Big blooms of coccolithophores along UK south coast

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Sentinel-3 data provided perspective on big blooms of coccolithophores that formed off the southern coast of the UK in late June/early July 2019.

Big blooms of coccolithophores along UK south coast
Date & Time
27 June–15 July 2019
Sentinel-3A & B
Level-1 OLCI radiances in RGB, Sea surface temperature time series

By Ben Loveday (PML) and Hayley Evers-King (EUMETSAT)

Coccolithophorid species, such as Emiliana huxleyi, are a type of calcifying of plankton that bloom in the mid to high latitudes during spring and summer. Sentinel-3 ocean colour data, combined with sea surface temperature time series from the Copernicus Marine Environmental Monitoring Service (CMEMS), can observe and help to explain why these calcium carbonate lith forming plankton might form such massive blooms.

Although individual coccolithophores are tiny (Figure 1), these spectacular blooms are one of the most visible features in ocean colour images. OLCI sensors on board the Sentinel-3 satellites regularly capture these blooms (Figure 2).

Figure 2: The calcium carbonate outer bodies of coccolithophores are highly reflective, making them appear as bright, cloud-like patches in the sea. This animation shows nine images captured by the OLCI sensors on board Sentinel-3A and B from the end of June and through July.

As these species form calcium carbonate shells, they play an important role in the ocean (and global) carbon cycle, and may be affected by both climate change and ocean acidification.

It is, therefore, vital to understand what drives the formation of these blooms at particular times and locations and to understand how the role of coccolithophores in the ocean may change and how this might, in turn, feed back on ocean biogeochemistry and climate.

The processes influencing phytoplankton growth are complex, and include physical factors such as solar radiation and temperature, and biological factors, including physiology, competition and predation.

The aforementioned physical factors combine to influence the level of ‘stratification’ or how stable the water column is. A stable water column can allow phytoplankton, like coccolithophores, access to light that they can use to photosynthesise and grow. Work by scientists has shown that these factors can explain a large part of the variability in coccolithophore bloom formation (Raitsos et al., 2006).

Figure 3
Figure 3: The sea surface temperature anomaly in June–-July 2019 using a baseline of 2002–2018 daily data from CMEMS. The data was averaged over the area shown in Figure 2.

Deriving an anomaly (how much a current measurement differs from a longer term average) of sea surface temperature (Figure 3) indicates that the waters that these blooms formed in, were warmer than average, and, coupled to weak winds, may have been more stratified than usual. This may help to explain why these large blooms formed when and where they did.

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