Plankton are migrating due to climate change (and it could wreak havoc on fish)
Watch: Plankton Are Beginning to Migrate and That Could Have Catastrophic Consequences
Ocean warming is causing plankton to move towards the poles as the oceans warm up – and it could have unforeseeable knock-on impacts, including on fish stocks.
Plankton also play an important role in removing CO2 from the atmosphere – which could be threatened as our warming world drives them towards the poles.
Plankton are tiny ocean creatures carried by tides and currents.
Phytoplankton (plant-like plankton) use photosynthesis to fix carbon from carbon dioxide, making them a key driver of the oceanic carbon cycle.
They are also food for zooplankton (animal plankton), which nourish creatures including blue whales.
Lead author Fabio Benedetti said: “In some areas of the ocean, we will see a rise in species numbers that may, on the face of it, seem positive.
"But this boost in diversity could actually pose a serious threat to the existence and functioning of well-established marine ecosystems at higher latitudes.”
Read more: Melting snow in Himalayas drives growth of green sea slime visible from space
The researchers put together a new global dataset to create distribution maps for more than 860 species of phytoplankton and zooplankton based on various statistical algorithms and climate models.
The researchers believe that the number of zooplankton species could increase in subpolar seas by up to 40% in 2081-2100, but that they could also decrease in tropical oceans.
At very high temperatures – above 25C – phytoplankton and zooplankton respond differently to warming: phytoplankton diversity continues to increase, while zooplankton diversity decreases.
This would lead to a reduction in zooplankton diversity in the tropics.
Plankton species from the tropics and subtropics would shift polewards and replace species that are adapted to cooler waters.
Researchers expect the biggest changes to occur in oceans at high and temperate latitudes – precisely those regions that are crucial for CO2 fixation and fisheries.
The researchers simulated the effects of climate change on the size structure of two important plankton groups, the diatoms and copepods.
Read more: A 1988 warning about climate change was mostly right
Using the simulations, the scientists demonstrated that the quality of the habitat increases for smaller organisms, while it decreases for the larger ones.
Plankton communities could change, and so too the relative proportions of small and large species: smaller organisms would become more abundant and numerous, especially at high and temperate latitudes, while larger organisms decrease in number.
According to the researchers, this would affect the ecosystem services that plankton provide – and could have a negative impact on fish yields.
Read more: Why economists worry that reversing climate change is hopeless
Plankton also play an important role in oceanic carbon fixation. Some of the carbon fixed by phytoplankton sinks to the deep ocean and is effectively removed from exchange with the atmosphere.
For example, the Arctic Ocean is currently home to phytoplankton that are larger than those in tropical seas. Many of these have shells, and their excretions are also larger and heavier.
As a result, both dead organisms and their excrement sink faster and to greater depths before the carbon they contain is decomposed back to CO2.
Dissolved in deeper waters, this CO2 remains trapped in the depths for long periods of time due to density stratification and the slower circulation of the deep ocean.
If smaller species replace larger ones, this transfer of carbon to the deep ocean will decrease.
Benedetti says, “The only thing we can determine right now is how important certain areas of the ocean are today in terms of different ecosystem services and whether this provision of services will change in the future.”
Watch: Who is 'Insulate Britain'?
