes uninhabitable for other organisms. The decay of dead algae within a bloom, for
“There are various reasons why a satellite is handy but not the be all and end all”
example, can rapidly reduce the availability of dissolved oxygen in the water, creating hypoxic ‘dead zones’ that are unable to sustain fish and other aquatic life.
Such harmful algal blooms can, of course, inflict large scale financial losses on the aquaculture industry. Indeed, farmed fish are particularly at risk as they are unable to avoid the location of a bloom as a wild fish might.
In 2016, a bloom of Pseudochattonella killed approximately 15 per cent of Chile’s annual salmon stock, worth an estimated US$800 million.
To minimise the risk of such devastating fish kills in the future, efforts are being made to improve the toolkit available to monitor, predict and mitigate the occurrence of harmful algal blooms.
HAB monitoring programmes often involve the routine collection of water samples to be analysed in the laboratory.
Traditional methods for detecting harmful algae in these samples (for example, microscopy, immunological screening for toxins) have more recently been complemented with a suite of highly sensitive molecular probes and assays that are capable of identifying individual HAB species at very low densities.
Outside the laboratory, a range of in situ monitoring tools are also being developed. A Scottish
consortium, for example, is currently working on an optical sensor system that can be deployed at aquaculture sites to detect the presence of harmful algae.
In a recent press release, Chris Hyde, chief commercial officer at consortium member OTAQ, said that this new technology ‘will fundamentally automate the (detection) process and provide accurate information about plankton numbers 24 hours a day’.
Remote sensing via satellite has also emerged as a valuable tool for monitoring harmful algal blooms. This form of data collection may offer several advantages over laboratory based methods, as it can occur in real-time, is less labour intensive and can be conducted on a larger scale.
However, Professor Keith Davidson of the Scottish Association for Marine Science (SAMS) explains that remote sensing via satellite should not be regarded as a silver bullet for bloom monitoring either, as some algae (particularly toxin producing species) are harmful at much lower densities than can be detected by satellite.
‘A harmful algal bloom can consist of just a few hundred cells per litre, depending on the species,’ said Professor Davidson. ‘But to detect a bloom by satellite, you’d probably need ten thousand or a hundred thousand cells per litre.’
Satellites are also limited in their ability to detect algae located below the sea surface.
‘If there is stratification of the water column, you might get a thin layer of algae sitting at 10m depth, for example, which a satellite won’t be able to detect. So there are various reasons why a satellite is handy but not the be all and end all.’
Professor Davidson and colleagues at SAMS have collaborated with partner institutions across Europe on two successive projects (ASIMUTH, and now PRIMROSE) to develop and refine a system for forecasting HABs that threaten the aquaculture industry across the Atlantic Arc in Europe.
Forecasts are generated by incorporating data from HAB monitoring programmes (for example, microscopy counts and toxicity levels) and satellite remote sensing into mathematical models of physical and biological processes.
The team at SAMS prepare HAB forecasts for Scotland, which are shared with users online (https://www.habreports.org/) alongside regional HAB monitoring data.
Professor Davidson describes these HAB forecasts as ‘similar to a weather forecast, in that it gives a picture of what’s coming up in the next week or so’.
This early warning can provide the aquaculture industry with a valuable opportunity to mitigate the impact of a bloom, through actions such as the relocation of enclosures or early harvesting.
Looking forward, the expansion of such early warning systems beyond Europe could be key to ensuring that the global aquaculture industry can respond to the growing threat posed by harmful algal blooms in a timely and effective manner.
Below: This RGB composite image from Sentinel-2A taken on 7 August 2015 has a spatial resolution of 10m. It shows an algal bloom in the central Baltic Sea.
Above: Mussels on Loch Fyne. Far left: Queen scallops. Opposite Image of pseudo-nitzschia seriata complex- a type of diatom responsible for producing domoic acid which if consumed via intoxicated shellfish will cause amnesic shellfish poisoning (ASP). They can form chains in excess of 20 cells long. (Photos: SAMS)