A slight increase in jellyfish sightings is an ominous harbinger of climate change to come

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Have you seen a jellyfish on a recent trip to the seaside? British bathers are more likely to spot one now than in the past, as rising sea temperatures due to climate change have brought more of these gelatinous animals into northern European waters.

Jellyfish don’t swim like fish. They belong to plankton: a diverse group of sea 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 (less than two millimeters) and can only be seen under a microscope.

Although largely invisible, plankton are the basis of the ocean food web, eaten by fish, seabirds and even whales. Species that do not eat plankton, such as seals, will eat organisms that do. Broadly, phytoplankton (single-celled algae which, like terrestrial trees and shrubs, are predominantly green in color and use chlorophyll for photosynthesis) produce half the oxygen we breathe.

Increasingly abundant jellyfish are just one example of the many ways plankton reflect the influence of climate change on the ocean. My research team has discovered that the species that make up plankton communities in the North Atlantic are also change as sea temperature increases.

We analysed plankton data collected using nets and bottles throughout the Northeast Atlantic over the past 80 years. We have found that the larvae of crabs, starfish, sea urchins and lobsters are increasingly common, while shrimp-like crustaceans called copepods (an essential food source for fish, seabirds and even basking sharks) are in decline.

These are big changes among some of the smaller life forms, and they will affect the entire marine food web, as well as humans. You have to understand these changes to adapt to them. It could mean new fishing practices – and even diets.

In the wake of a jellyfish

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-skinned” group to which starfish and sea ​​urchins), which later settle on the seabed and mature into their familiar adult forms. Zooplankton and phytoplankton communities are very diverse and contain species in all sorts of weird and wonderful shapes.

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

The seasonal calendar of plankton abundance in the North Sea has also quirky, including around the UK. While the seasonal cycle of phytoplankton is governed by sunlight and therefore has not changed, the time of year when certain species of zooplankton are most abundant now comes earlier, as shorter and warmer winters cause the eggs of some species to hatch earlier. This has resulted in a lag between the spring bloom of phytoplankton and the annual peak of zooplankton abundance that gorges on it.

These changes have meant that the amount and type of food available to larval fish (which are themselves zooplankton 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.

Fisheries managers must 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 also have to adapt their diets as traditional species, such as cod in the UK, become scarce.

The jellyfish you now see in UK waters may have once been rare, but they follow a (largely unseen) crowd that is disrupting marine food webs and changing the kind of fish you could buy and eat locally. The next time you watch the hypnotic movement of one of these beautiful creatures as it pulsates through the water, think about the changes its arrival portends, both for the ocean and for yourself.

This article was originally published on The conversation by Abigail McQuatters-Gollop at the University of Plymouth. Read it original article here.

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