So, what is marine plankton bloom?

In this article Sally Campbell explains the why we have been experiencing the bright blue seas around Arran over the last month. Featured image shows Lamlash Bay waters 24th June 2021. Photo credit: Sally Campbell.

Plankton are a diverse collection of microscopic plants and animals that live in the sea.
Plankton blooms sometimes termed algal blooms, are a common phenomenon anywhere in the ocean describing the explosive population growth driven by certain favourable conditions. The initiators are often phytoplankton, microscopic plants that float in the upper, sunlit layers of the ocean. When large numbers of phytoplankton are concentrated in one area, surface water colour changes, as we have seen around Arran this last month. Algal blooms are often initiated by a nutrient, such as nitrogen or phosphorus acting as a fertilizer entering the aquatic system and causing excessive growth. An algal bloom affects the whole ecosystem. Consequences range from the benign feeding of higher trophic levels in the food chain, such as jellyfish, shellfish, and herring to more harmful effects like blocking sunlight from reaching other organisms, causing a depletion of oxygen levels in the water, and, depending on the organism, secreting toxins into the water. The process of the oversupply of nutrients leading to algal growth and oxygen depletion is called eutrophication.

While the growth-limiting nutrient is typically nitrogen or phosphorus, it can also be iron, vitamins, or amino acids There are several mechanisms, both natural and human intervention for the addition of these nutrients in water. In the open ocean and along coastlines, upwelling from both winds and topographical ocean floor features can draw nutrients to the sunlit zone of the ocean. Along coastal regions such as the Clyde and Arran, salmon aquaculture, land agriculture, city drainage, sewage, fertilisers and pesticides in the water also cause algal blooms.

Lamlash Bay 24th June 2021

The trophic level of an organism describes the position it occupies in a food web or chain. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. A food web starts at trophic level 1 with primary producers such as phytoplankton. It can then move to grazers/ filter feeders at level 2; Zooplankton are the animal component of the planktonic community. They are heterotrophic meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton. Many types of zooplankton migrate deeper into the water during the day and come up at night. The migration of species appears to be dependent on location rather than particular species types. All plankton migrate differently based on factors like age, sex and the season. The amount of light is probably the major factor governing the extent of migratory behaviour. Carnivores such as fish occupy level 3 or higher, and typically finish with apex predators at level 4 or 5, which of course also includes humans !

So, what happened in Lamlash Bay? What do we know?

Satellite approx. 12 June PML


Satellite week 20 June NEODASS


Appearance from the shore initially looks like a lake below a glacier. In this case ice wears away the rock below releasing a very fine sediment on melting that becomes suspended in meltwater making the water appear cloudy, and turquoise in colour. In Lamlash Bay and now the whole Clyde, we have seen a similar phenomenon in these present conditions as tiny calcareous shelled plants called coccoliths have grown in population suspended in the water! This phytoplankton bloom in the Clyde is probably initially mostly these coccoliths but also dinoflagellates Ceratium and possibly Karenia. The first images from Satellite based remote sensing of the bloom in the Clyde, received by Plymouth Marine Laboratory showed it developing around the south of Arran. The main objective of this Sentinel-3 mission is to measure sea surface topography, sea and land surface temperature, and ocean and land surface colour with high accuracy and reliability to support ocean forecasting systems, environmental monitoring and climate monitoring. The colour confirmed by the Plymouth laboratory examination suggests a mix of chalky coccoliths and an active bloom of Emiliania Huxley, a species of coccolithophore found in almost all ocean ecosystems from the equator to sub-polar regions. PML also suggested the dinoflagellate Karenia, which have been followed for a couple of weeks. However, in a sample taken from Lamlash Bay in July 2020, Ceratium was the dominant dinoflagellate. All these species names simply illustrate the complexity of plant and animal life in the natural world that is often disrupted by man’s intervention.

Turbulent storms churn the ocean in winter, adding nutrients to sunlit waters near the surface. This sparks a reproduction and feeding frenzy each spring as days lengthen and water warms, that can give rise to massive blooms of phytoplankton. Tiny molecules found inside these microscopic plants harvest vital energy from sunlight through photosynthesis, turning carbon dioxide dissolved in seawater into energy and excreting oxygen. The natural pigments, called chlorophyll, allow phytoplankton to thrive. It is thus claimed in certain quarters that blooms act to compensate as a sink for excess carbon responsible for global warming. The phytoplankton includes all the plant-like organisms, and they contribute directly to the food availability in surface waters for zooplankton, by building up their cells and food reserves directly from the carbon dioxide and salts in solution in the sea. Of prime importance are nitrates and phosphates. Most obvious in regard to size and numbers in samples of British phytoplankton are the diatoms, and next in importance are the dinoflagellates. Coccoliths are small flagellated planktonic stages with a calcareous capsule, which are attached to the substratum. These play an important part in the biochemical processes of the sea and are also important in that they are the organisms primarily responsible for the laying down of chalk in Upper Cretaceous times, as in the White Cliffs of Dover. This is a geological period that lasted from about 100 to 66 million years ago.

Phytoplankton cell size ranges over several orders of magnitude: from less than 2 micrometres (μm) in equivalent spherical diameter up to 200μm; 1000 μm equals one millimetre. So, one micron (μm) of length is equal to one millionth of a metre. We are being told by divers and sailors out in Lamlash Bay that there is little clarity of water, 1 metre maximum, so maybe that is influencing the almost complete absence of gannets diving for food in this locality at the present time.

What are these phytoplankton?

Evidence from simple optical microscopic examination suggests that what we are seeing in Lamlash Bay are coccoliths. These are the individual plates of calcium carbonate formed by coccolithophores, single-celled algae such as Emiliania huxley which are arranged around them in a coccosphere. Coccolithophores are spherical cells about 5–100µm across, enclosed by these calcareous plates, which are about 2–25µm across.


Diatoms are algae that live in houses made of silica rather than calcium. They are single celled algae. They are the only organism on the planet with cell walls composed of transparent, opaline silica. Diatom cell walls are ornamented by intricate and striking patterns of silica. Diatoms are commonly between 20-200 microns in diameter or length, although sometimes they can be up to 2 millimetres long.


Dinoflagellates are single-celled algae, mostly marine plankton, but they also are common in freshwater habitats. Their populations are distributed depending on sea surface temperature, salinity, or depth. Dinoflagellates are unicellular plankton, ranging in size between 2µm and 2mm in diameter, with complex life cycles involving asexual, sexual, motile, and non-motile life stages as well as resting cysts, depending on species.

Images show Tripos commonly known as Ceratium. Microscope image from Lamlash Bay water sample, courtesy of Arran resident Colin Cowley

Ceratium can cause blooms which deplete the resources and nutrients of the surrounding environment. These dense algal blooms also deplete the dissolved oxygen in the water, which is known to cause fish kills. In this bloom in 2021, Protoperidinium granii is one of the common dinoflagellates, known especially in colder waters.

Protoperidinium granii

Lamlash Bay microscopic image. Courtesy of Colin Cowley

Karenia consists of unicellular, photosynthetic, planktonic organisms found in marine environments. They are best known for their dense toxic algal blooms that can cause considerable ecological and economic damage; some Karenia species cause severe animal mortality. One species, Karenia brevis, is known to cause respiratory distress and neurotoxin shellfish poisoning (NSP) in humans. In general, more phytoplankton means more food and oxygen for everything else in the ocean, but occasionally plankton blooms are hazardous to human or marine life. These are known as HABs, Harmful Algal Blooms. What are the health effects of HABs? Depending on the type of algae, they can cause serious health effects and even death. For example, eating seafood contaminated by toxins from algae called Alexandrium can lead to paralytic shellfish poisoning (PSP), which can cause paralysis and even death. The dinoflagellate produces saxitoxin, which is a highly potent neurotoxin. If consumed, this toxin can cause PSP. By ingesting saxitoxin, humans can suffer from numbness, ataxia, incoherence, and in extreme cases respiratory paralysis and death. My mother always said. “Never eat shellfish when there is no R in the month”, which meant no shellfish, such as mussels, oysters, cockles, winkles and whelks, in May, June, July and August, as these are the months of phytoplankton blooms and some of those may well be HABs.

Karenia brevis

So what do we think is happening around us ?!

This Spring and Lamlash Bay

The Spring bloom has come late this year due to the climate/weather. Lots of wind earlier and a long spring of wave disturbance (ferry cancellation a guide!) and cold temperatures; turnover of water, rivers running high due to the rain, bringing dissolved nutrients and fine materials into estuaries and sea resulting in a bloom when, with wind and wave turnover these nutrients and plankton material are nearer the surface. This will also include nutrients from salmon farms’ waste (copper, zinc, phosphates, nitrates, carbon). Then conditions in the bay become ideal: calm, lots of hot sun, as mid-summer, long sunny days so thermocline gets established. The thermocline is the transition layer between the warmer mixed water at the sea’s surface and cooler water below. This has developed over the last 3 weeks and has maybe reached its peak at the latter end of June. When the wind returns, and sun obscured by rain or cloud the accumulated warmth will be dispersed into the deeper layers so swimmers will find water more uniform lower temperature. Usually, there is a spring phytoplankton bloom (very late this year!), which then feeds the zooplankton bloom, then the phytoplankton nutrients get “used up”, oxygen becomes depleted, there is a windy patch, water mixes, and the bloom retreats. Blooms typically last until late spring or early summer, when nutrient stocks are then in decline and predatory zooplankton start to graze.

Bodies of water are made up of layers, determined by temperature. The top surface layer is called the epipelagic zone, or “sunlight zone”. This layer interacts with the wind and waves, which mixes the water and distributes the warmth. At the base of this layer is the thermocline. A thermocline is the transition layer between the warmer mixed water at the surface and the cooler deep water below. It is relatively easy to tell when you have reached the thermocline in a body of water because there is a sudden change in temperature. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep-water temperature. In the sea, the depth and strength of the thermocline vary from season to season and year to year. It is semi-permanent in the tropics, variable in temperate regions, such as the UK, often deepest during the summer when sun is stronger, days longer and less wind, and shallow to absent in the winter.

Many zooplankton settle out, so young fish, shellfish move to shallow bottom sea adult habitats. Much of the phytoplankton dies off, drops to seabed where next year the nutrients of shell of dead coccolithophores will be raised again by wind and warmth. The usual patterns in these latitudes are a spring big bloom followed by early autumn smaller bloom. If you think about it, we sometimes experience weather disturbances at end of summer followed by “Indian summer” of warmth, no wind and a last hurrah before the equinoxial gales! Surface water is at its warmest here in September.

So, we have seen an incredible colour of the sea, remarked on far and wide as being a rare phenomenon. It is all part of the complexity of our marine environment, affected by weather, tides, nutrients in the water and that includes human interference. The species of all types of plankton will change through the summer. Copepods are becoming more numerous as zooplankton feeds on the phytoplankton. This week small sandeels are in the rockpools which feed primarily on plankton, tiny gurnards and small common gobies also present, whilst moon jellyfish are on the shore in Brodick, and small jellyfish in the water of Lamlash Bay, so already the composition of the inshore waters is changing.

Enjoy the blue ocean whilst it lasts!

Common Plankton in Lamlash Bay. Collecting and photos by Colin Cowley

Ceratium tripos


Protoperidium species




Larger group of copepods


Dinophysis species is shown beside a human hair, to show scale

Many thanks to Colin Cowley, for information and photos of plankton

Sally Campbell
June 2021

Newell, G.E. and Newell, R.C. (1964) Marine Plankton a practical guide. Hutcheson
Hardy, A. (1964) The Open Sea: Its Natural History. Part 1. The World of Plankton. Collins

In Memory of Dr Richard (Dick) Newell January 1939-March 2021

PS Most recent information:

If anyone is interested in watching harmful algal blooms, the Food Standards Scotland
publish results of tests carried out where there is commercial shellfish harvesting. This is to issue warnings where there is a risk to humans from eating shellfish. The results are published every 4 weeks. Shellfish filter feed and accumulate toxins. Where there are classified waters, shellfish are tested regularly.

The first table results of the monitoring of harmful phytoplankton and the second table shows Biotoxin levels.
For example, in just the last week the levels of Paralytic Shellfish poisoning (PSP) have reached dangerous levels at Bay of Skaill Westray – Orkney Isles, Loch Inchard – Sutherland, and Loch Leurbost – Lewis and Harris.
In this same week the levels of Amnesic Shellfish poisoning (ASP) have reached dangerous levels in Loch Sunart – Lochaber, and Inner Loch Torridon – Ross and Cromarty.

30 June 2021