Why are there more jellyfish in the sea and what does this mean?

Did you see a jellyfish on a recent trip to the seaside? UK beachgoers are more likely to spot one now than in the past, as rising sea temperatures due to climate change have ushered more of these gelatinous animals into the waters around northern Europe.

Jellyfish don’t swim like fish. They belong to the plankton: a diverse group of marine creatures that drift through the sea, floating wherever the currents take them. Jellyfish are among the few types of plankton visible to the human eye. Most plankton are tiny (smaller than 2mm) and can only be seen with a microscope.

Although largely invisible, plankton are the base of the ocean food web, eaten by fish, seabirds and even whales. Species that don’t eat plankton, like seals, will eat organisms that do. Globally, phytoplankton (single-celled algae which, like trees and shrubs on land, are mostly green in colour and use chlorophyll to photosynthesise) produce half of the oxygen we breathe.

Increasingly abundant jellyfish are just one example of the many ways that plankton are reflecting climate change’s influence on the ocean. My research team has found that the species making up North Atlantic plankton communities are also shifting as sea temperatures rise.

We analysed plankton data collected using nets and bottles throughout the northeast Atlantic over the past 80 years. We found that the larvae of crabs, starfish, sea urchins and lobsters are becoming more common, while shrimp-like crustaceans called copepods (a critical food source for fish, seabirds and even basking sharks) are declining.

These are big changes among some of the smallest forms of life, and they will affect the entire marine food web, as well as humans. We must understand these changes in order to adapt to them. That could mean new fishing practices – and even diets.

Zooplankton (the animal subset of plankton) consists not only of copepods and jellyfish, but also the larval stages of fish, crustaceans and echinoderms (the “spiny skin” group that starfish and sea urchins belong to) which later settle to the sea floor and mature into their familiar adult forms. Both zooplankton and phytoplankton communities are highly diverse, containing species of all sorts of weird and wonderful shapes.

Since the 1960s, colder-water zooplankton species have been retreating towards the Arctic, followed by warmer-water species that are also tracking rising sea temperatures northwards. The warmer-water zooplankton species which now dominate northern European waters are generally smaller and less nutritious than the cold-water species they have replaced.

The seasonal timing of when plankton are abundant in the North Sea has also shifted, including around the UK. While the seasonal cycle of phytoplankton is driven by sunlight and so hasn’t changed, the point in the year when some zooplankton species are most abundant now arrives earlier, as shorter and warmer winters cause the eggs of some species to hatch sooner. This has meant a mismatch between the spring phytoplankton bloom and the annual peak abundance of the zooplankton that gorge on it.

These shifts have meant the quantity and type of food available to larval fish (which are zooplankton themselves but eat smaller zooplankton) is changing in the North Atlantic. Warm-water species such as bluefin tuna and anchovies are now commonly found in northern European waters, while cod, herring, whiting and sprat, all important commercial fish species, have declined in number.

Fishery managers need to work with scientists to set quotas that ensure these new species are fished sustainably, while coastal fishing communities may have to catch new species as familiar ones decline. The public may have to adapt their diets too as traditional species, such as cod in the UK, become scarcer.

The jellyfish you now see in UK waters might have once been a rarity, but it’s following a (largely invisible) crowd that is upending marine food webs and changing the kind of fish you might buy and eat locally. The next time you watch the hypnotic motion of one of these beautiful creatures as it pulses through the water, think of the changes its arrival portends, both for the ocean and yourself.