Coccolithophores belong to a diverse group of phytoplankters (class Prymnesiophyceae) that is widespread in the oceans. The ecological group that includes Emiliana is particularly abundant in upwelling and temperate sub-polar regions . Most coccolithophores are not themselves harmful or toxic, but they are of ecological importance, particularly for carbon cycling and sequestration . They typically produce an exoskeleton consisting of several calcium carbonate plates called coccoliths . These coccoliths are not opaque (phytoplankton require light for photosynthesis), but they scatter and polarise light. Coccolith shedding occurs during the later stages of the bloom life cycle, when the cells are threatened, for example by pathogen pressure . During this stage, the coccoliths are shed and accumulate in the surrounding water. The visual effect is to turn the sea a milky turquoise colour, visible to the human eye and in satellite imagery.

The function of the coccoliths is unclear. They are believed to be protective, either against grazing, against viral or bacterial attack, or as a refractor of light that acts as a sunshade in excessively bright conditions . show that coccoliths initially provide some protection from viral attack but once shed can mediate such attacks. suggests that coccolith production may have evolved originally as an efficient mechanism for intracellular Ca2+ detoxification at a time of elevated seawater Ca2+ concentrations (e.g. during the Cretaceous and Jurassic periods).

Summer 2021 saw milky, turquoise-coloured waters caused by algal blooms in two locations off Scotland, in the Clyde Sea on the west coast and also to the east of the Shetland Islands (Fig. ). These blooms were visually striking and so unusual that they were reported in the news (e.g. ). Transmission-electron-microscope analysis of a water sample from the Clyde Sea (Fig. d) confirmed the algae to be a morphotype of coccolithophore, Emiliania huxleyi (morphotype B). Blooms of this organism are common in spring and early summer in the North Atlantic and occur in some years in the northern North Sea and the western English Channel. However, such striking occurrences have not been reported from the Clyde Sea for many years. Colleagues of one of the authors (PT) recall no such event since 1983, when they remember sampling turquoise waters and coccolithophores in sea lochs of the Firth of Clyde. In this paper we use observations and reanalysis data to look for unusual environmental conditions in 2021 that might have allowed these blooms to thrive.

Figure  shows a bloom on the eastern side of Shetland. There is no information on the species present, but the brightness and turquoise colour of the bloom suggest coccolithophores. Coccolithophore blooms in the North Sea are not unusual. An unusual feature of the 2021 bloom was that it came so close inshore. Examination of imagery for the years 2017 to 2020 from Plymouth Marine Laboratory (PML) (described in ) and of a dataset of coccolithophore blooms for 1998 to 2016 in finds no other examples where blooms intrude among the islands and bays on the eastern side of Shetland.

show that often blooms develop further south and east in the North Sea in spring or early summer and are advected by an anti-clockwise circulation sometimes reaching as far north as Shetland. Sometimes a bloom originates in the north, along the northern edge of the continental shelf, and is advected southwards. Bathymetry in Fig. c shows the location of this shelf edge. The European Slope Current flows eastward along this edge, bringing North Atlantic water into the Norwegian Sea. Some of this water flows south past Shetland into the North Sea. Imagery for July 2021 (Fig. c, d) seems to show the bloom is of the latter kind, originating along the northern edge of the shelf.

To confirm the bloom's origins and to understand why 2021 was unusual, we used the OceanParcels software package to simulate the trajectory of virtual particles in the ocean. Particles were initially positioned at 1 m depth along the slope current (Fig. ). These locations were chosen based on where the speed of the current (from a reanalysis mean climatology for 1993 to 2021) exceeded 0.2 ms-1. OceanParcels modelled 3D movement of the particles, advected by daily mean currents from the NWS reanalysis (product ref. no. 1, Table ) and using a 3 h time step. This was done separately for each year from 1998–2022, starting with particles in initial positions on 3 April and running forward 3 months to predict positions on 3 July (Fig. ).

In some years nearly all the particles move off beyond the edge of the plots (1999, 2002, 2003, 2009, 2022). In most years all the red particles (those initially at the eastern end of the slope current) disappear in this way. In 2021 it is these red particles that end up close to the eastern side of Shetland on 3 July. The year 2021 was unusual, although not unique, in that easterly winds during spring drove surface currents that pushed particles westward for part of that time. Figure  shows 10 m winds (from ERA5) and surface currents (from NWS reanalysis) for May 2021 and for a May climatology. To the east and north of Shetland, climatological winds are westerly, and the surface current is westerly or northerly. For May 2021, winds are north-easterly. These winds induce easterly surface currents (Ekman transport effect).

The years 2012, 2016, and 2019 also experienced easterly winds in spring or early summer (based on ERA5 reanalysis) and similarly show large numbers of red particles still within the plot region on 3 July. Other years see large numbers of other-coloured particles come close inshore, in particular 2007, 2017, and 2020. In the satellite imagery (2017–2020) and Kondrik catalogue (1998–2016), none of these years show coccolithophore blooms that reach into the bays and inlets of eastern Shetland. Imagery for 2020 (not shown) comes the closest, with a bloom 10–20 km away from the coast. The 2020 particle tracking has orange particles in this region, originating from further west on the shelf edge.

We conclude that in some years blooms around Shetland form in water coming from the shelf edge. We cannot conclude that bloom development is always linked to specific locations and timings of source water along the shelf edge.

To understand how the bloom appeared so close inshore, we examined the daily particle trajectories for 2021 (not shown). Particles move south down the eastern side of Shetland during the second half of June. In late June and early July there is a brief period of easterly winds, and the particles are driven in towards the coast. Brief easterlies are not unusual, but these coincided with coccolithophore-laden water near the coast. We suggest that coincidences of timing and weather in 2021 created the unusual phenomenon of a visible coccolithophore bloom on the eastern Shetland coast.

Analysis of the 2021 bloom in the Clyde Sea is hampered by a paucity of observations. Weather stations provide data on the atmospheric conditions. Satellite instruments provide estimates of SST and of chlorophyll, with caveats discussed below. We have found no in situ measurements of conditions within the Clyde Sea itself. Biogeochemical reanalysis data are available , but their ability to accurately simulate the Clyde Sea is hampered by a lack of data on river discharge. Freshwater input and nutrient input from rivers are important variables for the biogeochemistry here. In this paper we analyse the data that are available and build from them a plausible storyline. That storyline starts with diatom growth in early spring.

Phytoplankton are of special interest in the waters around Scotland, where aquaculture and fishing are major industries. Through primary production, phytoplankton form the base of a food chain that sustains marine fauna . Blooms of phytoplankton can be harmful to other marine life and can produce toxins dangerous for human consumers of seafood . There is thus increasing interest from policy-makers to understand the diversity dynamics of phytoplankton communities in Scotland and other parts of the North West Shelf (NWS) and to understand its influence on industries and diversity status assessments .

This paper presents hypotheses to explain two unusual blooms. We suggest that the bloom on the eastern side of Shetland originated in Atlantic water brought north of Shetland by the European Slope Current. The water's passage eastward was retarded by a period of anomalous easterly winds in May, and it was later steered towards the Shetland coast by a shorter period of easterly winds. The timing was such that there were abundant coccoliths present when this water was close inshore. The bloom within the Clyde Sea appears to have developed in place. We hypothesise that environmental factors may have combined to create suitable conditions in the Clyde Sea. A cold and dry April could have restricted spring growth of diatoms, leaving nutrients available for a summer bloom of coccolithophores. A wet and stormy May might also have added to the nutrients.

Our explanations are based on limited evidence (SST and chlorophyll estimates from satellites, modelling by the reanalysis). We do not have in situ measurements from within the blooms to confirm our hypotheses.

For both blooms, we propose the weather as a key factor. Other studies also identify the importance of the weather for algal blooms. looked at unusually strong blooms of the biotoxin-producing dinoflagellate Dinophysis on the western side of the Shetland Islands in the summers of 2006 and 2013. They found these blooms coincided with periods where the winds, usually more southerly, became westerly. They suggested the westerly winds advected Dinophysis populations onshore, resulting in an increase in diarrhetic shellfish toxin levels in farmed mussels (Mytilus edulis).

There is evidence that the distribution of coccolithophores has expanded polewards in recent decades , due either to changes in ocean temperature or dissolved inorganic carbon. Growth of E. huxleyi is also known to be impacted by major changes in ocean pH . Changes in pH observed in seas around the UK are not large enough to explain recent variability in coccolithophore abundance in Scottish waters.

Changing weather patterns have the potential to influence the occurrence of unusual phytoplankton blooms in coastal waters. These changes have the potential to impact higher trophic levels in the marine ecosystem. A better understanding of the processes and dynamics involved will help in forewarning, preparation, and development of adaptation measures for these changes.

This paper shows how use of satellite data and model reanalysis can help to meet the challenge of assessing major events in UK waters, despite a sparsity of in situ observations. However we could be more confident in our findings if we had more information about environmental conditions. More complete data on river discharge would help in simulating biogeochemical and ecosystem variables in the Clyde Sea and other inshore water bodies. Widespread routine monitoring of nutrient levels and phytoplankton components could help greatly in understanding future blooms.