Firth of Clyde bloom. Credit: Lucy Wallace

Big bloom in the Firth of Clyde

15-17 June, 21 June 11:35 UTC & 29 June 2021 10:38 UTC

Credit: Lucy Wallace

Firth of Clyde bloom. Credit: Lucy Wallace
Firth of Clyde bloom. Credit: Lucy Wallace

Ocean colour data reveals the extent of beautiful blue waters associated with a phytoplankton bloom observed by hikers around the Isle of Arran in June 2021.

Last Updated

21 February 2023

Published on

03 August 2021

By Hayley Evers-King (EUMETSAT), Peter Miller (Plymouth Marine Laboratory), Ben Loveday (Innoflair) and Lucy Wallace (Arran Wild Walks)

On 12 June, Lucy Wallace from Arran Wild Walks, noticed unusually bright patches of water offshore of several bays around the Isle of Arran, Scotland. Over the next few days, while running coastal wildlife watching trips, she noticed increasing bright turquoise patches of water, contrasting with the usual dark blue of the ocean around the island. By 16 and 17 June, all the waters surrounding the island were tropical blue, prompting many questions from locals.

Firth of Clyde bloom. Credit: Lucy Wallace
Figure 1: Photo of blue waters around Arran on 12 June. Credit: Lucy Wallace.

Lucy shared a photo (Figure 1) of the waters around the Isle of Arran on Twitter and was subsequently connected to Hayley Evers-King  and Peter Miller, who have been able to provide a satellite perspective of the bloom using data from the Copernicus Sentinel-2 and Sentinel-3 satellites from 15-29 June (Figures 2, 3 & 4).

Sentinel-2 True Colour with enhanced contrast, 21 June
Figure 2: Sentinel-2 MSI perspective of Firth of Clyde on 21 June 11:35 UTC, True Colour with enhanced contrast. Processed by NEODAAS, using ACOLITE atmospheric correction.
Sentinel-3 OLCI True colour with enhanced contrast, 29 June
Figure 3: Sentinel-3 OLCI perspective of the Firth of Clyde from 29 June. True Colour RGB with enhanced contrast.

The chalky blue colour of the water suggested the presence of a distinct type of phytoplankton — coccolithophores. These tiny organisms build and shed calcium carbonate armour known as liths, which scatter light and give the water its milky blue colour, visible to both human eyes and satellite instruments. Although unusual near the shore, coccolithophores are very common, particularly in mid to high latitude oceans.

Using multi-spectral data from Sentinel-3 OLCI allows for a more detailed interpretation of the colour. An enhanced colour image composite (Figure 4) allowed Peter Miller to suggest that the bloom was a mix of an active bloom of coccolithophores (likely Emiliania huxleyi), detached liths, and, possibly, some Karenia mikimotoi. This is because the resulting colour was not the bright turquoise a bloom would be if only containing the liths, but also contained a mixture of green/brown colours associated with absorbing phytoplankton, and the yellow colour of suspended sediment.

Sentinel-3B 3-day composite 15-17 June
Figure 4: Enhanced 'true colour' view of the bloom in the Clyde, Sentinel-3B 3-day composite 15-17 June. This image enhances the blue-green colours of the spectrum, so is not quite true colour. Processed by NEODAAS.

Interpreting images to provide information about blooms, particularly whether they may or may not be associated with species that can cause human and environmental health problems, is the subject of several projects and commercial services, provided by Peter and colleagues at Plymouth Marine Laboratory. For example, Scottish Karenia Watch is a service for salmon farming companies for which PML provides weekly updates of novel harmful algal bloom (HAB) risk maps for Karenia mikimotoi species, based on their characteristic colouring of the water; a dense bloom of that species can kill farmed fish.

The ocean colour techniques undergo continuous development via European and UK research programmes, most recently the Interreg Atlantic Area project Predicting the Impact of Regional Scale Events on the Aquaculture Sector (PRIMROSE) and Interreg Channel project Sentinel-3 satellite products for detecting Eutrophication and Harmful Algal Bloom events in the French-English Channel (S-3 EUROHAB).

While bright, chalky waters are a very clear indicator of coccolithophores, satellite data can’t fully confirm whether there are or aren’t certain species present in a bloom. Combining the satellite data with information from water samples analysed with microscopy can help provide a more complete understanding of what is happening in the water. Using all available information, is vital for making decisions about bathing water quality, food safety, and fisheries and aquaculture operations, when blooms occur.

Additional content

HABreports: Online Early Warning of Harmful Algal and Biotoxin Risk for the Scottish Shellfish and Finfish Aquaculture Industries (Frontiers in)
HAB report website (HAB)