At the very surface of the ocean there lives a thriving ecosystem teeming with small plant-like organisms known as phytoplankton. You may not give much thought to these tiny creatures and their lives, but the whole ocean ecosystem depends on their survival. They produce almost 50% of the world’s oxygen, yet are under threat from the changing climate.


So what are phytoplankton?

Phytoplankton are microscopic organisms, only around the size of a pinhead (usually 0.02 mm to 0.2 mm), and most are invisible to the naked eye. Although they create their energy in the same way as plants, by using chlorophyll through the process of photosynthesis, they are actually algae, a protist like seaweed.

Phytoplankton are very important organisms, distributed in large quantities around almost all of the globe. They don’t just produce oxygen, they are the base of the ocean’s food chain fuelling almost all other life in the ocean, which means they determine how much fish we can catch. These unseen creatures are just as important a resource for humanity as terrestrial ecosystems such as rain forests.

Phytoplankton are often comprised of a single cell, though there are some multi cellular forms. There are currently around 5,000 species of phytoplankton known to science.

Different shapes of phytoplankton
Phytoplankton come in all sorts of amazing shapes. Image source: Sailors for the Sea
Map of chlorophyll distribution in oceans globally
Phytoplankton are found in virtually every part of the ocean, but are concentrated mostly around coastlines. The distribution and amount of phytoplankton can be globally tracked by measuring chlorophyll concentrations in the water. Image source: SeaWiFS Project










Though you may not have seen the extraordinary shapes of individual phytoplankton, you probably have seen the greenish tinge of seawater in the spring, like in the picture below. These occur during phytoplankton “blooms”, when the populations rapidly increase in the spring.

Comparison of water colour before and after a spring bloom
The contrast between the colour of water before and during a phytoplankton bloom is extraordinary. Image source: NASA Goddard YouTube Video


The effect of these blooms is even more startling if you see them from a distance. In satellite images, such as the one below, you can see the light reflecting off the giant clusters of cells in the water. When the populations of phytoplankton increase in such great magnitudes as this, it also spurs increases in the populations of other creatures down the food chain, from tiny fish to large marine mammals.

Phytoplankton bloom around Britain
The lighter-coloured areas around the British coast are the phytoplankton blooms reflecting light. Image source: European Environment Agency





How do phytoplankton survive out in the ocean?


Since phytoplankton are photosynthetic organisms, they depend on light to create their energy. This means that phytoplankton are restricted to very near to the water’s surface, as the strength of the light entering the water decreases exponentially with depth. Phytoplankton are only found in the photic zone, where enough light enters the water to allow photosynthesis to take place.

Phytoplankton don’t just float around at the very surface of the water all day, especially if the sun’s rays are very powerful. Too much UV light can damage phytoplankton, so they move into deeper water when the light is too strong for too long. By vertically migrating down the water column, they can avoid harmful light intensities.

Very little light gets through the water column in the winter, limiting the growth of phytoplankton. But in the spring when light intensities increase the populations undergo a massive growth very quickly, with phytoplankton cells dividing around once a day.


When the water’s surface is heated during the summer, the water column becomes stratified as the heated water is less dense, and sits on top of the dense cold layer of water. This means that the surface layer containing the phytoplankton is separated from the underlying waters, with no mixing between the two layers.  All the nutrients in the narrow surface layer become quickly depleted as no nutrients are brought up from the waters below. This causes phytoplankton populations to decline in the summer despite the available light energy.


It’s not just the physical conditions of the water which challenge phytoplankton. They also undergo massive predation pressure. Other animals in the water can eat up to 73% of the phytoplankton that are produced each day in the summer.

You may think that phytoplankton, tiny organisms at the mercy of the waves, are pretty defenceless and an easy meal for other creatures in the water. But phytoplankton have evolved some interesting ways to avoid predation. Phytoplankton can produce their own light as a defence, known as bioluminescence. Bioluminescence is light produced by living organisms, using a chemical called luciferin in a biochemical reaction to create light energy. These kind of chemical reactions use a lot of energy, so phytoplankton only produce bioluminescence when threatened. It is used for protection against predators, both startling and disorientating potential predators, or attracting larger organisms to prey on any creatures threatening the phytoplankton like a “burglar alarm”. Of course, these defensive measures do not always work. But it’s a good thing they don’t; phytoplankton are the base of the ocean’s food chain, feeding up into almost every other form of life living in the sea.

bioluminescence from glowing plankton in tide line on beach, with stars above,  Vaadhoo Island, Raa Atoll, Maldives
The phytoplankton use bioluminescence to create sparkling blue waves. Image source: National Geographic, Doug Perrine


What lies ahead?

Phytoplankton have a tough life, and climate change affecting the ocean environment makes it even harder. Phytoplankton are at the start of a massive marine food chain, the majority of the ocean depends on them.

The changing climate on earth is reducing phytoplankton populations, as well as spurring the growth of blooms of harmful algal blooms which are toxic to humans and other animals. This is not good news at all, as what affects phytoplankton affects the rest of the ocean, and those who depend on it for their survival. Sometimes even the tiniest creatures can have the most colossal impact on our lives.

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