Science

Where plankton fits in the Earth’s ecosystem

Organism gives sea its distinctive smell and promotes creation of clouds, thereby playing a significant role in global climate

A diatom: they are among the most important and prolific microscopic sea organisms and are an important part of plankton: Illustration from Life Under The Lens
A diatom: they are among the most important and prolific microscopic sea organisms and are an important part of plankton: Illustration from Life Under The Lens
Padraic Fogarty
Thu Nov 21 2024 - 06:00

That forests on land are a crucial element of the climate and biodiversity battle is pretty well appreciated. The magic ingredient here is chlorophyll which the trees and other plants use to activate the transformation of carbon dioxide and sugars into forms of carbon which can be stored over long periods, such as wood.

However, much less appreciated is the fact that chlorophyll is also present in tiny plants that float in the sea. This may not amount to much except for the fact that the sea covers more than 70 per cent of our planet’s surface, and so the sheer volume of this oceanic plant matter is responsible for producing fully half of the oxygen we breathe.

The collective term for organisms that live in the ocean but which are unable to swim against a current is plankton (stronger swimmers are known as nekton). Each spring, as the Earth tips towards the sun, the increased light and warmth of the surface waters trigger a massive bloom of plant plankton, technically referred to as phytoplankton. The blooms can be so big as to turn the waters green in streaks across the oceans that are visible from space.

This explosion in plant matter is followed by a wave of grazing by animal plankton, or zooplankton. These animals are mostly tiny and include the embryonic forms of all kinds of creatures from crabs and urchins to barnacles and fish but can also be big: some of the jellyfish can be many metres in length.

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The zooplankton themselves form the chief food source for a great variety of larger animals. Certain fish, such as sand eels and sprats, feed exclusively on plankton, themselves becoming food for larger fish and seabirds, such as puffins, as well as marine mammals right the way up to the great whales. The plankton is the basis for the entire marine food web, but their wonders do not stop there.

Although most plankton are small, their existence has been appreciated for some time. Some species produce their own light, known as bioluminescence, and produce the most stunning displays, rather like a biological version of the northern and southern lights. Charles Darwin noticed it when travelling on the Beagle in the 1830s, noting that “as far as the eye reached, the crest of every wave was bright, and the sky above the horizon, from the reflected glare of these livid flames”.

Beginning in 1899, the German biologist Ernst Haeckel, transcended the boundaries of art and nature with his extensive colour illustrations of plankton, including an entire volume devoted to the radiolaria, a group of single-celled phytoplankton, then revealed in great detail by the latest microscopes. His illustrations revealed their spiked, sputnik-like forms and latticed exoskeletons in exquisite detail. They caused a sensation at the time, inspiring works of painting and architecture which were a part of the Art Nouveau movement.

In the 1960s, James Lovelock believed that the plankton were a part of a planetary system of self-regulation which he christened “Gaia”. Plankton produces dimethyl sulphide, a vapour that is credited with giving the sea its distinctive smell but one that also promotes the creation of clouds, thereby playing a significant role in global climate.

In winter, as plankton blooms die back, their tiny skeletons drift towards the ocean floor in a perpetual snow that transfers minerals from the atmosphere to the sediments of the deep, essentially locking them away.

As such, the plankton are not only fundaments of the ocean’s ecology but are critical in the cycling of carbon through Earth’s systems that have, until now, helped to stabilise the climate. But the scale of extraction of marine life from the world’s oceans as a result of fishing, the run-off of pollutants from the land, particularly nitrogen and phosphorous, as well as the burning of fossil fuels is now seeing that stability break down.

In September, at the UN General Assembly, scientists launched a Global Plankton Manifesto, calling for “immediate global recognition and action to protect these vital organisms” and to “leverage new technologies and existing data sets to develop plankton-based solutions to support life on our planet”.

“Lots of things are affecting change in our ocean but if we’re thinking about plankton, the biggest driver of change is climate” says Abigail McQuatters-Gollop, associate professor of marine conservation at the University of Plymouth, UK.

This means warmer temperatures and changes to how water mixes between the deep and the shallows, and so how nutrients are transferred between these two areas. If seas are calmer, then nutrients do not make it to the surface waters where phytoplankton can avail of them. Since the 1950s the plankton in the northeast Atlantic has shifted 1,000km northward. “It’s not the individuals that are shifting northward, as they don’t live very long,” she says. “It’s the habitat, that is becoming more influenced by the tropics. In the waters around northern Europe, we’re starting to find species that weren’t here at all 60 years ago, and at the same time we’re missing species which are more suited to colder waters”.

We mustn’t view the plankton as a generic soup, as the diversity and abundance of individual species, and groups of species, within marine food webs matter, just as it does on land. So, although the overall volume of plankton in northeast Atlantic waters is relatively stable, this masks a significant decline in certain groups, which in turn can resonate through food webs, particularly if those species have less nutritional value to other animals higher up the food chain.

Last year, Ospar, an international body charged with monitoring the waters around western Europe, published a status report, of which McQuatters-Gollop was a lead author, warning the status of the Celtic Seas (ocean roughly west of Britain) was “not good”. It noted that the “planktonic larvae of benthic invertebrates, such as crabs and sea urchins, have increased in abundance” while “other zooplankton which provide the crucial link between primary production and fish have experienced long-term declines in abundance which could resonate higher up the food web”. The report refers to “widespread changes in water column dynamics and nutrient availability driven by both climate change and reductions in nutrient inputs”.

Without plankton, we would lose a big source of oxygen and although McQuattors-Gollop says that there is no sign that this is happening, she cautions that “it could be a future consequence, somewhere down the line because of climate change. By absorbing carbon dioxide, the plankton have helped protect us from climate change, but there’s only so much they can do. If we don’t have plankton communities that can produce oxygen and support fish populations that are important food sources, then we are in trouble as humans.”

What to do? McQuattors-Gollop says that “the thing we really need to manage is our carbon because climate change is the overwhelming driver of change in the plankton in the northeast Atlantic”. If plankton are no longer supporting the fish populations that are important food sources for humans, we can address this by managing fishing pressure. “Maybe those fish stocks need a break,” she says. Indeed, allowing fish and whale populations to recover, alongside restoring marine ecosystems would, because the animals store carbon in their bodies, increase the capacity of the carbon storage capacity of the sea.

Such is the potential for plankton to sequester carbon that there have been proposals to use them in geoengineering experiments. Having determined that iron is a limiting nutrient in the growth of the phytoplankton, showering the ocean surface with iron filings would, in theory, trigger plankton blooms, and so act as a form of carbon capture.

However, McQuattors-Gollop sounds a cautious note: “It is dangerous, we don’t know what the outcome will be. If you ruin an entire ecosystem, what could the unintended consequences be? There are things we need to do first before we start manipulating the entire ocean. Conservation, protection and restoration of ‘blue carbon’ habitats is a much safer place to start.”